Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session M04: Flash Oral Presentations: Multiphase Flows |
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Chair: Petia Vlahovska, Northwestern University Room: 6e |
Monday, November 25, 2019 3:20PM - 3:21PM |
M04.00001: Local analysis of the clustering, velocities and accelerations of particles settling in turbulence Mohammadreza Momenifar, Andrew D. Bragg We use Direct Numerical Simulations (DNS) and 3D Vorono\text{\"i} tessellation to analyze the local dynamics of small inertial particles in isotropic turbulence, considering the effect of Taylor Reynolds number ($R_\lambda$), Froude number ($Fr$), and Stokes number ($St$). In line with previous results using global measures of particle clustering, we find that for small Vorono\text{\"i} volumes, the behavior is strongly dependent upon $St$ and $Fr$, but only weakly dependent upon $R_\lambda$, unless $St>1$. However, larger Vorono\text{\"i} volumes (void regions) exhibit a much stronger dependence on $R_\lambda$, even when $St\leq 1$. This, rather than the behavior at small volumes, is the cause of the sensitivity of the standard deviation of Vorono\text{\"i} volumes to $R_\lambda$ that has been previously reported. Particle acceleration results indicate a non-trivial effect of gravity, while results for the fluid acceleration at the particle position call into question the sweep-stick mechanism for clustering. Comparing the local dynamics of particles in clusters to all particles in the flow reveals that while their kinetic energies are nearly the same, the clustered particles settle much faster on average, and this difference grows significantly with increasing $R_\lambda$. [Preview Abstract] |
Monday, November 25, 2019 3:21PM - 3:22PM |
M04.00002: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 3:22PM - 3:23PM |
M04.00003: Feedback control of a combined two-fluid/electro-spray coaxial injector via real-time measurements and Principal Component Analysis Rodrigo Osuna-Orozco, Nathanael Machicoane, Peter Huck, Alberto Aliseda We present an experimental study of the physics of gas-assisted atomization combined with electro-spray. We leverage a low dimensional representation of the spray, from light attenuation measurements, to implement feedback control in real-time. The laminar liquid stream is injected through a long straight metallic needle at the center of the turbulent gas jet. The liquid is electrically charged by a very strong electric field at the nozzle exit. We apply an external electric field along the spray development region, characterizing the ability of an external sideways force on the individual droplets to modify the structure of the spray in the midfield. We characterize the break-up dynamics using high-speed visualizations in the near field and the droplet sizes and velocities in the midfield using light interferometry. In the implementation of real-time control, we use optical attenuation of light traversing the spray downstream of the nozzle in the mid-field. Low dimensional representations of the radial liquid volume fraction profiles allow for real-time control of the spray based on actuation on the gas total and angular momentum as well as on the external electric field. [Preview Abstract] |
Monday, November 25, 2019 3:23PM - 3:24PM |
M04.00004: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 3:24PM - 3:25PM |
M04.00005: On The Relationship Between Internal Flow and Jet Dynamics in a Charge Injection Atomizer William Doak, Paul Chiarot Atomization of a dielectric micro-jet is achieved using an electrohydrodynamic (EHD) charge injection process. The atomizer is comprised of a grounded nozzle plate and an internal high voltage probe, with electric potentials up to 20 kV, concentric with the emitting orifice. Dielectric fluid flows through the cavity between the electrodes, impeding electron transport from the probe to ground and imparting charge to the fluid. When the jet is uncharged, it breaks up via an axisymmetric (Rayleigh-Plateau) instability. Once charged, a non-axisymmetric (bending) instability, in addition to the axisymmetric instability, is observed in the jet. Both instabilities modes grow with increasing jet charge density: the intact jet length shortens and the bending amplitude increases. We have found that changes in the jet instability modes are related to the EHD flow induced inside the nozzle. A transparent nozzle was built and the flow was seeded with micro-spheres to study this phenomenon. High speed microcopy and digital image correlation was used to measure the EHD flow at different jet stability conditions. We found that transitions in the internal flow are associated with changes in the jet stability regimes. [Preview Abstract] |
Monday, November 25, 2019 3:25PM - 3:26PM |
M04.00006: Using 3D-printed analogues to understand the aerodynamics of atmospheric ice particles Mark McCorquodale, Chris Westbrook Parameterisations representing the aerodynamics of ice particles in the atmosphere are required for weather and climate models. However, the aerodynamics of natural ice particles is poorly understood on account of the substantial variability in both shape and Reynolds number of particles that occur within the atmosphere. We report results from a laboratory study in which the aerodynamics of 3D-printed ``analogues'' of ice particles are investigated. Particles fall through a 1.8m tall column of quiescent viscous liquid at Reynolds numbers matching those of snowflakes in the atmosphere, between approximately 5 and 5000. Free-falling analogues are recorded using a system of synchronised digital cameras, which facilitate the digital reconstruction of the trajectory of the particles. We use this data to test commonly used parameterizations of the drag coefficient of natural ice crystals, and find that the drag coefficient is typically under-predicted at large Reynolds numbers (Re\textgreater 200), leading to predicted fall speeds being overestimated, by as much as a factor of 2. We identify that the reduced accuracy of existing parameterizations at high Reynolds number is associated with the onset of unsteady motions as the particle falls. [Preview Abstract] |
Monday, November 25, 2019 3:26PM - 3:27PM |
M04.00007: Hindered coalescence and break-up with insoluble surfactants Carolina Vannozzi For Capillary numbers (Ca) greater than 0.05, the thin film between two drops undergoing a flow-induced head-on collision, in the creeping flow regime, thins to a steady state thickness hss, as shown in [1]. Here, we analyze numerically this phenomenon in the presence of surfactants. For trace amount of surfactants, hss diminishes with decreasing surfactant interfacial diffusivity (Ds) and , unlike Nemer et al.'s analysis for non-diffusing surfactants[1], surfactants are still present in the dome region. For higher surfactant concentration, hss is present only for very high Ds, while, as Ds decreases, the film drains continuously. As Ca increases the two drops break-up into 4 district drops. Interestingly, for low surfactant concentrations, the dome region will thin, becoming a neck. This will continuously thin and stretch as the two extremities depart from each other following the streamlines, while keeping hss constant. This process will proceed until instabilities arise in the neck. Whereas, for high surfactant concentrations, the dome region will thin only at the center and the extremities will keep draining in place. [1] Nemer et al. 2004 Phys. Rev. Lett.~92. [Preview Abstract] |
Monday, November 25, 2019 3:27PM - 3:28PM |
M04.00008: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 3:28PM - 3:29PM |
M04.00009: Particle aggregates via droplet evaporation on superhydrophobic fractal-like substrates. Carola Seyfert, Eva Krolis, Erwin J.W. Berenschot, Arturo Susarrey-Arce, Niels Tas, Alvaro Marin Sessile droplets on superhydrophobic substrates are common in nature and technology. In the case of droplets containing solid particles, the evaporation of the solvent turns into an effective tool to aggregate any non-volatile content. The high contact angles and unpinned contact lines of the droplets, induced by hydrophobicity, can lead to a complete recovery of the solid solute in form of aggregates. Under the right conditions, the solid remainder takes the form of highly compact and spherically shaped aggregates, featuring a minimal contact area with the supporting substrate. We investigate the evaporation of colloidal droplets on a new kind of superhydrophobic, micro-structured substrate, featuring fractal-like glass pillars. Such substrates present an intricate geometry with non-flat top surfaces of the pillars. Different sizes and concentrations of monodisperse polystyrene particles lead to various shapes of particle aggregates after the evaporation of the solvent. [Preview Abstract] |
Monday, November 25, 2019 3:29PM - 3:30PM |
M04.00010: Colloidal crystallization in cylindrical geometry: Effect of particle wettability on banding Tejaswi Soori, Ying Sun A colloidal drop constrained within a capillary tube subjected to evaporation results in crystallization and particle banding. Past studies have shown the width and spacing of bands to increase with concentration for cylindrical geometries. The radius of curvature $R$ and capillary length $\ell_{c}$ for drops over planar substrates are usually of the same order of magnitude, while for capillary tubes the radius of curvature $R$ is equal to the capillary radius $r$ which can be much smaller than the capillary length $\ell_{c}$. Taking advantage of this fact, we use colloidal drops containing particles with different wetting properties in capillary tubes of radii $r=$ 400 and 750 $\mu$m at initial concentrations $\phi=$ 0.1 and 0.5 \% wt. We perform all evaporation experiments at isothermal, controlled humidity conditions and use an in-house MATLAB code to analyze the images captured via a CCD camera to measure the transient quantities like contact angle and contact line position. In this talk, we report the results quantifying the effect of particle wettability on deposition dynamics, contact line motion, and banding. [Preview Abstract] |
Monday, November 25, 2019 3:30PM - 3:31PM |
M04.00011: Determining time scales for directed assembly of particles by shear flow and electric field Minami Yoda, Andrew Yee, Hajime Onuki, Yoshiyuki Tagawa Suspended colloidal polystyrene particles assemble into structures called ``bands'' when subject to shear flow and a dc electric field. These bands exist only within a few $\mu$m of the wall, and have been observed over a wide range of conditions in combined Poiseuille and electroosmotic ``counterflow'' through microchannels, even at particle volume fractions as low as 33 ppm. There appear to be three stages in this process: 1) Accumulation, where particles are concentrated near the channel wall; 2) Band formation, with a relatively large number of unstable bands; and 3) Stable bands. The time scales for these stages were determined from evanescent-wave visualization images acquired at different streamwise channel locations. The standard deviation in the image grayscales was used to estimate the time required to reach the band formation stage, and compared with a similar time scale based instead on the time when the first band is observed. The mean grayscale, which was used instead to estimate the time to reach the stable bands stage, appears to have an exponential growth during the accumulation stage, and reaches its maximum value during the band formation stage, before decreasing to a relatively constant value in the stable bands stage. [Preview Abstract] |
Monday, November 25, 2019 3:31PM - 3:32PM |
M04.00012: Defining dimensionless parameters for electrohydrodynamic field-directed nanowire assembly. Rustom Bhiladvala, Mahshid Sam We present an approach for defining dimensionless parameters based on competing forces on nanoparticles in an electrohydrodynamic assembly process. These forces can either direct or disrupt the assembly process in different assembly situations. We define dimensionless parameters to maximize the ratio of directive to disruptive forces. This work is motivated by promising scientific capabilities that have been demonstrated using nanostructured devices at laboratory scale. Their translation to useful devices over centimeter to meter squared area at reasonable cost, can be facilitated by methods of field-directed assembly of nanostructures. Capillary and viscous forces, dielectrophoresis and dipole-dipole interaction, electrode polarization and electroosmosis, are involved during assembly. These depend on values of physical properties of the nanoparticles and the suspension fluid, design of the electrode pattern and the potential and frequency chosen for the electric field. The value of some of these physical parameters can each simultaneously affect forces that direct and disrupt nanostructure assembly. The tedious trial and error involved in parameter selection is an impediment to nanomanufacturing. A systematic approach to eliminate it is the motivation for this work. [Preview Abstract] |
Monday, November 25, 2019 3:32PM - 3:33PM |
M04.00013: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 3:33PM - 3:34PM |
M04.00014: Electrorotational instabilities of a drop in a uniform DC electric field Petia Vlahovska, Jeremy Koch, Michael Miksis In a uniform electric field, a weakly conducting drop bearing zero net charge initially adopts a prolate or oblate spheroidal shape, with both the shape and flow axisymmetrically aligned with the applied field - a classical result from G.I. Taylor. At higher field strengths we find two symmetry-breaking instabilities: a high viscosity drop undergoes Quincke rotation (the global flow acquires a rotational component), while low viscosity drops only develop a secondary flow -- a series of surface vortices -- in a belt along the drop equator. We explore these phenomena experimentally in a silicone oil/castor oil system to map the region of the vortices-belt instability as a function of fluid viscosity and electric field strength. [Preview Abstract] |
Monday, November 25, 2019 3:34PM - 3:35PM |
M04.00015: Chaotic dynamics of a Quincke rotor in 3D Gerardo Pradllo, Hamid Karani, Petia Vlahovska The Quincke effect is an electrohydrodynamic instability which gives rise to a torque on a dielectric particle in a uniform DC electric field. The equations which describe the resulting rotation are known to map onto the Lorenz equations (Peters et al, Chaos (2005)), predicting the existence of a second bifurcation upon which rotation is no longer steady. In this presentation we discuss the dynamics of the Quincke rotor in 3D, using the recently discovered hovering state (Pradillo et al, Soft Matter (2019)) at high electric fields. We experimentally confirm the existence of chaotic motion and demonstrate the presence of periodic regimes in this previously unexplored 3D system. Our experimental results are compared to the solutions of the fully three-dimensional model. [Preview Abstract] |
Monday, November 25, 2019 3:35PM - 3:36PM |
M04.00016: Electrohydrodynamic instability of a suspended liquid film Mohammadhossein Firouznia, David Saintillan Electrohydrodynamic instabilities at liquid-liquid interfaces are of high importance due to their relevance in various microfluidic applications. In this work, we analyze the stability of a system of three superposed layers of two immiscible liquids subject to a normal electric field. Following the leaky-dielectric model, the interfaces admit free charge while the bulk remains electroneutral. Interfacial charge dynamics is captured by a conservation equation accounting for Ohmic conduction, advection by the flow, and finite charge relaxation. We use this model to perform a linear stability analysis and uncover regimes of instability in terms of the relevant dimensionless groups of the problem. [Preview Abstract] |
Monday, November 25, 2019 3:36PM - 3:37PM |
M04.00017: Modulation of the streaming potential and slip characteristics in electrolyte flow over liquid filled surfaces Bei Fan, Prabhakar Bandaru A significant enhancement in the streaming potential (Vs) was obtained in experiments considering the flow of electrolyte over liquid-filled surfaces (LFSs), where the grooves in patterned substrates are filled with electrolyte immiscible oils. Such LFSs yield larger Vs (by a factor of 1.5) compared to superhydrophobic surfaces, with air-filled grooves, and offer tunability of electrokinetic flow. Through changing the filling oils, it is shown that the density, viscosity, conductivity, surface tension as well as the dielectric constant of the filling oil in the LFS, determine Vs. Considering the concentration polarization phenomenon for nonhomogeneous charged LFS, the hydrodynamic slip length is inversely proportional to the dielectric constant of the filled oil. Relating the hydrodynamic slip length to the obtained Vs offers insight into flow characteristics, as modulated by the liquid-liquid interfaces in the LFS. [Preview Abstract] |
Monday, November 25, 2019 3:37PM - 3:38PM |
M04.00018: Low-voltage onset of electrokinetic mixing at heterogeneous charge selective interfaces Anne Benneker, Burcu Gumuscu, Ernest Derckx, Rob Lammertink, Jeffery Wood Electrokinetic mixing of the concentration boundary layer adjacent to charge selective interfaces can enhance the performance of electrodialysis systems and fuel cells by increasing the transport of ions towards the interface. Traditionally, electrokinetic instabilities occur at relatively high potentials, significantly reducing the system efficiency. In this work, we experimentally show that altering the topology of the charge selective interface can induce electrokinetic mixing at low potentials using patterned charge selective hydrogels. Fluorescence microscopy for mapping local ion concentration is combined with electrical characterization to unravel the development of ion depletion zones and the onset of electrokinetic mixing. For different geometries, we find that the development of these depletion zones is distinctly different as a result of the distribution of the field lines through the different geometries. Enhancement in the total transport is observed with increasing system heterogeneity as a result of electroosmotic contributions to the charge transport, starting a low applied potentials. This indicates that inducing non-uniform electric fields using membrane topology variations and spacers is a promising route for increasing charge transport to the interface. [Preview Abstract] |
Monday, November 25, 2019 3:38PM - 3:39PM |
M04.00019: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 3:39PM - 3:40PM |
M04.00020: Contact angle effect on the drop impact onto a solid substrate Zhen Jian, Christophe Josserand, Stephane Zaleski, Pascal Ray As drop impacts onto a solid substrate, abundant outcomes such as splashing, bouncing can be observed under different conditions including gas, liquid and solid properties. We study the contact angle effect on the splashing of drop impact by direct numerical simulation with open source codes Gerris and Basilisk. For a typical partial wetting substrate with a contact angle of $90^\circ$, we obtain a phase diagram of splashing or not by varying the density and viscosity ratio between gas and liquid. Splashing can be either created by changing the contact angle to smaller than $90^\circ$, or eliminated with a contact angle larger than $90^\circ$ with all the other properties constant. The wettability plays a role in the formation of splashing during drop impact on solid. [Preview Abstract] |
Monday, November 25, 2019 3:40PM - 3:41PM |
M04.00021: An Evaluation of Droplet Breakup Characteristics for Low- and High-Speed Vehicle Impacts Michael Kinzel, Jason Turner, Brendon Cavainolo, Caroline Anderson In this work, computational fluid dynamics (CFD) predictions are used to simulate and study the evolution of a droplet in its approach of aerodynamic surfaces. The CFD effort is based on the volume-of-fluid (VOF) method in a formulation that contains the compressibility of both the liquid and gas. Using this formulation, the calculations of the breakdown of the droplet on impingement are directly simulated. These studies include evaluations of both low-speed subsonic and high-speed hypersonic interactions. These conditions are drastically different as low-speed cases involve a graduate build-up of slip velocity between the gas and droplet. Whereas high-speed impacts involve shock interactions. The traditional scaling parameter used in this scenario is the Weber number, but the present CFD indicate that cavitation also plays an important role. For this reason, the effort explores cavitation scaling parameters for high-speed droplet breakup as well. The overall effort presents prediction results, interpretation of the results, and evaluation of the cavitation scaling parameter. [Preview Abstract] |
Monday, November 25, 2019 3:41PM - 3:42PM |
M04.00022: Contact line friction driven droplet transport over asymmetric sawtooth surface microstructures Yaerim Lee, Gustav Amberg, Junichiro Shiomi Directional control of droplet motion over a dry solid surface has broad application areas ranging from printing to heat exchangers and microfluidic devices. In this study, the macroscopic motion of liquid droplets was manipulated directionally by oscillating the structured substrates with asymmetric sawtooth shapes in microscale. The horizontally oscillating substrates propelled water droplets of few microliters preferentially in the down-hill direction relative to the sawtooth geometries. Above a threshold in oscillation acceleration, the water droplet started to travel to the down direction gradually increasing the travel speed. The opposite directionality is observed at beyond resonance frequencies with stretched liquid-solid contact line. The droplet travel speed depending on the surface geometries were formulated from the theory of contact line movement describing the local departure of the contact angle from its static value in terms of actual surface shapes. The imbalance of the hindrance from line friction can transport water droplet at a maximum speed of 22 mm/s with a small oscillation amplitude of 0.5 mm. The contact line friction driven droplet transport was valid even when the substrate was tilted from the horizontal by 15 degrees. [Preview Abstract] |
Monday, November 25, 2019 3:42PM - 3:43PM |
M04.00023: Flow field near Contact Lines : Role of Inertia Anjishnu Choudhury, Charul Gupta, Harish N. Dixit The dynamics of contact line involves the movement of two immiscible fluids in contact with a solid surface. An analytical solution for Stokes flow near a moving contact line was solved by Huh \& Scriven (JCIS, 1971) which suggests a simple classification of the flow field based on viscosity ratio, $R$ and contact angle, $\theta$. But experiments by Savelski et al. (JCIS, 1995) and Ito et al. (Tran. Vis. Soc. Japan, 2009) find flow fields which differ from predictions of Huh \& Scriven’s theory. Our work is an attempt to resolve this contradiction by exploring the kinematics near the contact line through accurate simulations and scaling laws. We focus our attention on flow fields in a region of $O(l_c)$, the capillary length. Excellent agreement between simulations and theory is found in the viscous limit. A careful examination of the parameters used in the reported experiments suggests that $Re$ based on $l_c$ is not small, hence inertial effects may not be negligible. We observe a stagnation point on the interface at length $l_i$ for moderate $Re$ and at $O(1)$ $We$. The obtained flow field potentially resolves the contradiction between theory and experiments. Experiments are underway to validate these flow fields and will be discussed during the meeting. [Preview Abstract] |
Monday, November 25, 2019 3:43PM - 3:44PM |
M04.00024: Droplet baseball: skirting droplets rebounding from a rigid wall Jacob Hale, Alyssa Fisher, Momoka Goto, Anh Le, Mason Lee Droplets of 1 cst silicone oil, with an initial velocity component tangent to the surface of a bath of the same fluid, will roll along the surface for hundreds of milliseconds, slowing exponentially until eventual coalescence. A rigid vertical wall, placed in the bath perpendicular to the path of the droplet, forces early coalescence either on the meniscus formed on the wall or after rebounding from the wall. By moving the wall along the line of motion of the droplet, a transition point is found in the droplet's trajectory in which, at a fixed wall position, the droplet may either coalesce within less than one droplet diameters distance from the turning point or travel many droplet diameters. We find a clear bi-modal distribution in droplet travel distance at this transition point and present a physical model to explain this interaction. [Preview Abstract] |
Monday, November 25, 2019 3:44PM - 3:45PM |
M04.00025: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 3:45PM - 3:46PM |
M04.00026: Surface Chemistries Dramatically Influence Cassie-to-Wenzel Transitions in Doubly Reentrant Cavities Sankara Narayana Moorthi Arunachalam, Zain Ahmad, Ratul Das, Jamilya Nauruzbayeva, Himanshu Mishra Surfaces and membranes that can robustly entrap air on immersion in liquids have proven valuable for drag reduction and desalination. Typically, the ability to entrap air is engendered by coating rough surfaces/membranes with water-repellent chemicals. Recently, it has been demonstrated that if microstructures comprising doubly reentrant cavities (DRCs), i.e., cavities that broaden below the inlets and whose walls have T-shaped cross-section, are carved on a surface, can entrap air despite the hydrophilicity of the material. Here, we studied wetting transitions in DRCs with a variety of surface chemistries and compared them with simple cylindrical cavities (SCCs). We realized arrays of DRCs and SCCs on SiO$_{\mathrm{2}}$/Si wafers using photolithography and dry etching processes, and modified surface chemistries of silica to realize water (intrinsic) contact angles, $\theta _{\mathrm{o}}$, 0\textdegree , 40\textdegree , 112\textdegree . We found that life-times of Cassie-states could vary from 10$^{\mathrm{-3}}$-10$^{\mathrm{7}}$ s. The mechanisms underlying the wetting transitions including, the diffusion of the trapped air, the capillary condensation, liquid imbibition, and the release of the trapped air as a bubble will be explained. [Preview Abstract] |
Monday, November 25, 2019 3:46PM - 3:47PM |
M04.00027: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 3:47PM - 3:48PM |
M04.00028: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 3:48PM - 3:49PM |
M04.00029: Dynamics of spontaneous emulsification at particle-laden interfaces: a coupling between interfacial microstructures and interfacial flows. Parisa Bazazi, Hossein Hejazi Interfaces between two immiscible liquids are omnipresent in nature and industrial process including enhanced oil production, CO2 sequestration, and drug delivery. A thermodynamically stable dispersion of liquids, i.e. microemulsions, may develop at the fluid interfaces under specific circumstances such as having close to zero interfacial tension. In this work, we investigate the possibility of spontaneous emulsification at the particle-laden oil-water interfaces and examine the extent to which the phenomena are analogous to those of surface active molecules. It is shown that the nanoparticle-micelle complexes are formed at the interface of an aqueous drop submerged in an oleic phase promoting spontaneous emulsifications and double emulsion formation. Thus, the interface is covered with less than one-micron droplets of emulsified phase. The microemulsion detachment from the interface and their non-uniform distributions trigger fluid circulations inside the aqueous drop. The induced interfacial flow alters the drop shape, contribute in fluid mixing and consequently enhance the emulsification process. We quantify the emulsification rate and the fluid motions at the interfaces and provide plausible physical mechanisms describing the fluid dynamics of the process. We ultimately demonstrate the possibility of generating interfaces with specific microstructures and interfacial properties only by tuning the particle-surfactant concentrations. [Preview Abstract] |
Monday, November 25, 2019 3:49PM - 3:50PM |
M04.00030: Particle monolayer growth in evaporating salty colloidal droplets Myrthe Bruning, Laura Loeffen, Alvaro Marin When pinned colloidal sessile droplets evaporate, the well-known coffee stain effect will occur: particles accumulate along the contact line and form a ring. However, when small amounts of salt are added to the droplet, interesting phenomena occur that alter the particle agglomeration process drastically. As a consequence of the inhomogeneous evaporation along the droplet interface, salt accumulates at the contact line. Since salt increases the surface tension, an interfacial Marangoni flow is generated. This surface flow is directed from the apex of the droplet towards the contact line. It overcomes the bulk capillary flow, resulting in a reversed flow direction as compared to the classical coffee-stain case. Interestingly, despite the flow reversal, particles still accumulate at the contact line. However, in this case particles arrive at the contact line along the liquid-gas interface of the droplet and form a monolayer there. The structure of this particle layer is studied in detail using laser scanning confocal microscopy, which allows us to access the full 3D-position of all particles at the interface and gain insight into their distribution. In this work we will study the dramatical effect that the salt concentration has on the particle layer structure. [Preview Abstract] |
Monday, November 25, 2019 3:50PM - 3:51PM |
M04.00031: Effect of a second component in organic droplet evaporation: initially present versus absorbed during the process Sahar Andalib, Ali Alshehri, Pirouz Kavehpour Evaporation of organic liquid has numerous applications ranging from bio-diagnostics to coating technology. Multi-component droplets are often encountered in industrial applications. Even, evaporation of a pure liquid droplet into an environment containing a second substance can result in a multi-component droplet. Despite their omnipresent nature, mechanisms underlying multi-component droplets are not yet fully understood. Present work studies the similarities and differences of the effect of a second component during evaporation of a droplet of an organic solvent. In the first case the second component was present in the droplet from the beginning of evaporation process, while in the second case the second component gets absorbed or adsorbed during the process of evaporation. Analysis of experimental data provides valuable insight into wetting and spreading phenomena of multi-component systems. [Preview Abstract] |
Monday, November 25, 2019 3:51PM - 3:52PM |
M04.00032: Using Electrospray to Probe the Interfacial Flow of Evaporating Fluid Masses Paul Chiarot, Aref Ghafouri, Timothy Singler, Xin Yong The ability to measure flow along the surface of an evaporating fluid mass remains a challenge. Tracers are typically dispersed throughout the bulk of the fluid, which means the interfacial transport cannot be easily isolated using particle imaging techniques. In this research, nanoparticles, acting as Lagrangian tracers, are delivered to the interface of sessile drops and rivulets using electrospray atomization. The microdroplets produced by electrospray evaporate in-flight, which leaves behind dry particles that adsorb at the target interface and do not desorb into the bulk. Using this technique, we captured the flow induced on the surface of the fluid masses during evaporation. The flow structure changed dramatically when surfactant was present or if the bulk solvent was a binary (water / alcohol) solution. For these cases, solutal Marangoni flow was induced along the fluid interface, producing flow patterns that were significantly altered from the pure (aqueous) solvent case. [Preview Abstract] |
Monday, November 25, 2019 3:52PM - 3:53PM |
M04.00033: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 3:53PM - 3:54PM |
M04.00034: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 3:54PM - 3:55PM |
M04.00035: Boiling in acoustically levitated nanofuel droplets Khushboo Pandey, Saptarshi Basu Inclusion of metal/metalloid particles in conventional hydrocarbon fuels has unearthed the possibilities of nexgen energetic fuels. Recent research stride on nanofuel (Base Fuel $+$ Nanoparticles) combustion has shown a unique pathway of droplet secondary atomisation, stemming from heterogeneous boiling. Presence of nanoparticles (NPs) aids internal ebullition in nanofuel droplets. Rupture and expulsion of the bubbles generate high-speed ligaments which further undergo tip break-up. In the current work, we report detailed analyses of evaporation and atomisation characteristics of nanofuel droplets in a contactless environment (acoustic levitation) under external radiative heating. We explore the critical parameters for bubble incipience by varying base fuel vapour pressure, initial droplet size, and the input laser power. A time scale analysis considering orthokinetic NP aggregation, evaporation lifetime, and bubble growth rate is presented to elucidate the mechanism of internal boiling. A theoretical non-dimensional time scale ($\tau $*) is devised for calculating the lower limit for droplet size necessary to exhibit internal ebullition. REFERENCE Pandey K., and Basu S. `How boiling happens in nanofuel droplets' Physics of Fluids, 2018, 30, 107103. [Preview Abstract] |
Monday, November 25, 2019 3:55PM - 3:56PM |
M04.00036: Effect of vapor pressure on the performance of a Leidenfrost engine Prashant Agrawal, Gary Wells, Rodrigo Ledesma-Aguilar, Glen McHale, Anthony Buchoux, Khellil Sefiane, Adam Stokes, Anthony Walton, Jonathan Terry Friction is a hindrance to effective mechanical energy conversion, especially at microscales where significant wear occurs due to a high surface-area to volume ratio. Recently, we have developed the concept of a virtually frictionless heat engine, employing levitating liquids (or solids) on turbine-inspired substrates to convert thermal energy to mechanical motion. The levitation is achieved via the Leidenfrost effect, where a liquid drop levitates on its own vapor when it contacts a substrate heated to temperatures significantly above the liquid's boiling point. The vapor layer virtually eliminates friction and allows evaporating drops to self-propel on asymmetrically textured substrates. In this work, we operate this Leidenfrost heat engine continuously and control its output power by mechanically altering the vapor layer thickness. This is done by replenishing the liquid and supporting the rotor using a bearing assembly. We observe an increase in the power output despite the added bearing friction. The design principles described here can be extrapolated to develop mm and sub-mm scale engines for applications in extreme environments with naturally occurring low pressures and high temperature differences. We acknowledge funding from UK EPSRC (EP/P005896/1 and EP/P005705/1) [Preview Abstract] |
Monday, November 25, 2019 3:56PM - 3:57PM |
M04.00037: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 3:57PM - 3:58PM |
M04.00038: PIV measurements of a dilute suspension flow over and through various porous media models Theresa Wilkie, Eileen Haffner, Parisa Mirbod Porous media has become more prevalent in various engineering applications. This study's aim is to provide insight on how different properties of porous media would affect a dilute suspension flow. Specifically, a suspension fluid containing neutrally buoyant, non-collodial, non-Brownian, rigid, spherical particles with a volume fraction of 3{\%} was examined as it passed over and through various porous media models with porosities of 0.7, 0.8, and 0.9 and thicknesses of 5mm and 3mm. All Reynolds numbers were kept very low to neglect inertial effects. Particle image velocimetry (PIV) technique was used to obtain two-dimensional velocity vectors for planes on top and within the porous media models. To quantify the effect of different porosities and the porous media thickness on a dilute suspension the slip velocity and shear rate analyzed at the suspension-porous interface. We also compared our experimental results with the predicted model, and found a good agreement. [Preview Abstract] |
Monday, November 25, 2019 3:58PM - 3:59PM |
M04.00039: Porous walls impact on suspension flows SeyedMehdi Abtahi, Marco Rosti, Parisa Mirbod, Luca Brandt Suspension flows are encountered in various industrial applications including blood flow, transport of slurries, and pharmaceutical industries. We study suspensions of rigid particles in a plane Couette flow with porous walls. To tackle the problem at hand, we perform three-dimensional Direct Numerical Simulations of a plane Couette flow with a suspension of neutrally buoyant rigid particles simulated with an Immersed Boundary Method, while the flow inside the porous walls is simulated using the volume-averaged Navier-Stokes equations. We show that the porous walls produce a shear-thinning effect in the suspensions and this behavior originates from the interactions between the rigid particles, the porous walls and the carrier fluid: non-zero velocity fluctuations at the interface between the porous layers and the clear fluid regions are triggered by the presence of the particles, which in return migrates towards the bulk of the channel. We found that the effect grows with the particle volume fractions and with the permeability of the porous material. At the end for comparison, we also present how the interaction between porous walls and a particle suspension affects a fully developed plane turbulent channel flow. [Preview Abstract] |
Monday, November 25, 2019 3:59PM - 4:00PM |
M04.00040: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 4:00PM - 4:01PM |
M04.00041: Versatile particle delivery into dead-end pores enabled by reactive solutes Xiaoyu Tang, Nan Shi, Anirudha Banerjee, Todd Squires Particle delivery/extraction in dead-end pores is critical in many applications such as drug delivery and oil recovery. However, convective flow is suppressed due to geometrical confinement, while particle diffusion is prohibitively slow. Diffusiophoresis, in which solute concentration gradient drives particle migration, provides a promising strategy. The solute gradient is commonly imposed by introducing a solution of higher or lower concentration and relying on solute diffusion. However, single solute offers very little control over the particle delivery speed. Here, we present a new and versatile strategy to create solute gradient for diffusiophoretic particle migration: chemical reaction. Different model systems are demonstrated including acid-base and precipitation reactions. We will demonstrate that the speed and concentration of the particles can be controlled by varying reactant concentration ratio and reactant diffusivity ratio. Different particle delivery pattern can be achieved: non-focusing or focusing, where particles are delivered in a concentrated band. A theoretical model for the particle delivery dynamics agree well with the experiment. Diffusiophoresis under reactive solute gradient opens up new possibilities to manipulate particle migration in many configurations and applications. [Preview Abstract] |
Monday, November 25, 2019 4:01PM - 4:02PM |
M04.00042: Transport phenomena in porous structures in interaction with the atmospheric boundary layer Saurabh Saxena, Neda Yaghoobian This study lays emphases on the underlying physics of mass and heat transfer within porous structures found in the atmospheric boundary layer. The transport process in the porous media happens due to the variation in the structure surface temperature (induced by the variable solar fluxes) and instabilities in the atmospheric boundary layer turbulent flow. The study is inspired by the longstanding problem of the aeration function of soil-based animal-built structures in nature. Direct numerical simulation (DNS) is used to simulate the flow past and through porous media. The Navier--Stokes equations are modified using the Darcy-Brinkman-Forchheimer model to represent the porosity effect. To examine the effect of the natural and forced convection on the transport phenomena within the porous body, the system is further exposed to the diurnal surface temperature variation that is computationally simulated using detailed surface energy balance analyses. [Preview Abstract] |
Monday, November 25, 2019 4:02PM - 4:03PM |
M04.00043: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 4:03PM - 4:04PM |
M04.00044: Stress and velocity fluctuations in photoelastic granular avalanches Nathalie Vriend, Amalia Thomas We study granular avalanches using a custom-built narrow chute where we release 2D photoelastic disks down an incline. Using high-speed imagery, we are able to obtain position and velocity data from particle tracking, and the full stress tensor, including normal and shear stress components, from the photoelastic response of interacting particles. Even though the avalanche is steady-state in time and space, minute fluctuations in velocity and forces away from the mean directly influence the rheology and fluidity. In this study, we analyze the correlation between velocity fluctuations and stress fluctuations in both the quasi-steady layer (close to the rough base) and the flowing layer (near the free surface). We correlate the fluctuations with direct measurements of the measured non-local properties within this photoelastic avalanche. [Preview Abstract] |
Monday, November 25, 2019 4:04PM - 4:05PM |
M04.00045: Shear banding and shear jamming in homogeneously sheared granular material Yiqiu Zhao, Jonathan Bar\'{e}s, Hu Zheng, Joshua E. S. Socolar We experimentally study the generation and evolution of a shear band during quasistatic shearing of a 2D granular material using a novel split-bottom Couette apparatus in which a layer of photo-elastic disks rests on a base consisting of 21 independently controllable concentric rings. The rings rotate at different rates to generate a uniform basal shear profile. Previous experiments using this setup [arXiv:1904.10051] showed that a steady localized shear band is generated at sufficiently large strains when the packing fraction is higher than a critical value $\phi_c\approx 0.78$, which lies between the minimum shear jamming density and the isotropic jamming density. In the present work, we focus on the evolution and structure of the shear band. We find that the width of the shear band is independent of the global packing fraction above $\phi_c$, and we analyze the spatial variations in the local packing fraction, the force network structure, and the particle flow field throughout the shearing process. [Preview Abstract] |
Monday, November 25, 2019 4:05PM - 4:06PM |
M04.00046: A Simple Photo-resistor sensor for detection and characterization of droplets Mohammad Shahab, Anshul Verma, Raghunathan Rengaswamy This work describes a method which utilizes the difference in optical properties (i.e. transmittance) of continuous and dispersed phase to detect droplets in a milli-channel made of PDMS using a sensor i.e. Light Dependent Resistor (LDR) and a light source i.e. Light Emitting Diode (LED). Due to low cost and footprint, LDR is a viable option. LDR also covers the entire spectrum in visible range and gives satisfactory response as opposed to planar diffused photo-diodes which respond only to a limited wavelength-band. Any change in the transmittance of the fluid flowing in the channel changes the intensity of light falling from the LED on LDR resulting in a resistance change in LDR. Droplet detection corresponding to the peaks in the sensor signal due to change in the resistance of LDR is established by comparing the sensor data with the video-processed data. After sensor validation, peak features such as peak height, width and saturation voltage are studied to measure droplet size, velocity and spacing. As an application of this multiphase flow detection, it has been shown that droplets can be classified on the basis of their chemical properties using Beer-Lambert law. [Preview Abstract] |
Monday, November 25, 2019 4:06PM - 4:07PM |
M04.00047: Binary Aerosol Composition Measurements using Ultra Small Angle X-ray Scattering Daniel Duke, Harry Scott, Anesu Kusangaya, Damon Honnery, Brandon Sforzo, Katarzyna Matusik, Alan Kastengren, Matthew Frith, Jan Ilavsky, David Lewis Nearly all consumer aerosols are binary mixtures of a product in a liquefied propellant. In the pressurised metered dose medical inhaler (PMDI) for example, drugs may be dissolved in ethanol if their solubility in the propellant is poor. The rate of change of liquid composition as the propellant flash-evaporates strongly affects precipitation of the inhaled particles. However, aerosol composition is difficult to measure in an optically dense flash evaporating spray. A novel approach to this problem is demonstrated using Ultra-Small Angle X-ray Scattering (USAXS) on a PMDI solution of 3.38 $\mu$g/$\mu$L ipratropium bromide, 85\% $^v/_v$ R-134a propellant and 15\% ethanol. The experiments were conducted at the 9-ID \& 7-BM beamlines of the Advanced Photon Source at Argonne National Laboratory. USAXS exploits the high electron density of R-134a relative to ethanol, which leads to a measurable change in X-ray scattering cross-section at the droplet surface. By combining USAXS with X-ray radiography and laser diffraction, the change in cross-section can be measured using Porod’s Law. From this, an ensemble average composition is determined. This new approach enables quantitative validation of binary droplet evaporation models and may lead to improvements in nozzle design. [Preview Abstract] |
Monday, November 25, 2019 4:07PM - 4:08PM |
M04.00048: Holographic astigmatic particle tracking velocimetry (HAPTV) Zhou Zhou, Santosh Kumar Sankar, Kevin Mallery, Cheng Li, Wensheng Jiang, Jiarong Hong The formation of twin images in digital inline holography (DIH) is the main issue that constrains the placement of the focal plane in the center of the sample volume for DIH-based particle tracking velocimetry (DIH-PTV) with a single camera. As a result, it is challenging to apply DIH-PTV for flow measurements in large-scale laboratory facilities and many field applications despite its low cost and compact setup. Here we introduce holographic astigmatic PTV (HAPTV) by inserting a cylindrical lens in the optical setup of DIH-PTV which generates distorted holograms. Such distortion is subsequently utilized in a customized reconstruction algorithm to distinguish tracers positioned on different sides of the focal plane located in the center of a sample volume. Our HAPTV approach is calibrated under different magnifications, and is implemented to measure a vertical jet flow and the channel flow in a large-scale water tunnel. The results are compared with measurements from conventional particle image velocimetry and show a good agreement. The work has demonstrated that HAPTV can achieve improved spatial resolution compared to the conventional astigmatic PTV, and also enable the implementation of DIH-based PTV to flows in field applications. [Preview Abstract] |
Monday, November 25, 2019 4:08PM - 4:09PM |
M04.00049: Machine learning based holography for flow diagnostics Kevin Mallery, Siyao Shao, Jiarong Hong Digital inline holographic particle tracking velocimetry (DIH-PTV) is a promising single camera technique for 3D flow diagnostics due to its low cost, compact setup, versatility, and improved performance as demonstrated in the recent work by Mallery {\&} Hong [Optics Express 27(13), 18069-18084]. However, both measurement precision and processing speed remain challenges limiting the broad adoption of DIH-PTV. We present approaches utilizing machine learning to accomplish the particle localization task with both high accuracy and speeds comparable to commercial PIV software. Our approach avoids the need for experimental ground truth measurements by utilizing existing hologram simulation and processing methods to construct a training data set to train multiple convolutional neural networks (CNNs). Our algorithm addresses issues particular to holographic imaging through a novel unification of network elements (including UNET, SWISH activation, and TV loss). We demonstrate that this network can match the results of the best conventional DIH-PTV processing on experimental data and improve speed by a factor of 100. We further demonstrate that this method increases the tracer concentration limits of holography, improving the spatial resolution of measurements. [Preview Abstract] |
Monday, November 25, 2019 4:09PM - 4:10PM |
M04.00050: Measuring the dispersion relation of stabilized Faraday waves using Bragg scattering. Paul W. Fontana, Mason Brown, Eileen Flesher Parametrically excited surface waves, or Faraday waves, are produced on the free surface of a liquid that is vibrated vertically above a critical amplitude. Unlike standing waves, Faraday waves are composed of collections of oscillating solitons, or ``oscillons." These oscillons are generally subject to random motion, but they can be stabilized into a robust square crystalline lattice by dissolving a small amount of a protein, bovine serum albumen (BSA), in the liquid medium. In the experiments presented here, we demonstrate the use of grazing incidence Bragg scattering to measure the lattice spacing of the Faraday wave crystal, and hence the dominant wave number of the Faraday wave. Visible laser light (543.5 nm) is reflected off stabilized Faraday wave crystals at grazing incidence, producing an interference pattern in the far field. Analysis of the interference pattern yields the wave number to high precision and also brackets the wave height. The wave number was measured for driving frequencies from 200 Hz to 900 Hz. The resulting dispersion relation is compared with that of gravity-capillary waves in the same medium. Applications of the Faraday wave crystal as a liquid-based, tunable 2D diffraction grating are being explored. [Preview Abstract] |
Monday, November 25, 2019 4:10PM - 4:11PM |
M04.00051: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 4:11PM - 4:12PM |
M04.00052: Drag forces of rectangular cylinders with different aspect ratios in a vertical soap film Song Pan, Xinliang Tian The soap film provides a convenient way to study the drag force coefficient of a rectangular cylinder in the quasi-2D flow while it has been already investigated with numerical (2D and 3D) and experimental (3D) methods. We measured tiny drag forces of different rigid-wire rectangular cylinders by varying their aspect ratios in a vertical flowing soap film. We placed the pre-impregnated rectangular cylinders on a hook inserted into the soap film. The aspect ratios are defined as ratios of the length of cross-stream directions to height in the streamwise. We find that the measured drag forces do not match with published 2D numerical results, indicating the soap film shows 3D features when the aspect ratio is small. [Preview Abstract] |
Monday, November 25, 2019 4:12PM - 4:13PM |
M04.00053: Gas Jet Blowing on a Falling Soap Film: Three Regimes of Interaction Maksim Mezhericher, Cedric Gerbelot, Eric Qiu, Antonio Perazzo, Yiguang Ju, Howard A. Stone Recently, several studies reported on the generation of soap bubbles by a gas jet blowing perpendicular to a falling soap film. However, an extensive investigation of the regimes of interaction between the air jet and the falling soap film has not been performed yet. In this work, we used hypodermic blunt needles with inner diameters between 110-260 microns as small cylindrical nozzles to create air jets blowing perpendicular to falling soap films with thickness in the range of 2-10 microns, surface tension 25-32 mN/m and viscosity 1-7 mPa*s. While gradually increasing the jet velocity, three regimes of interaction between the blowing jet and the soap film were observed: formation of a dimple in the film for jet velocities smaller than the threshold for bubbling; a bubbling regime over and above a threshold velocity; and a regime of immediate film rupture when a critical jet velocity was achieved. In the bubbling regime, tuning some parameters enabled production of monodisperse bubble aerosols. The onset of the regime of immediate film rupture was found to strongly depend on the nozzle diameter and weakly depend on the film thickness and its physicochemical properties. [Preview Abstract] |
Monday, November 25, 2019 4:13PM - 4:14PM |
M04.00054: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 4:14PM - 4:15PM |
M04.00055: A linear behavior between squeezing pressure and advancing length: when squeezing a viscous droplet into circular narrow confinement at capillary number $=$ 3/4 Christopher Landry, Xiaolin Chen, Zhifeng Zhang Particle squeezing through narrow constricted channels occurs in many processes throughout biomedical and chemical engineering fields. Applications range from lab-on-chip devices and pipette aspiration to transport through single pore as well as porous media. Recently, particle behaviors are compared based on particle properties and operation parameters. In the present research, an approximately linear relation between squeezing pressure and advancing length, in which the squeezing of a viscous droplet behaves like a linear solid particle, is found at Ca$=$3/4. Within the range of theoretical assumption, numerical modeling is conducted to validate the linear relation between the squeezing pressure and advancing length. This finding may have potential implications in cell/particle filtration, aspiration device design, enhanced oil recovery, etc. [Preview Abstract] |
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