Evaporation of particle-laden drops: The role of particle formation in situ

The evaporation characteristics of drops are relevant in many engineering applications including chemical deposition, cooling, spray, and combustion. The addition of particles and surfactants in liquid drops can change the evaporation rate where often the rate is found to be a decreasing function of particle concentration. Here, we place a highly concentrated solution of heptane with sorbitan monooleate (Span 80 at 5.0 wt.%) in contact with a mixture of sodium silicate solution and ammonium bicarbonate (the reagents). Consequently, an emulsion phase is spontaneously formed in heptane where the reagent-filled micelles (emulsion drops) undergo a sol-gel reaction, hence, forming silica particles in situ. We examine the evaporation dynamics of drops where silica particles are formed in situ and study the extent to which the effects are analogous to those of drops with ex situ dispersed silica particles. We monitor the shape of pendant drops of heptane laden with emulsions resulting in the history reduction of the droplet's radius. The drop lifetime depends on the age of the sample and is considerably altered with the origination of colloidal size particles from the emulsion templates.