Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A15: Drops: Complex Fluids |
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Chair: Gwynn Elfring, University of British Columbia Room: 3022/3024 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A15.00001: Drop impact experiment as a model experiment to investigate the role of oil-in-water emulsions in controlling the drop size distribution of an agricultural spray Clara Vernay, Laurence Ramos, Christian Ligoure, Jean-Paul Douzals, Rajesh Goyal, Jean-Christophe Castaing Agricultural spraying involves atomizing a liquid stream through a hydraulic nozzle forming a liquid sheet, which is then destabilized into droplets. Solution adjuvants as dilute oil-in-water emulsions are known to influence the spray drop size distribution. To elucidate the mechanisms causing the changes on the drop size distribution, we investigate the influence of emulsions on the destabilization mechanisms of liquid sheets. Model laboratory experiments based on the collision of a liquid drop on a small target are used to produce and visualize liquid sheets. With emulsion, the sheet is destabilized by the nucleation of holes in the sheet that perforate it during its expansion. The physico-chemical parameters of the emulsion, such as the concentration and the emulsion drop size distribution, are varied to rationalize their influence on the destabilization mechanisms. The results obtained with the drop impact experiments are compared to the measurement of the spray drop size distribution. The very good correlation between the number of nucleation events and the volume fraction of small drops in the spray suggests that experiments on liquid sheet are appropriate model experiments to gain an understanding of the physical mechanisms governing the spray drop size distribution. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A15.00002: Relaxation-induced coalescence with and without insoluble surfactants Carolina Vannozzi Rallison and Acrivos in 1978 investigated for the first time numerically the deformation and burst of a viscous drop in extensional flow. This seminal paper led to other important studies regarding the relaxation of viscous drops previously deformed in extensional flows, both for systems with and without surfactants. In this line of research we present the boundary integral simulations of the relaxation process of two viscous drops, previously undergoing a flow-induced head-on collision in an extensional flow, both with and without insoluble surfactants. The clean interface case showed that the acquired drop deformation induces a flow in the thin film between the drops as the interface relaxes back to restore the drop original shape [1]. Under certain circumstances, this flow thins the film, allowing drop coalescence. Surprisingly, this phenomenon, the so-called relaxation-induced coalescence, is possible even for collisions which would not lead to coalescence while the flow is active, and it thus influences the final drop size distribution of blends/emulsions. In the presence of trace amounts of insoluble surfactants relaxation-induced coalescence is still possible, but less likely, depending strongly on the surfactant diffusivity.\\[4pt] [1] Vannozzi, J Fluid Mech 2012. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A15.00003: Droplet impact on permeable meshes with yield stress fluids Brendan Blackwell, Randy Ewoldt Yield stress fluids can stick and accumulate where they impact. To understand coating of complex topography, we experimentally study the ability of droplets to accumulate on permeable solid meshes (rigid surfaces with small, evenly spaced openings). When inertial stresses are sufficiently high compared to the yield stress, a drop can pass through a mesh, breaking into smaller fluid particles with varying shapes, sizes, and velocities in the process. In contrast, when inertial stresses are sufficiently low compared to the yield stress, a droplet can stick to the mesh as though it were a solid surface. Drop size, impact velocity, mesh geometry, and rheological material properties are varied. Layers of multiple meshes are also examined, demonstrating a range of behaviors and the ability to coat internal aspects of complex topography. Dimensional analysis is performed to characterize material transmittance as a function of the input parameters. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A15.00004: Synchronous droplets as a test bed for pulsatory active fluids Georgios Katsikis, Manu Prakash Collective behavior in many-body systems has been studied extensively focusing on a wide range of interacting entities including: flocking animals, sedimenting particles and microfluidic droplets among others. Here, we propose an experimental platform to explore an oscillatory active fluid with synchronous ferrofluid droplets immersed in an immiscible carrier fluid in a Hele-Shaw configuration. The droplets are organized and actuated on a 2-D uniform grid through application of a precessive magnetic field. The state of our system is dependent on three parameters: the grid occupancy with fluid droplets, the grid geometry and the magnetic field. We study the long range orientational order of our system over a range of those parameters by tracking the motion of the droplets and analyzing the PIV data of the carrier fluid flow. Numerical simulations are juxtaposed with experimental data for prediction of the system's behavior. [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A15.00005: Complex traffic of drops in 2D microchannels: self-organization, periodicity and reversibility Danny Raj Masila, Raghunathan Rengaswamy In a 2D microchannel, drops slowdown and accelerate in the diverging and converging sections of the channel respectively. Drops entering the microchannel, approach each other when they slow-down in the diverging section. They start to interact hydrodynamically to form different layered structures depending on the spacing between the drops prior to entry into the channel. These patterns break in the converging end of the channel before the drops exit. We devised a multi-agent approach that was able to capture the dynamic pattern formation of drops inside the microchannel. The self-organized dynamic patterns formed are a function of the inlet spacing of the drops. These patterns due to complex drop traffic result in a non-linear outlet spacing between exiting drops. We present a study where we investigate how every spatial event inside the microchannel can result in a temporal signature in the outlet spacing of drops. Understanding the dynamic pattern formation also sheds light on the response of the microfluidic device to flow reversal. We observe that when the layering and breaking patterns of drops are similar the system is reversible. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A15.00006: Microscopic reversibility and memory in soft crystals undergoing large deformations Liat Rosenfeld, Claudiu Stan, Sindy K.Y. Tang In this study, we explore the transition from reversible to chaotic behavior in an oscillatory shear flow of water-in-oil emulsions. The emulsion was injected through a microchannel and was forced to rearrange due to a central constriction in the channel. We study the motion of the individual droplets and their neighbors in order to determine their ability to retain their original position after several cycles of oscillations. We have found that the emulsion exhibit behaviors that vary from complete reversibility to complete irreversibility depending on the volume fraction, velocity and strain rate. The reversibility, both in the trajectory and the deformation of every drop, is reproducible even when the drops undergo many rearrangement events over distances of \textgreater 150 droplet diameters. Moreover, the deformability of the drops and the high volume fraction are crucial conditions for the onset of reversibility. We provide here the first direct visualization and physical analysis of this phenomenon. This work is an important step in describing the flow of concentrated emulsions and suspensions in microchannels and is therefore crucial for understanding the behavior of droplets, bubbles and particles in droplet microfluidic applications. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A15.00007: Morphological dynamics of a falling drop in a magnetic field David S. Mart\'Inez, Miguel Angel Cabrerizo-Vilchez, Antonio Viedma, Alidad Amirfazli A ferrofluid drop was released and fell through the air; as it travelled through a thin coil a magnetic pulse (4-65 mT for 4 ms) was given. The drop either deformed or split into a multitude of smaller particles. For pulse amplitudes less than 9 mT the drop sequentially deformed to oblate and prolate ellipsoids; the dynamics of drop deformation in this case was modeled using a harmonic damped oscillatory function. Higher magnetic field pulse amplitudes resulted in drop taking the form of a cylindrical ligament; depending on the field strength various numbers of drops were ejected from ends of the ligament. The size of ejected drops decreased with increasing magnetic field strength. Ejected drops travelled with higher velocities as magnetic field strength was increased in a linear fashion. At field strengths of 65mT up to six drops were ejected from the ligament. Ejected drops where all spherical, and the ejection process was over in 22 ms. The cylindrical ligament eventually recovered to a spherical shape due to surface tension forces in 210 ms. Other interesting observations such as momentary interactions without coalescence between consecutive drops due to the wake effect will be discussed. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A15.00008: Fractal pattern formation in metallic ink sessile droplets Miloud Hadj-Achour, David Brutin We report a fingering instability that occurs during the spreading and evaporation of a nanosuspension droplet. The patterns has a fractal structure similar to those reported by N. Shahidzadeh-Bonn and al. (2008) for salt crystallisation, during evaporation of saturated Na2SO4 on a hydrophilic surface. The fingering instability has been widely studied for both Newtonian and non-Newtonian fluids. However, we describe for the first time that a fingering instability is observed for the spreading of a nanosuspension sessile droplet. We demonstrate that in certain cases, the contact line evolves through different spreading regimes according to J. De Coninck et al. (2001) with an enhancement in the evaporation rate due the formation of the fractal patterns. [Preview Abstract] |
Sunday, November 23, 2014 9:44AM - 9:57AM |
A15.00009: Relative humidity influence on the spreading dynamics of sessile drops of blood David Brutin, Wassim Bouzeid We studied the effect of relative humidity on the initial stages of spreading dynamics for drops of whole human blood. A range of relative humidities from 8{\%} to 90{\%} was studied. Drops of the same volume were gently deposited on ultra-clean microscope glass substrates. We show that the drop spreading is driven by two distinct regimes. The first is characterized by fast dynamics and competition between viscous forces and capillary forces, whereas the second regime is characterized by competition between viscous dissipation and evaporation and exhibits slower dynamics. At early stages of spreading, the power law r(t) $\sim $ t$^n$ (n $=$ 0.65) was observed regardless of the humidity. At later stages of spreading, the exponent of the power law r(t) $\sim $ t$^n$ (n $=$ 0.19) was found to be higher than that of Tanner's law because of the effect of humidity and Marangoni stresses. Spreading time and spreading dynamics were found to be related to relative humidity. This is explained by the adhesion of red blood cells to the substrate that is similar to the mechanism observed for nanofluid droplets. The mean velocity of the triple line followed the same behavior as Tanner's model, where the final wetting radius and the apparent contact angle are functions of relative humidity. [Preview Abstract] |
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