Session L7: Microfluids: Particles III - Droplets and Emulsions

Lateral migration of a droplet by optical force in a uniform flow

The lateral migration of a droplet under an optical force in a uniform flow was studied to show the separation characteristics in the cross-type optical separator. The initially spherical droplet was moved through the fluid flow using a loosely-focused Gaussian laser beam along the direction normal to the flow. To simulate such a system, the lattice Boltzmann method was adopted to obtain the two-phase flow field, and the dynamic ray tracing method was applied to the optical force calculation. The optical forces acting on the spherical droplets are found to be in good agreement with the theoretically predicted values. The trajectories of the droplets are obtained and compared with the experimental data. Simulations on various flow and optical parameters have been made. It is found that the lattice Boltzmann method, combined with the dynamic ray tracing, can be utilized to design optical manipulation systems.   

Effects of surfactants on the deformation of microfluidic drops

A microfluidic analog of the four-roll-mill experiment is used to study the deformation and breakup of microfluidic drops. The behavior of water drops flowing in mineral oil is quantified as a function of the capillary number, $Ca$, which is based on the oil viscosity, drop radius, flow shear rate and equilibrium interfacial tension, both in the presence and absence of surfactants. In the absence of surfactants the deformation of the drops increases linearly with $Ca$. If surfactants are added to the carrier oil then, for the same value of $Ca$, drops deform less if the flow velocity is larger. Moreover, for a given drop size in the presence of surfactants, drops begin to split at a threshold shear rate but stop breaking if the shear rate is increased beyond a second threshold. These observations are explained by a decrease in the surfactant concentration at the surface of the drop due to advection of surfactant molecules by the oil flow. This increases the interfacial tension, thus making the drop less deformable for higher flow velocities. We use the deformation of the drops to infer the mean interfacial tension and from this we quantify the surface concentration of surfactants at the drop interface.   

Droplet velocity in microfluidics Hele-Shaw cell: effect of the disjoining pressure

We present an experimental evidence of the disjoining pressure effect on the traveling velocity of highly confined viscous droplets in microfluidics. Two regimes are observed depending on the capillary number, $Ca=\mu_f U_d/\gamma$. Above a critical capillary number $Ca^*$ the droplet velocity $U_d$ follows $U_d \propto U_f Ca^{1/3}$, where $U_f$ is the velocity of the carrier liquid. However, for $Ca [Preview Abstract]

Formation and dynamics of partially non-wetting droplets in square microchannels

We experimentally study the formation and evolution of partially non-wetting droplets in microchannels made of glass and silicon. Droplets are generated by focusing pure water in an external phase of silicone oil using square microchannels. To probe the influence of the capillary number on droplet behavior in confined geometries, the oil viscosity is varied over four decades. For each fluid pair, the critical speed associated with the dynamic wetting transition is experimentally determined using a contact angle goniometer equipped with a high-speed camera. We discuss a variety of wetting phenomena in microchannels. In particular, we focus on the droplet lubrication transition from low to large capillary numbers and we examine droplet velocity as a function of droplet length, flow rates, and dynamic wetting properties.   

Controlled Gelation of Particle Suspensions Using Controlled Solvent Removal in Picoliter Droplets

Droplets in microfluidic devices have proven useful as uniform picoliter reactors for nanoparticle synthesis and as components in tunable emulsions. However, there can be significant transport between the component phases depending on solubility and other factors. In the present talk, we show that water droplets trapped within a microfluidic device for tens of hours slowly dehydrate, concentrating the contents encapsulated within. We use this slow dehydration along with control of the initial droplet composition to monitor gelation of aqueous suspensions of spherical silica particles (Ludox) and disk-shaped clay particles (Laponite). Droplets are generated in a microfluidic device containing small wells that trap the droplets. We monitor the concentration process through size and shape changes of these droplets as a function of time in tens of droplets and use the large number of individual reactors to generate statistics regarding the gelation process. We also examine changes in suspension viscosity through fluorescent particle tracking as a function of dehydration rate, initial suspension concentration and initial droplet volume, and added salt, and compare the results with the Krieger-Dougherty model in which viscosity increases dramatically with particle volume fraction.   

Geometrically-protected reversibility in hydrodynamic Loschmidt-echo experiments

We demonstrate an archetypal Loschmidt-echo experiment where thousands of droplets interact in a reversible fashion via a viscous fluid. Firstly, we show that, unlike equilibrium systems, periodically driven microfluidic emulsions self-organize and geometrically protect their macroscopic reversibility. This self-organization is not merely dynamical, it has a clear structural signature akin to the one found in a mixture of molecular liquids. Secondly, we evidence that above a maximal shaking amplitude both structural order and reversibility are lost simultaneously in the form of a 1st order non-equilibrium phase transition. Thirdly, we account for this discontinuous transition, in term of a memory-loss process.   

Generation of monodisperse particle-stabilized droplets with controlled particle loading at the interface

Common high-shear methods of generating particle-stablized emulsions have no direct control over the rate of droplet generation, the final droplet size distribution, or the composition of the interface, all of which are important to the interfacial and bulk rheology. In this talk, we present a method that allows for independent control over the droplet size generated and the degree of particle loading on the interface. Droplets are formed on demand at a T-junction with a desired size via a pressure-controlled generation scheme, then travel along the axis of a circular capillary containing a surface-active particle suspension. We will model the surface coverage as a function of residence time, then use light scattering methods to assess the degree of depletion of particles from the bulk and thus verify our model for two different systems: air-in-water foams stabilized by silica nanoparticles, and water droplets stabilized by silica in a continuous phase of cyclohexane. Thus, we show that by controlling channel geometry, applied inlet pressures, and residence time, we can directly control droplet size, volume fraction, and particle loading on the bubble interface, all of which are critical parameters relevant to the stability and rheology of a particle-stabilized emulsion or foam.   

Self assembly of droplets under shear

We produce droplets of colloidal size in microfluidic systems, using step emulsification generators. The mechanism of generation allows to produce droplet clusters under control. These clusters evolve in the presence of a shear. At small shears, and for adhesive droplets, the clusters adopt equilibrium configurations that maximize the number of contact points, consistently with observations made in fluids at rest. At larger shear, we observe a rich variety of configurations, stationary, long-live or oscillatory.   

Drag Force on Droplet in Filtration Process

The drag force is studied for particles and droplets with their axis tilted with respect to Stokes flow. Boundary singularity method is adopted to solve the problem. While in stationary flow the droplet is turned so as its axis is parallel to the flow. In fiber filtration problems droplet is either attached to a fiber at an arbitrary angle between flow and fiber or is recently detached from a fiber and still turned with respect to flow. In addition, fluid droplets may form by merging of unequal droplets or form asymmetric shapes under action of gravity. In many studies Stokes force acting on arbitrary shaped particle is determined by calculating radius of spherical particle with either volume or surface same as of an original particle, and using Stokes formula. Since the approach is heuristic, the correcting coefficient is used to adhere to a particular class of particle shapes. The magnitude of drag is changed substantially with the angle. Therefore the approach of equivalent volume sphere is not valid for determination of drag for considered cases. The pressure and shear stress at the particle surface are obtained to explain the behavior of the drag with its maximum at 45 degrees. The droplets attached to a fiber are considered and the effects of fiber on the drag are studied.   

Hindered Brownian motion of colloidal particles near a liquid-liquid interface

In this work, anisotropic hindered Brownian motion of colloidal particles in the vicinity of a liquid-liquid interface is experimentally quantified and compared with established theories. Evanescent wave-based particle tracking velocimetry is used to measure the three-dimensional Brownian motion of fluorescent microspheres near an interface between water and non-polar oil. The experimental results confirm that the mobility of particles suspended in the less viscous liquid is suppressed anisotropically, and differs from the hindered mobility of particles near a solid wall. The measured hindered diffusivities are in close agreement with the theoretical predictions.