Topological heterogeneity and evaporation dynamics of irregular water droplets

Water droplets sitting between wires are ubiquitous in nature and industry, often showing irregular (non-spherical) droplet shapes. To understand their topological singularity and evaporation mechanism, measuring volume changes of irregular water droplets is essential but highly challenging for small-volume water droplets. Here we experimentally explore topological heterogeneity and evaporation dynamics for irregular water droplets between wires with four-dimensional X-ray microtomography that directly provides images in three spatial dimensions as a function of time, enabling us to get three-dimensional structural and geometric information changes with time. We find that the topological heterogeneity of an irregular droplet is due to the local contact lines and the evaporation dynamics of an irregular droplet is governed by the effective contact radius. This study may offer an opportunity to understand how the topological heterogeneity contributes to the evaporation dynamics of irregular water droplets.

Evaporation and deposition of inclined colloidal 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.