Flow dynamics and size-based sorting of bidispersed colloidal particles in evaporating sessile water droplets: substrate heating and wettability effects

The present work investigates the size based self-sorting behavior of particles near the contact line (CL) region of an evaporating sessile aqueous droplet containing bidispersed (1 and 3 μm) colloidal particles. Experiments were performed on hydrophilic glass and hydrophobic (Poly)dimethylsiloxane (PDMS) substrates maintained at different temperatures ( T s = 23°C, 50°C, and 80°C). The role of substrate wettability and substrate temperature in the modulation of final deposition specifically, the self-sorting characteristics was examined. Various experimental techniques namely, high-speed visualization, 3D optical profilometry, fluorescence microscopy, and scanning electron microscopy were implemented to analyze the droplet evaporation and the morphology of the final deposition. As per the findings, in the case of hydrophilic substrate self-sorting existed at all temperatures. However, an appreciable improvement in the sorting quality was observed only at elevated temperatures. Particularly, for the case of T s = 80°C in which the final deposition demonstrated better sorting of 1 and 3 μm particles. Since, most of the region in the CL’s vicinity had a substantial reduction in the intermixing of 1 and 3 μm particles, thus effectively favouring the suppression of mixed region. In the case of nonheated ( T s = 23°C) hydrophobic substrate it was observed that a continuous depinning of the CL, in conjunction, with a weak radial flow led to complete intermixing of the bidispersed particles thus, lacking any self-sorting behavior. However, for the heated hydrophobic substrate cases, a substantial delay in the depinning of the CL existed. This, in turn, combined with the outward-driven capillary flow and recirculatory thermal-Marangoni flow resulted in the manifestation of self-sorting phenomenon. This study is expected to offer novel fundamental insights into the underlying mechanisms that can be effectively leveraged, to gain better control of the governing parameters for the realization of desired sorting characteristics.