Controlling particle deposit morphologies in drying nano-particle laden sessile droplets using substrate oscillations.

Sessile water droplets containing nano-silica particles are allowed to evaporate in the presence of driven substrate oscillations at chosen frequencies. Different mode shapes are observed at different oscillation frequencies. As reference, the evaporation of the same droplets is also observed under stationary conditions i.e. in the absence of any oscillations. For all cases, the deposit structures formed by the agglomeration of the nano-silica particles have been imaged. It has been observed that for the stationary droplets and for droplets whose oscillations are initiated close to the resonance of the lowest allowable oscillation mode, the structures are similar having larger spread over height, while for higher frequencies the structures are dome-like with more uniform outer dimensions. The possible reasons behind these structures are investigated using experimental techniques such as high-speed imaging of droplet oscillations, internal flow visualization and SEM imaging. Understanding of the underlying mechanisms behind the formation of these striking features is required for these methods to be applicable in larger scale drying operations or micro-device applications. Altogether a novel methodology has been presented and investigated for manipulating the morphological features in evaporating nano-particle laden sessile droplets.