Droplet evaporation on two-tier hierarchical micro-pillar array surface.

• Evaporation kinetics of droplet on two-tier hierarchical micro-pillar array surfaces are systematically studied. • Droplet evaporation on two-tier surface is superior to single-tier competitor when secondary pillars are on primary pillar sides or on substrate parallel to primary. • Droplet evaporation is deteriorated when secondary pillars are on primary pillar top due to liquid layer wrapped around primary pillar by capillary wicking. • Evaporation on surface with secondary pillars on substrate parallel to primary is dominated by triple-phase contact line on secondary pillars. • Geometrical effects of primary pillar including pillar side length, spacing and height and mechanisms are analyzed. As surface modification becomes a promising approach to thermal management, quantifying effects of surface structure geometrical dimension is of great significance to optimize heat transfer performance. In this paper, we emphatically report droplet evaporation on three types of two-tier hierarchical micro-pillar array (THM) surfaces. Utilizing the lattice Boltzmann model, evaporation characteristics including droplet morphology, triple-phase contact line (TPCL) behavior, liquid–vapor interface evolution and heat flux distribution are elucidated. In comparison to single-tier micro-pillar array (SM) surfaces, THM surfaces enable remarkable enhancement in heat transfer when secondary pillars are configured on the side of the primary pillar or configured parallelly to the primary pillar on the substrate, which is attributed to the synergistic effect of TPCL evaporation and liquid–vapor interface evaporation. The varying trend of droplet lifetime with the primary pillar geometrical dimension on all THM surfaces are also summarized, giving a guideline for optimization of heat dissipation by tailoring structures. [ABSTRACT FROM AUTHOR]