Thomas Cubaud

Transport of bubbles in square microchannels

Liquid/gas flows are experimentally investigated in 200 and 525 μm square microchannels made of glass and silicon. Liquid and gas are mixed in a cross-shaped section in a way to produce steady and homogeneous flows of monodisperse bubbles. Two-phase flow map and transition lines between flow regimes are examined. Bubble velocity and slip ratio between liquid and gas are measured. Flow patterns and their characteristics are discussed. Local and global dry out of the channel walls by moving bubbles in square capillaries are investigated as a function of the flow characteristics for partially wetting channels. Two-phase flow pressure drop is measured and compared to single liquid flow pressure drop. Taking into account the homogeneous liquid fraction along the channel, an expression for the two-phase hydraulic resistance is experimentally developed over the range of liquid and gas flow rates investigated.

Capillary threads and viscous droplets in square microchannels

We experimentally study the formation and evolution of threads containing more viscous liquids surrounded by less viscous, immiscible liquids through hydrodynamic focusing in square microchannels. Over a large range of viscosities and interfacial tensions, five characteristic regimes of flow behavior are identified: threading, jetting, dripping, tubing, and displacement. We locate the boundaries between these regimes on a flow map based on the capillary number of each fluid. In the jetting and the dripping regimes, the droplet size is measured and related to fluid properties, flow parameters, and geometry. The critical thread length before jetting droplets and the critical length of a viscous tail before breakup in dripping are also examined. This study classifies and defines regimes of thread instabilities that can be used to produce supra- and subchannel size viscous droplets in an elementary microfluidic geometry.

Scaling law in liquid drop coalescence driven by surface tension

This Letter reports experimental results on the coalescence of two liquid drops driven by surface tension. Using a high speed imaging system, we studied the early-time evolution of the liquid bridge that is formed upon the initial contact of two liquid drops in air. Experimental results confirmed the scaling law that was proposed by Eggers et al. based on a simple and yet elegant physical argument. We found that the liquid bridge radius rb follows the scaling law rb∝t1/2 in the inertial regime. Further experiments demonstrate that such scaling law is robust when using fluids of different viscosities and surface tensions. The prefactor of the scaling law, rb/t1/2, is shown to be ∝R1/4, where R is the inverse of the drop curvature at the point of contact. The dimensionless prefactor is measured to be in the range of 1.03–1.29, which is lower than 1.62, a prefactor predicted by the numerical simulation of Duchemin et al. for inviscid drop coalescence.

Dissolution of carbon dioxide bubbles and microfluidic multiphase flows

We experimentally study the dissolution of carbon dioxide bubbles into common liquids (water, ethanol, and methanol) using microfluidic devices. Elongated bubbles are individually produced using a hydrodynamic focusing section into a compact microchannel. The initial bubble size is determined based on the fluid volumetric flow rates of injection and the channel geometry. By contrast, the bubble dissolution rate is found to depend on the inlet gas pressure and the fluid pair composition. For short periods of time after the fluids initial contact, the bubble length decreases linearly with time. We show that the initial rate of bubble shrinkage is proportional to the ratio of the diffusion coefficient and the Henrys law constant associated with each fluid pair. Our study shows the possibility to rapidly impregnate liquids with CO(2) over short distances using microfluidic technology.

A Methanol-Tolerant Gas-Venting Microchannel for a Microdirect Methanol Fuel Cell

As a byproduct, gas is constantly generated from the electrochemical reactions of direct methanol fuel cells (DMFCs). In the anodic channel of a DMFC, the gas forms bubbles, which leads to bubble clogging and pressure buildup if the device is miniaturized. Bubble clogging increases the flow resistance in microchannels, calling for excessive power consumption for fuel delivery. Pressure buildup aggravates the undesired crossover of methanol. In order to solve those problems, this paper introduces a gas-venting microchannel that directly removes gas bubbles from the two-phase flows of gas and methanol solution without leakage. By employing a hydrophobic nanoporous membrane, successful venting is achieved for both water and methanol fuel with a concentration of as high as 10 M. The fuel is contained without leakage under overpressures of as high as 200 kPa for both water and 10-M methanol, fulfilling the requirement of the current- as well as next-generation microdirect methanol fuel cells. A 1-D venting rate model is developed and experimentally verified for elongated bubbles. The reported bubble removal approach is also useful for other microfluidic devices, in which the accidental introduction of gas bubbles is prevalent.

CO2 dissolution in water using long serpentine microchannels

The evolution of carbon dioxide bubbles dissolving in water is experimentally examined using long microchannels. We study the coupling between bubble hydrodynamics and dissolution in confined geometries. The gas impregnation process in liquid produces significant flow rearrangements. Depending on the initial volumetric liquid fraction, three operating regimes are identified, namely saturating, coalescing, and dissolving. The morphological and dynamical transition from segmented to dilute bubbly flows is investigated. Tracking individual bubbles along the flow direction is used to calculate the temporal evolution of the liquid volumetric fraction and the average flow velocity near reference bubbles over long distances. This method allows us to empirically establish the functional relationship between bubble size and velocity. Finally, we examine the implication of this relationship during the coalescing flow regime, which limits the efficiency of the dissolution process.

Folded micro-threads: Role of viscosity and interfacial tension

The shape and evolution of periodically folded threads are experimentally examined in a microfluidic network. The fluidic system is designed for the production and lubricated transport of very uniform folds. To investigate the influence of viscosity and interfacial tension on buckling deformations, multiphase flows are scrutinized using both miscible and immiscible fluid pairs. The parameters used to analyze folding morphologies include thread diameter, arc-length, fold amplitude, and wavelength. When fluids are immiscible, the onset of viscous folding is characterized as a function of the capillary number and the phenomenon of “capillary unfolding” where a corrugated thread straightens along the flow direction is demonstrated. The spatial transition from folding to coiling-like flow behavior of high-viscosity capillary threads is also shown.

Formation and dynamics of partially wetting droplets in square microchannels

We experimentally study the production and evolution of partially wetting droplets as a function of the dynamic advancing contact angle and the viscosity of the external phase in microchannels. The natural spreading properties of immersed droplets are measured using high-speed goniometry and their forced spreading behaviors are probed in confined microgeometries. Low- and high-viscosity microfluidic segmented flows are generated by focusing water in a continuous phase of silicone oil using square microchannels. The shape and stability of the lubricating film between droplets and channel walls permit the classification of typical flow regimes, including wetting, thin film, thick film, and constant film thickness. Hysteretic partially wetting systems are shown to exhibit two modes of droplets formation, namely dripping and rivulet. Small-scale multiphase flows are investigated as a function of capillary number, droplet size, concentration, and velocity. We also discuss the occurrence of dynamic wetting transitions and stick-and-slip motion of microfluidic droplets. This study shows the possibility to control droplet dynamic wetting behavior with flow rates of injection in microfluidic platforms.

Interacting viscous instabilities in microfluidic systems

We discuss the formation, evolution, and stability of microfluidic flows involving two or more miscible fluids that have different viscosities. When two liquids that have widely different viscosities are injected into a rigid microfluidic device, their flow streams can naturally rearrange to form lubricated threads or stratified flows depending on the geometry and history of injection. An overview of two-fluid and three-fluid flow configurations in microchannels having square cross-sections is given for a variety of injection geometries. Miscible viscous fluid threads in confined microsystems can experience a range of viscous instabilities, such as folding and swirling. We show that microfluidics can be used to cause two or more instabilities to interact and co-evolve in diverging microchannels, thereby creating a variety of complex flow patterns.

Regimes of miscible fluid thread formation in microfluidic focusing sections

We experimentally study the formation and stability of miscible fluid threads made of high-viscosity liquids using hydrodynamic focusing sections. Miscible core annular flows are useful for transporting viscous materials and can be destabilized for enhancing mass transfer. We delineate phase-diagrams of the generation of lubricated threads from low to large viscosity contrasts with various diffusion coefficients. Depending on fluid properties and flow rates of injection, stable microflows are classified into engulfment, thread, and tubing regimes. For low Peclet numbers, we examine thread dynamics when diffusive effects strongly alter basic flow structures and induce new flow configurations, including ultra-diffusive and diffusive instability regimes. Another unstable flow arrangement is investigated for moderate Reynolds numbers where small threads are rapidly destabilized in the inertial flow field of the sheath fluid near the fluid junction. This study provides an overview of stable and unstable flow regimes and their transitions during the formation of miscible viscous fluid filaments in square microchannels.