Influence of equilibrium contact angle on spreading dynamics of a heated droplet on a horizontal plate

Abstract

In most of researches about the droplet spreading on a substrate, one adopts aprecursor layer to relieve the stress singularity near the contact line without considering wall properties, which, however, is inapplicable for studying the relationship of the wettability with wall temperature. In this paper, the spreading of a heated droplet on the solid substrate, under the action of the three-phase contact line, is simulated. The influences of the wall temperature on wettability and droplet spreading are examined from the viewpoint of equilibrium contact angle. The simulated results show that when the wall temperature is uniform, the evolution of droplet spreading is dominated only by the gravity, illustrating symmetrical spreading characteristics. When the temperature gradient is applied to the wall, the combination of thermocapillary force and gravity drives the droplet into spreading, therefore the main part of the droplet migrates toward the low temperature region due to the Marangoni effect. The left contact line continually moves toward the left side while the right contact line first moves toward the right side, then turns to the left side after the receding time. The spreading range of the droplet is changed notably because of different travelling speeds of the contact line on both sides. With the increase of the temperature gradient, the Marangoni effect is promoted, resulting in a faster migration toward the low temperature region. A thin film is formed between the contact line in the hotter region and the bulk of the droplet, where the gravity and thermocapillary force dominate the spreading successively. The present simulation shows that the surface wettability is not only dependent on its chemical composition and geometrical morphology, but also closely related to wall temperature. When the sensitivities of the liquid-solid, liquid-gas and solid-gas interfacial tensions to temperature are all identical, the equilibrium contact angle between the droplet and the wall keeps constant, leading to a uniform wettability on the wall. When the liquid-solid interfacial tension or the liquid-gas interfacial tension is more sensitive to temperature than the other two interfaces, the equilibrium contact angle increases and the wettability tends to be worse, presenting a more hydrophobic substrate, which decelerates the spreading of the droplet with the contact line moving to the colder region. As the solid-gas interfacial tension is more sensitive to temperature than the other two interfaces, the equilibrium contact angle tends to lessen, and the contact line feels a more hydrophilic substrate (the droplet wets perfectly when the equilibrium contact angle decreases to zero), hence the spreading is enhanced. The present results indicate that the equilibrium contact angle plays a key role in the evolution of a heated droplet on a horizontal plate. The simulation conclusions can provide a theoretical basis for relevant experimental findings, which promotes the understanding of the relationship between wall temperature and its wettability.