Bridging-droplet transfer from solid to porous surfaces

Abstract

When the top of a sessile droplet is contacted by an opposing solid surface, the droplet can transfer depending on the wettabilities and relative velocity of the surfaces. What if the surface receiving the liquid was porous? High-speed imaging was used to capture the transfer of a droplet from a solid substrate to an opposing porous surface. The parameters that were varied include the wettability of the donor substrate, the pore size of the receiving surface and the droplet's volume and working fluid. Generally, the transfer process is split into two sequential regimes, wetting and wicking, with wicking being three orders of magnitude longer than wetting on average. The wetting regime is split into two sub-regimes, the donor-independent and donor-dependent regimes. The donor-independent regime follows the dynamics of droplet coalescence, starting in a mass-limited viscous regime followed by a capillary–inertial regime. The donor-dependent regime is driven by a global change in Laplace pressure across the liquid bridge, with the viscous wedge of the receding contact line being the rate-limiting factor. The wicking regime is governed by Darcy's law, completing the transfer process of the droplet.

Teflon AF–Coated Nanotextured Aluminum Surfaces for Jumping Droplet Thermal Rectification

“…We performed this comparative study because droplet jumping only occurs on SHPB surfaces, while droplet bridging has been demonstrated using HPB or SHPB surfaces. [ 7,64 ] Our measurements at large fill fractions of ϕ = 20% for H = 1.2 and 2.4 mm shown in Figure S3 (Supporting Information) found much lower G fwd and η for the HPB surfaces as compared to SHPB surfaces. These results indicate that droplet bridging and/or falling from a HPB surface are not effective condensate return mechanisms for the H and ϕ tested here.…”

“…Alternatively, liquid bridge confined boiling has been shown to handle larger heat fluxes up to 100 W cm −2 in wettability contrast-based vapor chambers and could offer a higher capacity if SHPB surfaces were rationally designed with local pinning sites. [64] To obtain h pc , the heat flow through the vapor space due to phasechange heat transfer was estimated as Q − Q o . Q o represents heat flow through the device due to parasitic conduction and radiation across the test section of the device and was obtained from a linear fit to forwardmode Q versus ΔT measurements of an uncharged JDTD (ϕ = 0%) that was unable to induce liquid-vapor phase change (see Figure S5, Supporting Information).…”

“…Alternatively, liquid bridge confined boiling has been shown to handle larger heat fluxes up to 100 W cm −2 in wettability contrast‐based vapor chambers and could offer a higher capacity if SHPB surfaces were rationally designed with local pinning sites. [ 64 ]…”

Jumping droplets

“…When the gap between plates is sufficiently small (typically $${H}_{{\rm{v}}}\sim 10\mbox{--}100$$ μm), dropwise condensate simply bridges across the gap to return to the wick upon reaching a critical size that scales with the gap height (Figure 10c). 101 In addition to being durable, this approach is attractive because the engineered gap height directly defines any desired value of $${D}_{{\rm{c}}}$$, in contrast to dropwise or jumping‐drop condensation where $${D}_{{\rm{c}}}$$ is fixed to a single value for a given surface. As a result, the heat transfer can be tuned.…”