Effects of substrate heating and particle size variation on the contact line pinning-depinning dynamics of evaporating sessile droplets containing mono-/bidispersed colloidal particles

We investigate the contact line (CL) pinning and depinning dynamics of evaporating sessile water droplets containing mono-/bidispersed colloidal particles on nonheated/heated hydrophobic polydimethylsiloxane substrate. Transient variation of droplet shapes and the liquid-gas interfacial temperatures are acquired using a high-speed camera and an infrared-thermography, respectively. On the nonheated substrates, it is seen that for droplets with mono- and bidispersed colloidal particles, there exists a notable difference in the CL pinning durations leading to a variation in the particle deposition patterns. A first-order model is illustrated based on the various forces acting on the particles near the CL. For the bidispersed cases, a variation in the inter-particle contact force in the presence of particles of different sizes is unveiled. Further, for the heated substrate cases, a temperature difference exists between the droplet's apex and the CL. Moreover, a stagnation zone develops near the CL region due to the combined effects of outward-driven capillary flow and circulatory thermal Marangoni flow. Consequently, the complete pinning of the CL with a ring-like deposit is manifested for the bidispersed case as opposed to an inner deposit for the monodispersed case.