We report on the self-propulsion of boiling droplets which, despite their contact with viscous, immiscible oil films, attain high velocities comparable to those of levitating Leidenfrost droplets. Experiments and model reveal that droplet propulsion originates from a coupling between seemingly disparate short and long timescale phenomena due to microsecond fluctuations induced by boiling events at the droplet-oil interface. This interplay of phenomena leads to continuous asymmetric vapor release and momentum transfer for high droplet velocities.
We have studied the limits of supersaturation in binary alkane mixtures and recombined crude oils at elevated pressures. Our study has interrogated these fluids using an optical cell with an internal volume of approximately 1 μL that includes a thermally pulsed wire which nucleates bubbles to "break" supersaturation, thereby acting as a reference. In parallel, we have made measurements in a larger and more conventional optical cell with an internal volume closer to 1 mL absent the aid of a nucleation device. For both, we demonstrate a maximum supersaturation of approximately 800 psi for conventional black oils. In contrast, supersaturation is demonstrated to be minimal for volatile oils. We show that the higher surface tension of conventional black oils favors surface-mediated nucleation, while the lower surface tension of volatile oils gives rise to nucleation that primarily takes place in the bulk. Among the surfaces studied, optically smooth high-pressure windows that permit observation of the nucleation process show the lowest tendency to nucleate bubbles.
Controlling cell adhesion to surfaces is an important, but difficult, problem. Current methods to control adhesion rely on surface functionalization, which have limited material choice to avoid cell toxicity and are typically cell specific. Herein, cell adhesion is modulated by using nanometric high‐k dielectric films. Voltage is applied across the dielectric film, changing the film surface's zeta potential, ζ. High performance dielectrics, HfO2 and SiO2, enables a change in the ζ polarity and magnitude over large, 100 mV, ranges by applying ≈1 V across the dielectrics with ≈1nW power draw. Freshwater Chlorella vulgaris and saltwater Nannochloropsis oculata, which have a negative ζ, are used as model cells. Cell adhesion is observed to be inhibited when both surface and cell ζ are negative and enhanced when surface ζ is positive and cell ζ are negative using microfluidic experiments. Finally, millimetric scale cell patterning is demonstrated by spatially modulating ζ with no observed toxicity to cells over 4 weeks.
In article number 2300732, Kripa K. Varanasi and co-workers, developed a voltage-controlled electrostatic approach for modulating the bio-adhesion of cells to surfaces. Surface zeta potential is controlled by the application of voltage across nanometric high-K dielectric film. By reducing algae cell adhesion and biofouling, the efficiency of photobioreactors for CO 2 capture is enhanced. This approach can be broadly applied to cell culture, medical implants, tissue engineering and bioreactors.
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