Session LF: Microfluidics: Drops and Capillarity

Lubrication failure of viscous threads in microfluidic chambers

We investigate lubricated transport of high-viscosity fluids in microfluidic systems. Using miscible liquids having disparate viscosities, we produce a viscous core transported within a less viscous annulus, i.e., a viscous thread, in a square microchannel. Downstream, the thread motion is studied in diverging-converging slit microchannels. We describe a variety of flow patterns resulting from the folding instability and examine the relationships between flow morphologies and system parameters including fluid viscosities, mass diffusion coefficient, flow rates, and micro-cell geometry. In particular, we demonstrate that small threads can traverse the extension without lubrication failure while large threads experience a significant dilation due to direct contact with the top and bottom walls. We also investigate the lubrication failure of capillary threads using immiscible fluids and show the possibility to manipulate contact lines.   

Folding of capillary threads in microfluidic networks

We examine the evolution of the folding instability of lubricated viscous threads in straight microchannels. Folds having a uniform wavelength can be produced using a diverging microchannel connected to three channels in parallel. This design allows for the detailed experimental study of the influence of viscosity contrast, interfacial properties, and flow rates on the structure of miscible and immiscible micro-threads. In particular, we focus on the spatial damping of the amplitude of sinuous capillary threads due to interfacial tension effects. This study shows methods for the interfacial control of high-viscosity fluids in microfluidic systems.   

Nanoemulsion Through Stretching-Folding Instability

A new kind of instability sets in when a oil filament is focused by surrounding water-flow through a thin constriction commonly used in fluid focusing devices. At sufficiently high flow rates the oil filament is forced into harmonic oscillations through interfacial forces. Past the constriction the liquid is suddenly slowed down which leads to rapid shortening of the filament's wavelength. At sufficiently high amplitudes the co-flowing water stream breaks up and pinches off micrometer and sub-micrometer sized droplets in a very repeatable manner, thus producing a water-in-oil emulsion. This pinch off is caused by a stretching and folding instability when the oscillating flow impinges into the quasi stagnant reservoir past the constriction. We will present high-speed movies at up to 300,000 frames per second resolving details of the very fast events. A simple model based on restoring interfacial forces is able to predict the kilohertz oscillation frequency observed.   

Droplet formation and storage using immiscible two liquids in a micro-channel with transverse micro-ribs

Such a transition in the micro-channel with micro-ribs was also studied, which may be significant in designing super-hydrophobic micro-channel. An in-depth study of the wetting transition in micro-channel with micro-ribs is carried out to scrutinize the condition of the wetting transition. And based on the optimized condition for the wetting transition, we investigate the flow characteristics of two immiscible liquids in the micro-channel in order to generate and storage the droplet. When the interface of immiscible liquids moves across the cavity between two neighboring micro-ribs, the oil phase may replace the water in the cavity, isolating the water phase in a corner of the cavity and forming a droplet. The isolated water volume directly affects the droplet size, which is determined by the speed of the interface, the geometry of the micro-ribs, and physical properties of the fluids, such as viscosity and surface tension. For the formation of uniform droplets without any daughter droplets, the synchronization of both contacts on the top of the forward micro-rib and the bottom of the cavity must be considered to find for optimal condition.   

Generating double emulsions W/O/W in PDMS systems for pharmaceutical applications

Vesicular systems, and especially water/oil/water multiple emulsions, present fascinating properties of pharmaceutical interest (release of fragile pharmaceutically active molecule, detoxification). Their implementation requires a strict control of their governing physical parameters, since their properties (stability, efficiency) crucially depends on the size of the internal droplets, and of the globules containing these droplets. However, traditional methods for generating emulsions with the help of high shear mixers do not allow to produce calibrated systems, thus limiting their characterization. It is well known that microfluidics provides a prominent tool to generate simple or multiple emulsions in a controlled way, with a very low diameter dispersion (below 5{\%}). In this talk, we will present preliminary results concerning the generation of such objects (sizes distributions, stabilities{\ldots}) and compare them to traditional methods.   

Scaling the drop size in coflow experiments

We performed extensive experiments with coflowing fluids in microfluidic devices. When the inner fluid is a liquid, two different types of regimes have been identified, dripping and jetting. Dripping is characterized by the fact that no long jets of the dispersed phase are formed. By contrast, when jetting occurs, the dispersed phase forms long liquid jets and drops are emitted right at the tip of the liquid thread. In the jetting regime, we could reproduce the {\sl widening} and {\sl stretching} regimes [Utada et. al. PRL 99 (2007)]. We have given a step further and provide a general expression to estimate the drop size in either regime as a function of measurable parameters. By contrast to the liquid case, when the inner liquid is a gas, we find that no long jets form, irrespective of the values of the control parameters, [Marin et. al. Coll. Surf. A (2009)].The crucial role of the axial strain exerted by the outer stream on the inner one to stabilize long fluid threads will be elucidated by means of BEM simulations, which show good agreement with experiments.   

Droplet break-up in microfluidic T-junctions at small capillary numbers

We perform experimental studies of droplet breakup in microfluidic T-junctions in a range of Capillary numbers lying between 4.10$^{-4}$ and 2 10$^{-1}$ and for two viscosity ratios of the fluids forming the dispersed and continuous phases. The present paper extends the range of Capillary numbers explored by previous investigators by two orders of magnitude. We single out two different regimes of breakup. In a first regime, a gap exists between the droplet and the wall before breakup occurs. In this case, the break up process agrees well with the analytical theory of Leshansky and Pismen [Phys. Fluids, 21(2), 023303 (2009)]. In a second regime, droplets keep obstructing the T-junction before breakup. Using physical arguments, we introduce a critical droplet extension for describing the breakup process in this case.   

Hydrodynamics and Heat Transfer of Discrete Droplets in Microfluidic Devices

Electrostatic manipulation of surfaces tension forces is now a standard fluid handling technique in microfluidic devices. In this investigation electrowetting on dielectric (EWOD) is employed in order to use discrete droplets for thermal management of compact micro systems. Both hydro- and thermodynamics of digitized droplets are investigated by experimental, theoretical and computational means. EWOD devices have been built on silicon substrates with highly doped layers replacing metal electrodes, and higher quality thermal oxides replacing the more expensive PECVD oxides. In parallel, an experimental test rig has been built to measure the heat transfer rate of the slug flow at a macro scale. Droplets at several length and speed are created systematically. Average heat transfer rates and Nusselt numbers in constant heat flux in a tube has been experimentally measured for continuous and discrete water flow cases and the results have been compared with numerical results.   

A numerical method for Stokes flow with capillary effects

A simulation tool is presented for capillary driven reacting and polymerizing low-Reynolds-number flows. The free surface is represented by a level-set function, but with extra terms added so that it remains sharp despite finite numerical diffusion. The governing equations are discretized with hierarchical finite elements. A splitting facilitates implicit time advancement of the nonlinear advection-diffusion transport equation. At small Capillary numbers, the pressure jump at the free surface due to surface tension makes the saddle-point discrete system for the velocity expensive to solve. A decomposition into two parts, facilitated by the hierarchical elements, significantly accelerates the overall solution. The first part governs the static system, which includes the sharp interface. This portion requires no incompressibility constraint and is therefore relatively easily solved using $p$ refinement for high accuracy. The second part is dynamic but smooth, so it can be solved in relatively few iterations despite the incompressibility constraint. This decomposition reduces the computational expense, by up to 95\% in the demonstration simulations, which are relevant to self-healing autonomic materials.   

Novel Method for Measuring Temperature of Microchannel Flows Using Polydiacetylene Sensor Droplets

Monitoring temperature in a microchannel flow is important when to use a microfluidic chip for biochemical analysis like cell culture. There are two typical methods: using thin film thermocouples (TFTCs) and Rhodamine B solutions. The former can measure temperature of a microchannel flow accurately, but requires complex fabrication processes and high costs. In addition, it disturbs microchannel flows and is contaminated readily by reaction between TFTC and flow solutions. On the other hand, Rhodamine B has the excellent sensor property that its fluorescent intensity is linear to a flow temperature. Unfortunately, however, it often adsorbs to microchannel surfaces like PDMS and has issues of bio-compatibility due to its hydrophobicity Thus, we propose a novel method to monitor temperature of a microchannel flow using polydiacetylene (PDA) sensor droplets. PDA, a conjugated polymer, has a unique property to transform its color from visible blue to fluorescent red by thermal stress. By monitoring the fluorescence intensity of PDA droplets in a microchannel, we found a linear relationship between the flow temperature and the fluorescence intensity in a certain temperature range.