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Analyses of microchannel and microgap cooling show that galinstan, a recently developed nontoxic liquid metal that melts at À19 C, may be more effective than water for direct liquid cooling of electronics. The thermal conductivity of galinstan is nearly 28 times that of water. However, since the volumetric specific heat of galinstan is about half that of water and its viscosity is 2.5 times that of water, caloric, rather than convective, resistance is dominant. We analytically investigate the effect of using structured surfaces (SSs) to reduce the overall thermal resistance of galinstan-based microgap cooling in the laminar flow regime. Significantly, the high surface tension of galinstan, i.e., 7 times that of water, implies that it can be stable in the nonwetting Cassie state at the requisite pressure differences for driving flow through microgaps. The flow over the SS encounters a limited liquid-solid contact area and a low viscosity gas layer interposed between the channel walls and galinstan. Consequent reductions in friction factor result in decreased caloric resistance, but accompanying reductions in Nusselt number increase convective resistance. These are accounted for by expressions in the literature for apparent hydrodynamic and thermal slip. We develop a dimensionless expression to evaluate the tradeoff between the pressure stability of the liquid-solid-gas system and hydrodynamic slip. We also consider secondary effects including entrance effects and temperature dependence of thermophysical properties. Results show that the addition of SSs enhances heat transfer.

We semi-analytically capture the effects of evaporation and condensation at menisci on apparent thermal slip lengths for liquids suspended in the Cassie state on ridge-type structured surfaces using a conformal map and convolution. An isoflux boundary condition is prescribed at solid–liquid interfaces and a constant heat transfer coefficient or isothermal one at menisci. We assume that the gaps between ridges, where the vapor phase resides, are closed systems; therefore, the net rates of heat and mass transfer across menisci are zero. The reduction in apparent thermal slip length due to evaporation and condensation relative to the limiting case of an adiabatic meniscus as a function of solid fraction and interfacial heat transfer coefficient is quantified in a single plot. The semi-analytical solution method is verified by numerical simulation. Results suggest that interfacial evaporation and condensation need to be considered in the design of microchannels lined with structured surfaces for direct liquid cooling of electronics applications and a quantitative means to do so is elucidated. The result is a decrease in thermal resistance relative to the predictions of existing analyses which neglect them.

Vertically oriented graphene (VG) has attracted attention for years, but the growth mechanism is still not fully revealed. The electric field may play a role, but the direct evidence and exactly what role it plays remains unclear. Here, we conduct a systematic study and find that in plasma-enhanced chemical vapor deposition, the VG growth preferably occurs at spots where the local field is stronger, for example, at GaN nanowire tips. On almost round-shaped nanoparticles, instead of being perpendicular to the substrate, the VG grows along the field direction, that is, perpendicular to the particles' local surfaces. Even more convincingly, the sheath field is screened to different degrees, and a direct correlation between the field strength and the VG growth is observed. Numerical calculation suggests that during the growth, the field helps accumulate charges on graphene, which eventually changes the cohesive graphene layers into separate three-dimensional VG flakes. Furthermore, the field helps attract charged precursors to places sticking out from the substrate and makes them even sharper and turn into VG. Finally, we demonstrate that the VG-covered nanoparticles are benign to human blood leukocytes and could be considered for drug delivery. Our research may serve as a starting point for further vertical two-dimensional material growth mechanism studies.

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