Bacteria hinder large-scale transport and enhance small-scale mixing in time-periodic flows.

Understanding mixing and transport of passive scalars in active fluids is important to many natural (e.g., algal blooms) and industrial (e.g., biofuel, vaccine production) processes. Here, we study the mixing of a passive scalar (dye) in dilute suspensions of swimming Escherichia coli in experiments using a two-dimensional (2D) time-periodic flow and in a simple simulation. Results show that the presence of bacteria hinders large-scale transport and reduces overall mixing rate. Stretching fields, calculated from experimentally measured velocity fields, show that bacterial activity attenuates fluid stretching and lowers flow chaoticity. Simulations suggest that this attenuation may be attributed to a transient accumulation of bacteria along regions of high stretching. Spatial power spectra and correlation functions of dye-concentration fields show that the transport of scalar variance across scales is also hindered by bacterial activity, resulting in an increase in average size and lifetime of structures. On the other hand, at small scales, activity seems to enhance local mixing. One piece of evidence is that the probability distribution of the spatial concentration gradients is nearly symmetric with a vanishing skewness. Overall, our results show that the coupling between activity and flow can lead to nontrivial effects on mixing and transport.

Formation and Stability of Thin Condensing Films on Structured Amphiphilic Surfaces.

We present a microamphiphilic surface to promote the formation of a thin, stable liquid film during condensation. The surface consists of a hydrophilic micropillar array with hydrophobic pillar tips and was made using photolithography, deep reactive ion etching, and liftoff. The hydrophobic tips prevent the liquid film from growing thick, thereby keeping the thermal resistance low without the cyclical growth and shedding process of dropwise condensation. The wetting behavior was modeled analytically, and the parameters required for film formation were determined and verified with ESEM experiments. When a surface filled with condensate and lacked a low-pressure zone for the water to leave, a rupture event occurred, and a large Wenzel droplet emerged to flood the surface irreversibly. A number of strategies were found to combat rupture events. Tilting the surface vertically and dipping in a liquid pool gave the condensate a low-pressure region and prevented rupture. Irreversible flooding can also be avoided by ensuring that the emerged droplet was a nonwetting, highly mobile Cassie droplet. Parameters for Cassie-stable amphiphilic surfaces were determined analytically, but the high aspect ratios required prevented the manufacture of these surfaces for this study. Instead a hierarchical design was presented that demonstrated emerged Cassie droplets without challenging the manufacturing limits of the microfabrication procedure. This design avoided Wenzel droplet flooding without the need for a designated low-pressure zone. Additionally, sites for Cassie emergence could be engineered by removing a single pillar from the array at a designated location.

Dewetting from Amphiphilic Minichannel Surfaces during Condensation.

Condensation heat transfer can be altered significantly by changing the texture and material of a surface to promote droplet removal and therefore lower thermal resistance. These designs are often expensive and fragile, however, and are fabricated using micro- or nanoscale features that are not easily implemented in real-world systems. Here, we present a novel macromachined amphiphilic surface that promotes droplet removal and resists permanent flooding via a spontaneous dewetting transition. While much of the research in condensation involves condensing on surfaces that are fully or mostly hydrophobic, droplets on the surface presented here nucleate and grow inside the structure on a hydrophilic material. The absence of any coating between the liquid and the conductive surface has the benefits of both decreasing thermal resistance and enhancing nucleation density. When the liquid grows to a critical size inside the channel, its elongated shape becomes unstable and spontaneously dewets to form rounded droplets on the hydrophobic fin peaks. The removal of liquid from the channels promotes new growth on the bare hydrophilic material, while the emerged rounded droplets can more easily shed from the hydrophobic fins. The dewetting phenomenon is shown experimentally and characterized analytically such that a desired critical water slug length could be designed by changing geometric parameters of the surface structure. The macroscale machined surface is also more durable than typical nanofabricated surfaces and easier to manufacture, making the surface more applicable to use in real-world systems. Spontaneous condensate dewetting on the amphiphilic structure is expected to enhance the study of inhibiting flooding on condensing surfaces and provide new pathways for droplet shedding techniques without a requirement for nanothin hydrophobic coatings.