DRYING DYNAMICS OF SESSILE DROPLETS ON SUBSTRATES OF DIFFERENT WETTABILITIES
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The interaction between a liquid and a solid substrate, as encountered in any wetting phenomenon, is relevant to a wide range of industrial and biological applications. The drying mechanism was first explored by Deegan and his co-workers through the “coffee-ring” effect.1,5,6 In the current study, the objectives are to understand the detailed evaporation mechanisms on surfaces of various hydrophobicities and how surface chemical heterogeneity, together with surface physical roughness can dictate the overall deposit patterns of sessile droplets. The dynamics of the three-phase contact line of sessile droplets containing poly(vinyl alcohol) adsorbed gold nanoparticles is experimentally investigated on three silica substrates – silicon wafer, quartz, and glass – with different surface roughness and chemistry. Substrate and particle surface chemistry modulate surface-particle and particle-particle interactions and affect how particles pack relative to their nearest neighbors. In addition, it was found that substrate surface chemistry and roughness play significant roles in controlling the extent of contact line pinning and affecting particle distribution in a deposit. Our hypothesis that the total intrinsic energy barrier for contact line pinning consisting of three components, U_pr, U_ch and U_(p-s), created by substrate physical roughness, chemical heterogeneity and particle-substrate interactions, respectively, has been tested successfully based on experimental observations. This extension of preexisting theory can be used as a guide to analyze more complicated systems and predict the behavior of an evaporating droplet on a solid substrate.