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Droplet evaporation on porous materials is a complex dynamic that occurs with spontaneous liquid imbibition through pores by capillary action. Here, we explore water dynamics on a porous fabric substrate with in-situ observations of X-ray and optical imaging techniques. We show how spreading and wicking lead to water imbibition through a porous substrate, enhancing the wetted surface area and consequently promoting evaporation. These sequential dynamics offer a framework to understand the alterations in the evaporation due to porosity for the particular case of fabric materials and a clue of how face masks interact with respiratory droplets.

Colloidal droplets on flat solid substrates commonly leave symmetric ring-like deposits due to coffee-ring flows during evaporation. On inclined substrates, droplet shapes may become asymmetric by gravity. On this basis, it is not clear how their evaporation dynamics and final deposits are changed depending on inclination. Here we explore evaporation and deposition dynamics of colloidal droplets on inclined substrates, mainly by controlling colloidal particle size, substrate inclination, and relative humidity, which are crucial to gravitational intervention and evaporation dynamics. We experimentally investigate two different flows with opposite directions: downward sedimentation flows by gravity ($$v_s$$ v s ) and upward capillary flows by evaporation ($$v_c$$ v c ). We find that the competition of two flows determines the formation of final deposits with a flow speed ratio of $$\alpha = v_s/v_c$$ α = v s / v c . Notably, for $$\alpha$$ α $$\ll$$ ≪ 1, evaporation-driven upward flows overwhelm sedimentation-driven downward flows, resulting in accentuated particle movement towards the top ring, which seems to defy gravitational intervention. We suggest a possible explanation for the flow speed dependence of final deposits in evaporating colloidal droplets. This study offers a framework to understand the intervention of inclination to the formation of final deposits and how to overcome the deposit pattern radial asymmetry, achieving symmetric deposit widths from inclined colloidal droplets.

The characteristics of several patterns left after the evaporation of a particle-laden liquid droplet are investigated by using a coarse-grained lattice model. The model includes both evaporative convection and the Brownian motion of weakly interacting particles. The model is implemented by using a Monte Carlo method to investigate the different deposit patterns near the contact line. It was found that different deposit patterns form depending on the interplay between the convective transport and the deposition of interacting particles. The patterns were analyzed by varying the ratio of the convective forces to the interaction forces as well as the size and the number of particles. It was also found that the ring-like patterns are formed when the convective potential dominates the interactions of particles, whereas either wave-like or island-like patterns form in the opposite case. Finally, the average thickness of the wave-like patterns is mainly determined by evaporation rates.