Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M15: Drops I |
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Sponsoring Units: DFD DSOFT Chair: Pedro Saenz, University of North Carolina at Chapel Hill Room: 210/212 |
Wednesday, March 4, 2020 11:15AM - 11:27AM |
M15.00001: Intricate instabilities in drying drops Srishti Arora, Michelle R Driscoll Complex fluid drops dry with a striking variety of patterns, from the ubiquitous coffee ring to a wide range and scale of cracks. Colloidal suspensions are often used in these experiments, and to date, much of this work has been focused on drops with a relatively low volume fraction of particles. The patterns are thought to be controlled by several factors such as surface wetting properties, evaporation kinetics, and particle interactions. Here, we present a study of the drying of a high-volume fraction suspension drop. We work with highly monodisperse silica and polystyrene colloids made in-house and study both the final pattern morphology, as well as the dynamics of crack growth. In this limit of high concentration, we see intriguing structures emerging: a single dimple appears near the center that then connects to growing radial fractures. The patterns we observe depend on both particle size and concentration. Furthermore, even for similarly sized particles, changing the material of the colloidal particle can qualitatively change the morphology of the crack pattern. |
Wednesday, March 4, 2020 11:27AM - 11:39AM |
M15.00002: Droplet traffic in microfluidic networks: Role of droplet collisions and occlusion Masoud Norouzi Darabad, Siva A Vanapalli, Mark W Vaughn The dynamics of droplet flows in microfluidic loop networks are governed by the frequency at which the droplets enter the loop. At low frequencies, droplets always choose the channel with lower hydrodynamic resistance, and droplets interact indirectly through their modifications to resistance of the channels. As the droplet frequency increases, direct droplet-droplet interactions in the junction also contribute to droplet decisions and may force the droplets to the channels with higher hydrodynamic resistance. Collisions are considered the main type of direct interaction between droplets. However, our study suggests that occlusion of the entrance of the channel by droplets also impacts their decisions, even when there is no collision and contact. We study droplet decision rules by use of droplet trajectories and distribution of the droplets in the junction and throughout the network. Also, application of machine learning techniques to identify decision rules and predict the droplet decisions is studied. |
Wednesday, March 4, 2020 11:39AM - 11:51AM |
M15.00003: Joint effect of advection, diffusion, and capillary attraction on the spatial structure of particle depositions from evaporating droplets Konstantin Kolegov, Lev Barash A simplified model is developed, which allows us to perform computer simulations of the particles transport in an evaporating droplet with a contact line pinned to a hydrophilic substrate. The model accounts for advection in the droplet, diffusion and particle attraction by capillary forces. The parameters chosen correspond to the experiments of Park and Moon [Langmuir 22, 3506 (2006)], where an annular deposition and snakelike chains of colloid particles have been identified. We find that the annular sediment is formed by advection and diffusion transport. The close packing of the particles in the sediment is possible if the evaporation time exceeds the characteristic time of diffusion-based ordering. The chains are formed by the end of the evaporation process due to capillary attraction of particles in the region bounded by a fixing radius, where the local droplet height is comparable to the particle size. At the beginning of the evaporation, the annular deposition is shown to expand faster than the fixing radius moves. However, by the end of the process, the fixing radius rapidly outreaches the expanding inner front of the ring. The snakelike chains are formed at this final stage. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M15.00004: Dynamics and stability of confined levitating droplets Stuart Thomson, Matthew Durey, John W. M. Bush, Ruben R. Rosales Millimetric droplets have been shown to bounce on the surface of a vibrating liquid bath or “walk” by means of self-propulsion through a resonant interaction with their own wave field. When confined to an annular ring, we show that single droplets are observed to exhibit a random walk like behaviour, while the collective dynamics of one-dimensional droplet lattices exhibit canonical features of driven dissipative oscillator systems, namely out-of-phase oscillations and solitary wave propagation. Our experimental results are supported by a stability analysis of an accompanying theoretical model. Some open areas of investigation of potential interest to the dynamical and non-equilibrium systems community will be discussed in conclusion. |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M15.00005: Watching Droplets Dry in Wet and Dry Climates Paul Lilin, Philippe Bourrianne, Irmgard Bischofberger The drying of sessile droplets of colloidal suspensions with particle volume fraction > 5% leads to the formation of fracture patterns. As water evaporates, a solidification front propagates from the edge of the droplet, leaving behind a thin close-packed deposit that eventually covers the entire wetted area. Evaporation-induced stresses generate radial cracks in the deposit that grow towards the droplet center, defining regular petals. These petals bend out of plane to form a blooming flower. Strikingly, while the distance between the cracks is independent of relative humidity, the bending behavior changes drastically when the relative humidity is increased; the curvature of the petals along their width becomes inverted, and the kinetics of the petal bending is modified. We discuss the relation between these changes in structure and kinetics and the deposit thickness. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M15.00006: How air deforms the free surface just before disk impact on a liquid bath Devaraj Van Der Meer, Utkarsh Jain, Anaïs Gauthier, Detlef Lohse When a flat disk impacts onto a liquid bath, a layer of air is trapped between the disk and the free surface, a phenomenon known as air cushioning. The air layer is pushed out radially at increasing speeds, causing the water surface to be lifted up towards the approaching disk. This qualitative observation is traditionally ascribed to Bernoulli suction occurring in the low-pressure region created by the large air velocities in the gap. Here, by means of a novel high-speed imaging technique that uses the free surface as a mirror, we quantitatively measure the time evolution of the free surface profile. We show that, whereas the manner in which the surface below the center of the disk is pushed down is consistent with potential theory, this is not the case for the elevation of the free surface below the disk's edge. Instead, the surface lifting appears to be initiated by a Kelvin-Helmholtz instability occuring under the edge of the approaching disk. |
Wednesday, March 4, 2020 12:27PM - 12:39PM |
M15.00007: Dispersion of magnetic beads in flowing droplets. Eric Brouzes, Evan Lammertse Microfluidic droplets have proven useful for the manipulation and sample preparation of single-cells. However, the droplet toolkit lacks a robust and high-throughput purification module that can maintain partitions. We have demonstrated a proof-of-principle of magnetic purification from flowing droplets using an external magnetic field to partition beads within droplets before asymmetric splitting. Here, we study the interplay between the opposing effects of the magnetic and viscous forces by systematically reporting bead aggregation as a function of design parameters and droplet velocity. We used image-processing to measure aggregate count, size, and shape, allowing us to discover how the distributions of these parameters correlates with changes in experimental conditions. Data have revealed distinct aggregation regimes, in which either the magnetic attraction or the viscous dispersion forces predominate. We further explored the effect of design parameters on the transitions between these regimes both experimentally and via modeling using the dimensionless Mason number (ratio of viscous to magnetic forces). This knowledge will drive the optimization of our droplet purification module with increased throughput. |
Wednesday, March 4, 2020 12:39PM - 12:51PM |
M15.00008: Computational modeling of droplet spreading and coalescence on fiber rails Fang Wang, Ulf Schiller Flow phenomena in porous media are relevant in many technological applications including emulsion filtration, gas diffusion membranes, and biomedical implants. Fiber materials can be used as filtration membranes with tailored permeability range and controllable pore size distribution. In this talk, we will present lattice Boltzmann simulations of droplet spreading on fiber rails. The simulation results reveal the dependence of the droplet morphology on the fluid volume, fiber size, and contact angle. The transition from a barrel shaped drop to a liquid column shows hysteresis depending on the capillary pressure. We further present preliminary results on capillary trapping of droplets on fiber rails and coalescence induced detachment. We will outline how the results can be used to determine the critical capillary number between trapping and squeezing. We will discuss how these insights can be used to design functional materials for coalescence filtration and other applications. |
Wednesday, March 4, 2020 12:51PM - 1:03PM |
M15.00009: Spreading and evaporation of sessile droplets Chloe Lindeman, Nicholas Schade, Sidney Robert Nagel We measure the contact angle at which a drop of liquid sitting on a solid surface de-pins as a function of both the concentration of solute in the drop and the amount of time the drop has been allowed to sit on the surface. We do this by removing liquid from a drop via a small hole in the substrate. This procedure does not disturb the liquid surface. However, viewing the drop from the side only allows a view of two diametrically opposed points of the contact line which can create artifacts in the drop width data. Instead, we monitor the footprint of the drop from behind the substrate, using total internal reflection, to obtain simultaneous information about the entire contact line. We find that the initial spreading of the drop on the surface for both upright and inverted sessile drops can last for several minutes but is largely independent of the solute concentration. However, during evaporation we find that the de-pinning angle is highly dependent on concentration and on the waiting time. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M15.00010: Electric Discharge Mediated Transient Interfacial Dynamics of a Sessile Liquid Droplet: Plethora of Hydrodynamic Features Bhaskarjyoti Sarma, Sunny Kumar, Amaresh Dalal, Dipankar Narayan Basu, Dipankar Bandyopadhyay We experimentally demonstrate a plethora of hydrodynamic features namely jetting, crowning, and bursting, engendered from capturing electrical discharge inside a sessile liquid droplet. The intense electrical energy deposited inside the conducting droplet in a capacitor like configuration designed with a pin-type anode and thin dielectric film-coated plate type cathode. The high-speed experiments reveal the formation of thermal bubbles from the electrodes and their subsequent evolution till collapse as the dominant mechanism underlying the rich hydrodynamics of the droplet. By tuning the properties of the liquid droplet and the electric field, we can tune the intensity of the discharge and ensuing bubble dynamics, thereby enabling the transition from one regime to another. The position of the electrode can also be tuned to alter the dynamics of shape evolution. The bursting phenomena result from the high electric field and/or high conductivity of the droplet, producing numerous micron size secondary droplets. The explored regimes, specifically bursting of a liquid droplet, are highly relevant for many industrial applications involving enhanced mass transfer such as spray generation, tissue ablation to name a few. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M15.00011: High density ratio Lattice Boltzmann simulations of immiscible drop collision Neeru Bala, Halim Kusumaatmaja, Ciro Semprebon The physics and dynamics of a ternary fluid system are of special interest for a variety of practical applications, including combustion engines, ink-jet printing, and oil recovery. For example, recent experiments demonstrated that if fuel and water droplets are colliding in a combustion chamber, the water can be encapsulated by the fuel. This causes micro-explosions and enhances the burning rate of the combustion chamber. In this contribution, I will present a numerical investigation of the collision between two immiscible droplets by employing a high density ratio (~103) Free energy Lattice Boltzmann model, which account for the Inertial effects[1]. I mainly focus on the transition from adhesion to bouncing for a wide range of dimensionless numbers (Weber and Ohnesorge number) and Impact parameter by varying relative surface tension, Impact speed, liquid viscosity and drop size. |
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