Marangoni flow and surfactant transport in evaporating sessile droplets: A lattice Boltzmann study

Abstract

The circulatory Marangoni flow can alter the contact line deposition in evaporating colloidal droplets with pinned contact line. Marangoni flow can be induced by surfactants or thermal effects. Although both cases have been exclusively investigated, the combined effect of surfactant-induced and thermal Marangoni flows is still unknown. The lattice Boltzmann method is utilized to simulate droplet evaporation and corresponding Marangoni flow. Five equations for hydrodynamics, interface capturing, vapor concentration, temperature field, and surfactant transport are intrinsically coupled with each other. They are simultaneously solved in the lattice Boltzmann framework. A geometrical method is proposed to pin the contact line at the triple point. First, evaporation-induced and thermal Marangoni flows are successfully captured. By incorporating surfactant-induced effects, interesting flow patterns are observed. Considering the combined effect of surfactant and temperature gradient, maximum surfactant concentration and maximum temperature (local minima for surface tension) are found at the top and the edge of the droplet, respectively. The maximum surface tension is consequently located between them, and double-circulation flow is observed. If the thermal effect is eliminated, surfactant local concentrations intermittently converge to steady values so that the edge concentration becomes higher than the apex concentration. Until reaching the steady state, there are two patterns that the flow alternates between: one in the direction of the thermal Marangoni flow and the other in the opposite direction.

Numerical Simulation of Solvent Evaporation in a Reactive Silver Ink Droplet Deposited on a Heated Substrate

“…The constant contact angle and unpinned contact line in the simulations are not critical in replicating natural evaporation because the evaporation of water−ethylene glycol mixtures does not change the geometric relationship between the Marangoni and capillary flows. 45 In natural evaporation, the contact angle is less than 90°, gradually decreasing due to the pinned contact line (Figure 10b). The Marangoni vortices change slightly with the fluid−air interface.…”

Effect of surfactant concentration on the evaporation-driven deposition of carbon nanotubes: from coffee-ring effect to strain sensing

“…This is the part that makes the physics of the problem even more complicated. 41 Although adding more surfactant to the CNT colloids before doing all the process of preparation will be helpful for their stability, if the surfactant is added excessively, the remaining free surfactants that are not adsorbed to the CNTs will cause a strong Marangoni flow. This inward Marangoni flow will impede the outward evaporation-induced flow, Marangoni eddies will be created, suppressing the coffee-ring effect; the effect that is desired in our study.…”

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

“…23,28 On the subject of the contact line deposition, it is known that the Marangoni effect can alter the flow inside the droplet 29 and reverse coffee-ring depositions. 30 While thermal Marangoni flow in pure water droplets at room temperature is not well understood yet, 31,32 the presence of surfactants can have a strong impact on the flow and contact line deposition. [33][34][35] However, the physics of the problem is very complicated in such a way that different types of surfactants can have different effects on the interfacial flow of evaporating sessile droplets.…”