Effects of surface wettability and liquid viscosity on the dynamic wetting of individual drops

Abstract

In this paper, we experimentally investigated the dynamic spreading of liquid drops on solid surfaces. Drop of glycerol water mixtures and pure water that have comparable surface tensions (62.3-72.8 mN/m) but different viscosities (1.0-60.1 cP) were used. The size of the drops was 0.5-1.2 mm. Solid surfaces with different lyophilic and lyophobic coatings (equilibrium contact angle θ(eq) of 0°-112°) were used to study the effect of surface wettability. We show that surface wettability and liquid viscosity influence wetting dynamics and affect either the coefficient or the exponent of the power law that describes the growth of the wetting radius. In the early inertial wetting regime, the coefficient of the wetting power law increases with surface wettability but decreases with liquid viscosity. In contrast, the exponent of the power law does only depend on surface wettability as also reported in literature. It was further found that surface wettability does not affect the duration of inertial wetting, whereas the viscosity of the liquid does. For low viscosity liquids, the duration of inertial wetting corresponds to the time of capillary wave propagation, which can be determined by Lamb's drop oscillation model for inviscid liquids. For relatively high viscosity liquids, the inertial wetting time increases with liquid viscosity, which may due to the viscous damping of the surface capillary waves. Furthermore, we observed a viscous wetting regime only on surfaces with an equilibrium contact angle θ(eq) smaller than a critical angle θ(c) depending on viscosity. A scaling analysis based on Navier-Stokes equations is presented at the end, and the predicted θ(c) matches with experimental observations without any additional fitting parameters.

Kinetics of spreading of synergetic surfactant mixtures in the case of partial wetting

“…In agreement with [2,7] It should be emphasized that the spreading exponent at t>1 s according to [19] was smaller than 0.1, that is surfactant solutions in this case spread slower than pure liquids. This small value have been ascribed in [19] to adsorption of surfactant onto solid/liquid and solid/air interface.…”

“…As usually a static advancing contact angle is referred to below as "equilibrium contact angle". Viscous stage of wetting was observed only for liquids with small contact angles with threshold value of equilibrium contact angle increasing with an increase of the liquid viscosity [2]. According to [2] for the mixtures of water and glycerol with viscosity below 35 mPa·s only inertial stage of spreading was observed on substrates with contact angle 63 O .…”

“…Viscous stage of wetting was observed only for liquids with small contact angles with threshold value of equilibrium contact angle increasing with an increase of the liquid viscosity [2]. According to [2] for the mixtures of water and glycerol with viscosity below 35 mPa·s only inertial stage of spreading was observed on substrates with contact angle 63 O . The spreading in those cases was completed within approximately 10 ms. For the mixtures with viscosity of 35 mPa·s and above the viscous stage was observed on the same substrate.…”

“…Spreading kinetics in the case of complete wetting consists of two stages. During the first, inertial stage, radius of spread drop increases proportionally to the square root of time, ~√ [1,2]. The duration of this stage is in the range of millisecond, with characteristic timescale given by (ρR 3 /γ) 1/2 .…”

Mixtures of catanionic surfactants can be superspreaders: Comparison with trisiloxane superspreader

“…8. Note, the inertial or viscous stages of spreading, which develop on shorter time scale [24,25] were not considered: only kinetics of spreading on the time scale from hundred milliseconds to seconds was investigated, i.e. during the superspreading stage [24].…”