Controlling salt deposition patterns using engineered substrates and thermal gradients.

Preferential salt deposition and spreading influence both natural processes and industrial applications. This study examines salt deposition patterns in evaporating sessile drops and within capillary menisci above a brine pool. Temperature- and concentration-dependent surface tension induces Marangoni flow, which transports high-concentration liquid to the cooler side of the droplet where nuclei form and grow. Substrates with mixed thermal diffusivities and externally imposed thermal gradients can alter salt deposition patterns by controlling the direction of Marangoni flow. When connected to a brine pool, salt deposition creates a salt coating that can spread on vertical substrates. Spreading begins when crystals remain confined against the substrate: small crystals nucleating higher in the transition zone of capillary menisci have a higher likelihood of staying in place due to larger capillary forces. Salt spreading on a vertical substrate can be halted by imposing a downward thermal gradient to reverse the Marangoni flow direction within the capillary menisci or by engineering the surface roughness to reduce wettability. Horizontally roughened surfaces impede salt spreading; vertically roughened surfaces see the formation of stable crystals within short induction times, followed by rapid spreading as corner flow facilitates solution transport along interconnected roughness and through the porous salt coating.