Evaporation and deposition of inclined colloidal droplets

Abstract

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.

Geometrically-controlled evaporation-driven deposition of conductive carbon nanotube patterns on inclined surfaces

“…Another determining factor is the gravitational sedimentation of the particles. Assuming a creeping flow, 58 the sedimentation velocity can be calculated by the well-known Stokes law:where ρ p and ρ f are the densities of the colloidal particles and fluid, respectively, D is the particle diameter, μ is the fluid viscosity, and ϕ is the substrate inclination angle. However, sedimentation velocity should be comparable to the capillary velocity to contribute to the deposition of CNTs.…”

“…Although due to the lower contact angle, the local evaporation rate is higher at the top contact line compared with the bottom one, and the sedimentation velocity will compete with the capillary flow and decrease the effective velocity toward the top contact line, resulting in a thinner deposited line. 57,58…”

“…Another determining factor is the gravitational sedimentation of the particles. Assuming a creeping flow, 58 the sedimentation velocity can be calculated by the well-known Stokes law:…”

Ultrafast Self-Assembly of Colloidal Photonic Crystals during Low-Pressure-Assisted Evaporation of Droplets

“…The Marangoni recirculating flow has complex influences on the deposition pattern . Besides, the deposition pattern can be affected by the Derjaguin–Landau–Verwey–Overbeek (DLVO) attraction between the particles and the substrate, the sedimentation effect of gravity, , and the interfacial capture effect . Therefore, the comprehensive effects of these complex convections and dynamic processes determine the morphology of the final deposition patterns.…”

“…20 Besides, the deposition pattern can be affected by the Derjaguin−Landau−Verwey−Overbeek (DLVO) attraction between the particles and the substrate, 21 the sedimentation effect of gravity, 22,23 and the interfacial capture effect. 24 Therefore, the comprehensive effects of these complex convections and dynamic processes determine the morphology of the final deposition patterns. Although a lot of work has been conducted to suppress the coffee ring and obtain uniform patterns by regulating the Marangoni effect, 25 wettability of substrate, 26 acoustic fields, 27 and particle shapes, 28 the disordered microstructures and a large number of defects are inevitable for these patterns.…”