Role of the channel geometry on the bubble pinch-off in flow-focusing devices

Droplet on demand system utilizing a computer controlled microvalve integrated into a stiff polymeric microfluidic device

An integrated microvalve fabricated in a stiff polymeric (polycarbonate) device allows for formation of droplets and bubbles of arbitrary volumes and at arbitrary times of emission. Predesigned protocols of formation of sequences of volumes, intervals between the droplets and their chemical compositions can be practically and reproducibly realised and controlled from a personal computer. The design of the microvalve enables its utilization in chips made in a range of other materials, including plastics and glass and in devices operated both with positive and negative pressure providing a useful module for a range of applications of microfluidic droplet systems.

Dynamic memory in a microfluidic system of droplets traveling through a simple network of microchannels

The flow of droplets through the simplest microfluidic network--a set of two parallel channels with a common inlet and a common outlet--exhibits a rich variety of dynamic behaviors parametrized by the frequency of feeding of droplets into the system and by the asymmetry of the arms of the microfluidic loop. Finite ranges of these two parameters form islands of regular (cyclic) behaviors of a well defined period that can be estimated via simple theoretical arguments. These islands are separated by regions of behaviors that are either irregular or cyclic with a very long periodicity. Interestingly, theoretical arguments and numerical simulations show that within the islands of regular behaviors the state of the system can be degenerate: there can exist a number of distinct sequences of trajectories of droplets, each stable and--in the absence of disturbances--continuing ad infinitum. The system can be switched between these cyclic trajectories with a single stimulus.

Microfluidic fabrication of stable nanoparticle-shelled bubbles

We introduce a microfluidic approach to generating monodisperse, stable nanoparticle-shelled bubbles using air-in-oil-in-water (A/O/W) compound bubbles as templates. The oil phase of the A/O/W compound bubbles comprises a volatile organic solvent and a hydrophobic silica nanoparticle. Upon evaporation of the organic solvent, the nanoparticles in the oil layer form a stiff shell at the air-water interface, which drastically enhances the stability of the bubbles against dissolution and coarsening. On the basis of this approach, we demonstrate that it is also possible to generate functional bubbles stabilized by composite shells that are composed of mixtures of hydrophobic materials and nanoparticles with unique properties.

Flows around confined bubbles and their importance in triggering pinch-off

We describe the breakup of a confined gas thread in a cross-flowing stream of liquid at capillary numbers Ca < 10{-2}. The breakup is initiated, not by a Plateau-Rayleigh instability, but by liquid that flows from the tip of the thread to the neck where pinch-off occurs. This flow, faster than previously estimated, is driven by different curvatures at the tip and neck and runs through large gaps between thread and channel walls. Understanding how these curvatures evolve during bubble formation leads to accurate predictions of the moment of pinch-off.

Impact of inlet channel geometry on microfluidic drop formation

We study the impact of inlet channel geometry on microfluidic drop formation. We show that drop makers with T-junction style inlets form monodisperse emulsions at low and moderate capillary numbers and those with Flow-Focus style inlets do so at moderate and high capillary numbers. At low and moderate capillary number, drop formation is dominated by interfacial forces and mediated by the confinement of the microchannels; drop size as a function of flow-rate ratio follows a simple functional form based on a blocking-squeezing mechanism. We summarize the stability of the drop makers with different inlet channel geometry in the form of a phase diagram as a function of capillary number and flow-rate ratio.

Bubble pinch-off in a rotating flow

We create air bubbles at the tip of a "bathtub vortex" which reaches to a finite depth. The bathtub vortex is formed by letting water drain through a small hole at the bottom of a rotating cylindrical container. The tip of the needlelike surface dip is unstable at high rotation rates and releases bubbles which are carried down by the flow. Using high-speed imaging we find that the minimal neck radius of the unstable tip decreases in time as a power law with an exponent close to 1/3. This exponent was found by Gordillo et al. [Phys. Rev. Lett. 95, 194501 (2005)10.1103/PhysRevLett.95.194501] to govern gas flow driven pinch-off, and indeed we find that the volume oscillations of the tip creates a considerable air flow through the neck. We argue that the Bernoulli pressure reduction caused by this air flow can become sufficient to overcome the centrifugal forces and cause the final pinch-off.

Microfluidic crystals: dynamic interplay between rearrangement waves and flow

Microfluidic crystals are assemblies of miniature bubbles or drops flowing in channels. We explore here the flow of these crystals, when submitted to a given driving pressure. The flow velocity is linked to the finite number of elements in the channel width, and presents discontinuities when the crystal structure changes. At the transition from one structure to the other original dynamic features appear. The flow can self-regulate itself on a fixed velocity whatever the driving pressure, or, on the contrary, can spontaneously pulsate. All these features are predicted by simply considering the crystal's energy and friction, and looking at the propagation of structure rearrangements. We anticipate these results to improve the control over the structure of dense two-phase flows in microfluidic systems.

Spatiotemporal instability of a confined capillary jet

Recent experimental studies on the instability of capillary jets have revealed the suitability of a linear spatiotemporal instability analysis to ascertain the parametrical conditions for specific flow regimes such as steady jetting or dripping. In this work, an extensive analytical, numerical, and experimental description of confined capillary jets is provided, leading to an integrated picture both in terms of data and interpretation. We propose an extended, accurate analytic model in the low Reynolds number limit, and introduce a numerical scheme to predict the system response when the liquid inertia is not negligible. Theoretical predictions show remarkable accuracy when compared with the extensive experimental mapping.

Stability of a jet in confined pressure-driven biphasic flows at low Reynolds number in various geometries

We adress the question of the stability of a confined coflowing jet at low Reynolds number in various geometries. Our study is motivated by recent experiments in microfluidic devices. When immiscible fluids flow in microchannels, either monodisperse droplets or parallel flows are obtained depending upon the flow rate of the aqueous phase and the oil phase. In these experiments, the confining and the shape of the geometry play a fundamental role. In a previous paper [Guillot, Phys. Rev. Lett 99, 104502 (2007)], we analyzed the stability of the jet in the framework of the lubrication approximation at low Reynolds number in a cylindrical geometry, and we related the transition between the droplets regime and the jet regime to the absolute-convective transition of the Rayleigh plateau instability. In this work, the effect of the channel geometry and the jet position within the microfluidic device are discussed. New flow patterns are pointed out. Bidimensional jets are encountered in square and rectangular geometry. Contrary to jets occuring in circular geometry, these two-dimensional jets are absolutely stable. Focusing on situations where the inner fluid is more viscous than the outer one, we evidence a range of parameters where droplets are produced through a blocking and pinching mechanism. In this particular case, the flow is unstable, the growing perturbations are convected upstream. This induces the clogging of the channel by the internal phase and its pinching by the external one. In a future presentation we will give a comparison between this model and experimental data.