Session GM: Microfluids: General IV: Bubbles, Drops and Particles

Wettability dependent bubble dynamics in microfluidic networks

The routing of bubble or droplet traffic through microfluidic networks depends greatly on the hydrodynamic resistance in the individual branches of that network. We find that a confined bubble translating through a partially wetting liquid experiences significantly more friction than a bubble lubricated by a completely wetting liquid, with important consequences for the dynamic behavior. For our system, we observe symmetric bubble break up and alternating left-right routing at a microfluidic junction, as described previously by Link et al. For partially wetting liquids, we observe a much richer dynamic behavior, with asymmetric splitting and left-right routing with chaotic periodicity. We identify the contact angle as a key control parameter that determines the different regimes and we explore how the transitions between these regimes can be effected by tuning this parameter. The results of this work aid the prediction and control of bubble traffic through complex microfluidic networks. Link et al., Phys. Rev. Lett. 92 (2005) 054503   

Flow field inside a stationary microdroplet in a Hele-Shaw cell

We consider the flow field inside a water drop held stationary in a flowing external oil, experimentally and theoretically. The droplet is strongly confined in the vertical direction, making it take a ``pancake"-like shape. It is anchored in place by introducing a local variation in the channel height which reduces the free surface energy, with minimal modifications to its general shape. Two contrasting flow regions are visible inside the drop: a fast recirculation flow is observed near the droplet boundary, while a slower flow takes place in the central region. While the central flow is well described by the standard Hele-Shaw model, the flow near the droplet edge displays strong three-dimensional recirculation, pointing to a complex hydrodynamic coupling between the droplet and outer flows. These two regimes are characterized for different droplet geometries and external flow rates, and a theoretical justification for their existence is provided.   

The Microfluidic Thunderstorm

The so-called ``Kelvin's thunderstorm'' is a simple experiment demonstrating the spontaneous appearance of induced free charge in droplets emitted through a tube. As Lord Kelvin explained, the droplets acquire a net charge spontaneously during pinch-off due to the presence of electrical fields in their surrounding created by any metallic object. In his experiment, two streams of droplets are allowed to drip from separated nozzles into separated buckets, which are at the same time interconnected through the dripping needles. The implementation of such an effect in a microfluidic device could enhance the control of droplets and prevent undesired effects as coalescence. The phenomenon has been successfully reproduced in a simple microfluidic device, where the droplets could get charged to charge-to-mass ratios above the Rayleigh limit. Experimental measurements will be presented showing the dependence of the acquired charge in the droplets on different parameters as the flow rate or the liquid electrical conductivity. The concept certainly opens a door to a costless and accessible transformation of pneumatic pressure into electrical energy and to an enhanced control in microfluidic technologies.   

Droplet flows through periodic loop networks

Numerous microfluidic experiments have revealed non-trivial traffic dynamics when droplets flow through a channel including a single loop. A complex encoding of the time intervals between the droplets is achieved by the binary choices they make as they enter the loop. Very surprisingly, another set of experiments has demonstrated that the addition of a second loop does not increase the complexity of the droplet pattern. Conversely, the second loop decodes the temporal signal encrypted by the first loop [1]. In this talk we show that no first principle argument based on symmetry or conservation laws can account for this unexpected decoding process. Then, to better understand how a loop maps time intervals between droplets, we consider a simplified model which has proven to describe accurately microfluidic droplet flows. Combining numerical simulations and analytical calculations for the dynamic of three droplets travelling through N loops: (i) We show that three different traffic regimes exist, yet none of them yields exact decoding. (ii) We uncover that for a wide class of loop geometry, the coding process is analogous to a Hamiltonian mapping: regular orbits are destabilized in island chains and separatrix. (iii) Eventually, we propose a simple explanation to solve the apparent paradox with the coding/decoding dynamics observed in experiments. [1] M.J. Fuerstman, P. Garstecki, and G.M. Whitesides, \emph{Science}, 315:828, 2007.   

Patterning of non-spherical particles onto electrode surface: Study of orientation behavior under viscous fluid and AC electrokinetic forces

Recently we had proposed a technique called rapid electrokinetic patterning$^{ }$(REP), a tool that can manipulate colloidal particles near illuminated spot on an electrode surface. REP utilizes optical landscapes to create gradients in temperature allowing local changes in permittivity and conductivity of the fluid creating a microvortex. However, REP has been demonstrated till now only with spherical particles. We expand upon the initial disclosure of REP and conduct experiments with non-spherical beads. In the presence of linearly polarized field a non-spherical particle experiences frequency dependent alignment torques along three principle axis. This is mainly because of the different polarizability along each direction. In a fluid flow, a non-spherical particle would align itself in order to minimize the viscous drag. But characterizing the orientation behavior of non-spherical particles under the influence of both electric field and viscous fluid drag presents a unique physics problem. We observed the vertical orientation of the cylinders in the REP aggregation. We explore the mobility of the captured particles on the surface with respect to various physical parameters.   

Particles Dispersion on Fluid-Liquid Interfaces

In a previous study we have shown that when small particles, e.g., flour, pollen, etc., come in contact with an air-liquid interface, they disperse in a manner that appears explosive. This is due to the fact that the capillary force pulls particles into the interface causing them to accelerate to a relatively large velocity. The motion of particles in the direction normal to the interface is inertia dominated, and so they oscillate vertically about the equilibrium position before coming to a stop under viscous drag. This vertical motion of a particle causes a radially outward lateral (secondary) flow on the interface that causes nearby particles to move away. The dispersion on a liquid-liquid interface was relatively weaker than on an air-liquid interface, and occurred over a longer period of time. This was a consequence of the fact that particles became separated while sedimenting through the upper liquid and reached the interface over a time interval that lasted for several seconds. The rate of dispersion depended on the size of particles, the particle and liquids densities, the viscosities of the liquids involved, and the contact angle. The frequency of oscillation of particles about their floating equilibrium increased with decreasing particle size on both air-water and liquid-liquid interfaces, and the time taken to reach equilibrium decreased with decreasing particle size.   

Microfluidic enhanced conductive polymer microspheres for sensor applications

Methods and devices were developed to produce monodispersed, conducting, responsive polyaniline (PANI) particles for drug delivery and sensor applications. Liquid droplets are produced containing a dispersed phase carried through the device by the continuous phase. The two phases are immiscible. Each phase can be either oil or water based. The aniline monomer is contained within the dispersed phase while the oxidizing agent, ammonium persulfate (APS) is contained within the aqueous phase. The production of either solid (aniline, APS in dispersed phase) or shell particles (aniline in dispersed phase, APS in continuous phase) is possible. Droplets are formed by controlling the viscous and capillary forces at the interface. Droplet size is controlled by phase flow rates, the interfacial tension and viscosity ratio between the phases and the inlet geometry. PANI particles are produce via oxidative polymerization. The polymerization is pH dependent and the time of polymerization is monitored by the distance the droplets travel in the channel. The morphology and electrochemical characteristics of the particles resulting from these methods are studied.   

Transport and separation of micron sized particles at isotachophoresis zone boundaries

Conventionally, isotachophoresis (ITP) is used for the separation of ionic samples according to their electrophoretic mobilities. At the zone boundaries large gradients in concentration and electric field occur. These gradients may be utilized to transport and separate small particles, as we demonstrate experimentally. We show that polymer beads of 5~micron diameter dispersed in a high mobility leading electrolyte are picked up and carried along by an ITP zone boundary that is formed between a low mobility trailing electrolyte and the leading electrolyte. Additionally, it is shown that different types of beads can be separated in that way. In particular, beads of 1~micron diameter are not carried along by the transition zone, so that a separation from 5~micron sized beads is feasible. We have identified two different effects that contribute to the force acting on the particles. Firstly, there is an electric dipole force due to the electric field gradient, secondly, a electro-hydrostatic force is generated that induces a pressure gradient. Therefore, the resulting protocol for particle separation bears some resemblance with dielectrophoresis that also utilizes electric dipole forces. An apparent advantage of our technique over dielectrophoresis lies in the fact that no microstructured electrodes or other types of microstructures are needed to create the electric field gradient.   

3D inertial migration of microspheres in a microchannel flow: transition from radial to angular focusing

Particles suspended in a square channel flow tend to be focused near the centers of each channel face due to the inertial migration. We found a transition in the particle focusing mode, from radial to angular, using digital holographic microscopy technique. 3D positions of microspheres (d=7, 15$\mu $m) in a square microchannel (H=100$\mu $m) were measured at channel Reynolds number R$_{C}$=4.7$\sim $120. With increasing R$_{C}$, the particles migrated in radial direction in advance and then started to focus angularly toward the four equilibrium positions at around $R_{C}>$60. The degree of angular focusing was also found to be linearly increased as the product of shear-rate and particle diameter increases.   

Arrangement of high-viscosity droplets in diverging/converging microchannels

We experimentally examine the dynamics of highly viscous droplets in a two-dimensional pore model, i.e., a plane diverging/converging microfluidic chamber. Upstream from the chamber, regular trains of droplets are formed in the dripping and jetting regimes using a hydrodynamic focusing section into a square microchannel. This method permits the steady generation of elongated droplets (dripping) and small spherical droplets (jetting). We measure the velocity and trajectory of drops as they traverse the pore for various fluid viscosities and flow rates. In particular, the variation of the flow velocity in the diverging/converging channel produces a broad range of self-assembly phenomena. We focus on the formation crystal-like structures by hydrodynamic coupling by tracking individual droplets and measuring the droplet stream envelope in the chamber. The individual and collective droplet behaviors are analyzed based on the droplet concentration and injection flow rates required for initiating coalescence and building ordered or disordered droplet assemblies.