Use of a porous membrane for gas bubble removal in microfluidic channels: physical mechanisms and design criteria

We demonstrate and explain a simple and efficient way to remove gas bubbles from liquid-filled microchannels, by integrating a hydrophobic porous membrane on top of the microchannel. A prototype chip is manufactured in hard, transparent polymer with the ability to completely filter gas plugs out of a segmented flow at rates up to 7.4 microliter/s per mm2 of membrane area. The device involves a bubble generation section and a gas removal section. In the bubble generation section, a T-junction is used to generate a train of gas plugs into a water stream. These gas plugs are then transported towards the gas removal section, where they slide along a hydrophobic membrane until complete removal. The system has been successfully modeled and four necessary operating criteria have been determined to achieve a complete separation of the gas from the liquid. The first criterion is that the bubble length needs to be larger than the channel diameter. The second criterion is that the gas plug should stay on the membrane for a time sufficient to transport all the gas through the membrane. The third criterion is that the gas plug travel speed should be lower than a critical value: otherwise a stable liquid film between the bubble and the membrane prevents mass transfer. The fourth criterion is that the pressure difference across the membrane should not be larger than the Laplace pressure to prevent water from leaking through the membrane

Microdevices for extensional rheometry of low viscosity elastic liquids : a review

Extensional flows and the underlying stability/instability mechanisms are of extreme relevance to the efficient operation of inkjet printing, coating processes and drug delivery systems, as well as for the generation of micro droplets. The development of an extensional rheometer to characterize the extensional properties of low viscosity fluids has therefore stimulated great interest of researchers, particularly in the last decade. Microfluidics has proven to be an extraordinary working platform and different configurations of potential extensional microrheometers have been proposed. In this review, we present an overview of several successful designs, together with a critical assessment of their capabilities and limitations

Faceted drops on heterogeneous surfaces

We report an experimental study of the shape of large liquid drops spreading on surfaces with patterns of wettability. A heterogeneous surface exhibits multiple energy minima of free energy for the three-phase system. In particular, when the distribution of defects on the substrate is periodic (square or hexagonal), it is possible to initiate a transition between circular and faceted drops. We describe the surface growth of these liquid drops as a geometric avalanche process released by surface tension

Spreading of Large Drops on Patterned Surfaces

International audienceWe report an experimental study of the shape of large liquid drops spreading on surfaces with patterns of wettability. The patterns are sets of disks of a material less wettable than the rest of the plane substrate. When few defects are present, or when the average distance between defects is large compared to the capillary length, the drop edge is a set of circular arcs connecting the pinning points. When the density of defects is smaller than the capillary length, the contact line shape is more complex and we analyze its morphology through a kind of box counting method. The roughness of the contact line is maximum when the average distance between defects is comparable to two times the defects size. Setting the non-wetting defects on a periodic or almost periodic array produces drops with faceted edges. The transition between circular and faceted drops can be understood as a competition between the tension of the drop edge and the local force exerted on the contact line by the defects. Although the drops are grown at a fixed flow rate, the motion of the contact line is irregular, with jumps between pinning sites. In some cases, the jumps of the contact line are correlated in space, leading to avalanchesand large scale motions of the drop

Spreading of Large Drops on Patterned Surfaces

We report an experimental study of the shape of large liquid drops spreading on surfaces with patterns of wettability. The patterns are sets of disks of a material less wettable than the rest of the plane substrate. When few defects are present, or when the average distance between defects is large compared to the capillary length, the drop edge is a set of circular arcs connecting the pinning points. When the density of defects is smaller than the capillary length, the contact line shape is more complex and we analyze its morphology through a kind of box counting method. The roughness of the contact line is maximum when the average distance between defects is comparable to two times the defects size. Setting the non-wetting defects on a periodic or almost periodic array produces drops with faceted edges. The transition between circular and faceted drops can be understood as a competition between the tension of the drop edge and the local force exerted on the contact line by the defects. Although the drops are grown at a fixed flow rate, the motion of the contact line is irregular, with jumps between pinning sites. In some cases, the jumps of the contact line are correlated in space, leading to avalanchesand large scale motions of the drop

Cassie and Wenzel: Were they really so wrong?

The properties of superhydrophobic surfaces are often understood by reference to the Cassie-Baxter and Wenzel equations. Recently, in a paper deliberately entitled to be provocative, it has been suggested that these equations are wrong; a suggestion said to be justified using experimental data. In this paper, we review the theoretical basis of the equations. We argue that these models are not so much wrong as have assumptions that define the limitations on their applicability and that with suitable generalization they can be used with surfaces possessing some types of spatially varying defect distributions. We discuss the relationship of the models to the previously published experiments and using minimum energy considerations review the derivations of the equations for surfaces with defect distributions. We argue that this means the roughness parameter and surface area fractions are quantities local to the droplet perimeter and that the published data can be interpreted within the models. We derive versions of the Cassie-Baxter and Wenzel equations involving roughness and Cassie-Baxter solid fraction functions local to the three-phase contact line on the assumption that the droplet retains an average axisymmetry shape. Moreover, we indicate that, for superhydrophobic surfaces, the definition of droplet perimeter does not necessarily coincide with the three-phase contact line. As a consequence, the three-phase contact lines within the contact perimeter beneath the droplet can be important in determining the observed contact angle on superhydrophobic surfaces