Effects of Substrate Heating and Wettability on Evaporation Dynamics and Deposition Patterns for a Sessile Water Droplet Containing Colloidal Particles

摘要

Effects of substrate temperature, substrate wettability and particlesconcentration are experimentally investigated for evaporation of a sessilewater droplet containing colloidal particles. Time-varying droplet shapes andtemperature of the liquid-gas interface are measured using high-speedvisualization and infrared thermography, respectively. The motion of theparticles inside the evaporating droplet is qualitatively visualized by anoptical microscope and profile of final particle deposit is measured by anoptical profilometer. On a non-heated hydrophilic substrate, a ring-likedeposit forms after the evaporation, as reported extensively in the literature;while on a heated hydrophilic substrate, a thinner ring with an inner depositis reported in the present work. The latter is attributed to Marangoniconvection and recorded motion of the particles as well as measured temperaturegradient across the liquid-gas interface confirms this hypothesis. The thinningof the ring scales with the substrate temperature and is reasoned to strongerMarangoni convection at larger substrate temperature. In case of a non-heatedhydrophobic substrate, an inner deposit forms due to very early depinning ofthe contact line. On the other hand, in case of a heated hydrophobic substrate,the substrate heating as well as larger particle concentration helps in thepinning of the contact line, which results in a thin ring with an innerdeposit. We propose a regime map for predicting three types of deposits namely,ring, thin ring with inner deposit and inner deposit - for varying substratetemperature, substrate wettability and particles concentration. A first-ordermodel corroborates the liquid-gas interface temperature measurements andvariation in the measured ring profile with the substrate temperature.