Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session JC: Drops and Bubbles VIII |
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Chair: Michael Rother, University of Minnesota Room: Salt Palace Convention Center 150 G |
Monday, November 19, 2007 3:35PM - 3:48PM |
JC.00001: Sedimenting droplets with a reactive interface Angela M. Jimenez, Sigolene Lecuyer, Howard A. Stone, Andrew Belmonte Droplets of a CTAB (cetyl trimethylammonium bromide) solution sedimenting in oleic acid exhibit peculiar behavior and shape transformations. This is due to the fact that cationic and anionic surfactants interact at the interface, which results in the formation of a thin visco-elastic skin on the surface of the droplet. We study this phenomenon experimentally, as a function of CTAB concentration (2-60 mM) and droplet volume (3-50 $\mu$l). We report two main observations: 1) at low enough CTAB concentrations, droplets have a steady shape, and the sedimentation speed is increased compared to a pure water droplet of the same volume. This can be accounted for by small changes in the shape of the droplet, which is not spherical anymore. 2) at higher CTAB concentrations, droplets develop a thin ``tail'' that progressively grows as they fall down. We study the dynamics of this elongation and determine mechanical properties of the visco-elastic interface. [Preview Abstract] |
Monday, November 19, 2007 3:48PM - 4:01PM |
JC.00002: Effect of the interfacial properties of a suspended drop on the bulk flow Charles Eggleton, Andr\'es Gonzalez, Mustapha Jamal A liquid drop suspended in a distinct fluid is simulated. An adsorbed monolayer of surfactant molecules on the interface dynamically alters the surface tension distribution when the suspending liquid is set in motion. When the external flow is stopped the drop retracts back to its equilibrium spherical shape. The behavior of the surfactant monolayer is modeled using the Langmuir surface equation of state. Initial equilibrium surfactant concentrations ranging from zero to a highly packed monolayer are considered. External and internal flow field properties such as streamlines, the velocity field and the pressure field are analyzed at steady state and during the retraction of the drop in order to identify characteristic flow patterns as a function of initial monolayer surface concentration. [Preview Abstract] |
Monday, November 19, 2007 4:01PM - 4:14PM |
JC.00003: Thermocapillary interactions of spherical drops covered with incompressible surfactant Michael Rother An incompressible surfactant film occurs when very small changes in surfactant concentration correspond to finite changes in interfacial tension in a way analogous to the density-pressure relationship in incompressible fluids. In work on linear flows and Brownian motion, it has been shown\footnote{Blawzdziewicz, J., Wajnryb, E., and Loewenberg, {\it J. Fluid Mech.} {\bf 395} 29-59 (1999).} that in most cases incompressible surfactant holds for spherical drops. Surfactant coverage is thus nearly uniform, and the Marangoni number $Ma$ and surface P\'eclet number $Pe_s$ are combined into a single retardation parameter $A$ = $Ma~Pe_s$. Previously, we have presented results for arbitrary surfactant surface coverage in thermocapillary interactions of deformable drops. Herein, we use bispherical coordinates and multipole techniques to determine thermocapillary trajectories for two spherical drops covered with bulk-insoluble, non-ionic surfactant in the limit of small deviation in surfactant coverage, i.e., incompressible surfactant. The range of validity for incompressible surfactant theory is probed by comparison with our previous work. In addition, collision efficiencies are determined, and population dynamics are used to analyze the behavior of dilute dispersions of spherical drops in thermocapillary motion. [Preview Abstract] |
Monday, November 19, 2007 4:14PM - 4:27PM |
JC.00004: Drops and emulsions with complex interfaces Philipp Erni, Erich J. Windhab, Peter Fischer We study the behavior of emulsion drops in external flow fields, focusing on recent experimental work involving liquid interfaces covered with surface-active species, in particular adsorbed proteins and particles. Three different length scales are considered: (i) the rheology of complex interfaces is discussed for adsorbed polyelectrolyte surfactants with different molecular structure (compact and globular vs. random coil); (ii) the flow of single drops with macromolecular adsorption layers is studied in optical flow cells; (iii) dilute emulsions of drops are investigated using rheo-small angle light scattering (rheo-SALS). We discuss the results in the context of emulsion and drop models accounting for interfacial viscoelasticity, as well as with capsule suspension models for the case of rigid interfacial layers. Drops stabilized by adsorbed particles or globular proteins can be understood as capsules surrounded by a soft shell; their behavior on the single drop level is in many ways reminiscent of phenomena observed with red blood cells or vesicles, including non-linear drop shape fluctuations under creeping flow conditions. References: [1] Fischer P, Erni P. Curr Opin Colloid Interface Sci (2007, accepted) [2] Erni P et al., Appl Phys Lett 87, 244104 (2007) [Preview Abstract] |
Monday, November 19, 2007 4:27PM - 4:40PM |
JC.00005: Coalescence and breakup between a capillary switch and a droplet A. Hirsa, I. Martinovic, C. Lopez, O. Basaran, S. Ramalingam A capillary switch can be formed by overfilling a circular orifice in a plate with liquid, where the contact line is pinned at the rim by using a non-wetting substrate. For surface tension to be the dominant force in the system, the size of the hole has to be smaller than the capillary length (about a millimeter for water). Application of capillary switches has already been demonstrated in optics through a fast-acting adaptive lens. Here, we investigate how capillary switches can be used to detach a small packet of liquid from a substrate and subsequently reattach it elsewhere without the use of any moving parts. Overcoming the inherent adhesion force between the liquid packet and its substrate is the challenge. Experiments have been conducted on the coalescence and subsequent breakup of a switch and a droplet (pendant or sessile) which encapsulates the liquid packet. The coalescence and break-up processes are also analyzed computationally using finite element analysis and the predictions are compared with the experimental measurements. [Preview Abstract] |
Monday, November 19, 2007 4:40PM - 4:53PM |
JC.00006: Computational analysis of coalescence and breakup between a capillary switch and a pendant drop Santhosh Ramalingam, Osman Basaran, Carlos Lopez, Ines Martinovic, Amir Hirsa Although capillary switches come in a variety of configurations, one that has been receiving much attention recently consists of a continuous volume of liquid in which two drops are joined through a liquid-filled circular orifice in a plate. Recent research has focused on the bi-stable nature of such capillary switches and their potential application as optical MEMS devices. Here, we investigate the dynamics of coalescence and break-up between a capillary switch and a pendant drop hanging from a moving rod wherein the capillary switch and the pendant drop consist of the identical incompressible, Newtonian liquid. The dynamics are analyzed both using a fast, one-dimensional (1D) algorithm and also rigorously using a fully three-dimensional but axisymmetric, i.e. two-dimensional (2D), algorithm. The effects of surface tension, inertial, viscous, and gravitational forces on the dynamics of coalescence and break-up \textit{and} the subsequent steady-state shapes of the capillary switch and the pendant drop are investigated. Regions of the parameter space where the 1D algorithm is reliable are also identified. [Preview Abstract] |
Monday, November 19, 2007 4:53PM - 5:06PM |
JC.00007: Generation of micron-sized drops with viscous co-flows Alvaro G. Marin, Jose M. Gordillo In this presentation, the generation of micron-sized drops (smaller than 1.5 microns) will be shown to be obtained through a simple device, suggested by the numerical experiments by R. Suryo and O. A. Basaran (``Tip streaming from a liquid drop forming from a tube in a co-flowing outer fluid'', Phys. Fluids 18, 082102, 2006), in which two liquid streams co-flow coaxially under creeping flow conditions. Here we determine the appropriate experimental range of the dimensionless parameters in which tiny jets, over two orders of magnitude smaller than the drop from which they originate, are formed; this regime is achieved without resorting to the use of neither surfactants nor electric fields. In addition, the scaling for the drop diameter, which is provided as a function of the flow rate ratio and the viscosity ratio, predicts well the experimentally measured drop diameters, which range between 1.5 and 30 microns. Finally, we will also discuss on the analogies and differences between the present technique and some other alternative methods for the generation of microemulsions, namely, flow-focusing and electrosprays. In conclusion, we describe a feasible and cheap alternative experimental method for the generation of microemulsions that can be easily reproduced at any laboratory. [Preview Abstract] |
Monday, November 19, 2007 5:06PM - 5:19PM |
JC.00008: 3D Mixing Inside a Neutrally Buoyant Drop Driven by Electrohydrodynamic Flows Xiumei Xu, G.M. Homsy For a neutrally buoyant drop subjected to a uniform electric field, the internal flow is the well-known Taylor circulation. In Phys. Fluids 19 013102 (2007), we theoretically studied three dimensional mixing by periodically switching a uniform electric field through an angle $\alpha$. Periodically switching the field is equivalent to periodically changing the symmetry axis of the Taylor circulation. For $\alpha=0.5 \pi$, there is no chaotic mixing because the common heteroclinic trajectories form the separatrix of the flow. For other switching angles, chaotic advection is generated due to perturbations of the heteroclinic trajectory. Experimental investigations of mixing were carried out using a nearly isopycnic silicone oil/castor oil system. For $\alpha=0.5 \pi$, our experiments show the existence of symmetry planes. In addition, two blobs of particles are observed to maintain almost invariant shapes for very long time, indicating the absence of chaotic mixing, as predicted by the theory. For other switching angles, experiments show the penetration of symmetry planes by tracer particles. However it is difficult to draw definitive conclusions regarding chaotic mixing because of charge relaxation, long initial transients and drop translation effects. [Preview Abstract] |
Monday, November 19, 2007 5:19PM - 5:32PM |
JC.00009: Breakup of electrified drops Robert Collins, Jeremy Jones, Michael Harris, Osman Basaran A drop that is subjected to an electric field is a situation of common occurrence in nature and technology. Well known examples of drops subjected to electric fields include raindrops in thunderstorms and aqueous drops dispersed in an organic solvent in electrically-driven solvent extraction. A field of sufficiently high strength destabilizes such drops. The dynamics and disintegration of the drops are investigated here using a combination of simulation and experiment. The disintegration of the parent drop into daughter drops is analyzed and shown to accord well with experimental measurements. The implication of the findings to other fields, e.g. soft lithography, is also discussed. [Preview Abstract] |
Monday, November 19, 2007 5:32PM - 5:45PM |
JC.00010: Electrowetting-induced drop generation and control in a microfluidic flow-focusing device Florent Malloggi, Siva A. Vanapalli, Hao Gu, Dirk van den Ende, Frieder Mugele Recent upsurge in droplet-based microfluidic research is fueled by the potential application of drops as well-controlled environments for biochemical reactions, single cell analysis and fluid logical devices. Commonly pressure driven flows are used to create droplets continuously either in a flow-focusing or in T-junction geometry. While this approach provides high throughput capability, it is neither amenable to detailed on-demand generation of individual drops nor to dynamic control of surface wettability, which can dramatically affect the dynamics of two-phase microflows. Alternatively, electrowetting (EW)-on-dielectric is used to digitally manipulate drops. The EW provides exquisite control over individual drops and surface wettability. However, current implementations have low throughput and cannot readily be integrated with existing channel-based technologies. Here, we adopt a unified approach to create a soft microfluidic platform that harvests the power of both methods and offers the capability to address their limitations. We achieve this integration by incorporating EW into a flow-focusing device and demonstrate EW-controlled drop formation. We identify experimentally the range of voltages and driving pressures that yields EW-induced droplet generation. A theoretical description based on the balance of external pressures and voltage-controlled capillary pressures quantitatively accounts for the observations. Moreover we show that the smaller the geometric scales the more efficient the electrowetting control of drop generation. [Preview Abstract] |
Monday, November 19, 2007 5:45PM - 5:58PM |
JC.00011: Manipulation of compound drops by electric fields Marrivada Reddy, Asghar Esmaeeli Manipulation of tiny amounts of fluids in the form of drops is the central task in many lab-on-chips (LOB) applications. However, in certain applications such as blood analysis, there is a possibility of contamination of the discrete samples (drops) since they may leave their marks on the walls as they are pushed through the network of channels. One way to get around with this problem is to encapsulate the fluid sample by a neutral fluid. This necessitates a fundamental understanding of the behavior of the compound drops. While drops and particles can be manipulated by different forces, a particularly attractive method is the application of electric field as electricity is readily available and can influence the flow from a distance. Here, we propose to examine manipulation of compound drops suspended in another fluid by a uniform electric field. We use Direct Numerical Simulations (DNS) in conjunction with a one-field formulation to solve the governing Navier-Stokes and electrohydrodynamic equations. The goal is to understand the role of the controlling parameters, such as material properties and the strength of the electric field, on the deformation and orientation of the drops and the induced flow. [Preview Abstract] |
Monday, November 19, 2007 5:58PM - 6:11PM |
JC.00012: The Dynamics of Tipstreaming in Microfluidic Flow Focusing Devices Shelley L. Anna, Lynn M. Walker, Wingki Lee We have previously shown that a tipstreaming-like phenomenon occurs in microfluidic flow focusing experiments when dissolved surfactants are present in one of the liquid phases. This regime of droplet breakup leads to submicron droplets that are orders of magnitude smaller than the flow focusing orifice. The process is observed to be periodic, in which streams of tiny droplets alternate with the formation of a large (50 micron diameter) droplet at a frequency on the order of hundreds of cycles per second. In this talk, we report our observations of the dynamics of this process, including measurements of the relevant timescales for the formation of a cone-like interface, the drawing and disintegration of a fine thread, and the period with which the process repeats. We relate these timescales to dimensionless flow parameters such as the capillary number and the flow rate ratio, as well as the characteristic timescales for transport of surfactants to and along the interface. Through these observations and simple scaling analyses we suggest ways to extend the tipstreaming portion of the process to enhance the overall yield of submicron droplets. [Preview Abstract] |
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