Convective flow in coalesced droplets under temperature difference for uniform distribution of microspheres in vertical contact-separation

Abstract

Convective flow in coalesced droplets under temperature difference for uniform distribution of microspheres in vertical contact-separation Shinji Bono*a,b,c, Shintaro Okaid, Mary-Johanna Weir Weisse, Boris Stoebere,f, Satoshi Konishi*a,b,c,d We brought a water droplet and a droplet containing microspheres (MSs) into vertical contact and then investigated the transport of the MSs in the coalesced droplets under a temperature difference. We used optical coherence tomography to obtain the trajectories of the MSs in the coalesced droplets. We also observed a convective flow in the coalesced droplets after increasing the temperature of the lower substrate. Using wetting pattern substrates, we can separate the coalesced droplet under this temperature difference into two droplets and assessed the distribution of the MSs in the top and bottom droplets. The downward flow at the center of coalesced droplets and the upward flow at the liquid-air interface under a temperature difference suggests that the convective flow is driven by the Marangoni effect. The Marangoni convective flow in the coalesced droplet resulted in equally distributed MSs in the two droplets after separation.