Lossless Fast Drop Self‐Transport on Anisotropic Omniphobic Surfaces: Origin and Elimination of Microscopic Liquid Residue

Abstract

Surfaces enabling directional drop self‐transport have exceptional applications in digital microfluidics, chemical analysis, bioassay, and microreactor technology. While such properties have been obtained by engineering a surface with anisotropic microstructures, a microscopic liquid residue—though it might be invisible macroscopically—is generally left behind the transported drop, resulting in undesired transport loss and severely limiting practical applications of the surface. Here, the origin of microscopic liquid residue is studied by investigating directional drop self‐transport on anisotropic surfaces made of radially arranged omniphobic microstripes. It is revealed that the occurrence of a liquid residue is governed by a transport‐velocity‐dependent dynamic wetting mechanism involving the formation of entrained thin liquid films at high capillary numbers while the local dynamic receding contact angle vanishes. Rayleigh‐like breakup of the liquid films leads to the microscopic liquid residue. It is further shown that a liquid‐like coating featuring highly flexible molecular chains can effectively suppress the formation of entrained liquid films at high transport velocities, thereby facilitating lossless and fast drop self‐transport on anisotropic omniphobic surfaces.

Gradient Quasi‐Liquid Surface Enabled Self‐Propulsion of Highly Wetting Liquids

“…[24] The alkylsilane, featuring a much less mobile CCC skeleton (bond angle = 109 o ) than that of SiOSi, can be considered as a conventional rigid layer when grafted on the surface (Figure 1D,E). [21,36] In contrary to GQLS, GRS shows much higher CAHs to highly wetting liquids (Figure 1F and Figure S4, Supporting Information). For example, the CAH of Krytox101, FC40, and FC72 presented on GRS is 2.7 o , 4.5 o , and 0.8 o , while that on GQLS is 0.5 o , 0.4 o , and 0.2 o , respectively (Tables S1 and S2, Supporting Information).…”

Anisotropy-induced directional self-transportation of low surface tension liquids: a review

“…In 2019, Huang et al 77 carried out an extended investigation on the formation mechanism of microscopic liquid residues on similar re-entrant proles. They revealed that the liquid residue is governed by a dynamic wetting process rather than simply the contact angle hysteresis of the surface.…”

“…8c). 77 The analyte droplets spontaneously moved toward the center where the two droplets merged, and a microscope above the setup was used to monitor the reaction.…”

Covalently attached liquids as protective coatings

“…Droplet movement was studied on microstripes constructed on silicon wafer substrates. 51 The patterned surfaces were grafted with liquid-like perfluoropolyether brushes to eliminate microscopic liquid residue that is typically left by moving drops. The resulting behavior was termed 'lossless fast drop transport'.…”