Nonisothermal Spreading Dynamics of Self-Rewetting Droplets

Abstract

We experimentally studied the spreading dynamics of binary alcohol mixtures (and pure liquids for reference) deposited on a heated substrate in a partially wetting situation under nonisothermal conditions. We show that the spreading mechanism of an evaporating droplet exhibits a power-law growth with early-stage exponents that depend strongly and nonmonotonically on the substrate temperature. Moreover, we investigated the temporal and spatial thermal dynamics in the droplet using infrared thermography, revealing the existence of unique thermal patterns due to thermal and/or solutal instabilities, which lead to surface tension gradients, namely the Marangoni effect. Our key findings are that the temperature of the substrate drastically affects the early-stage inertial-capillary spreading regime owing to the nonmonotonic surface tension-temperature dependence of the self-rewetting liquids. At later stages of wetting, the spreading dynamics enters the viscous-capillary dominated regime, with the characteristic low kinetics mirroring the behavior of pure liquids.

Spreading and retraction dynamics of sessile evaporating droplets comprising volatile binary mixtures

“…Mamalis et al. (2018) also saw an increase in the spreading exponents with substrate temperature in their experiments with self-rewetting droplets. Additionally, when the temperature is increased, the number of fingers produced at the contact line (see figure 7 and § 4.3 for a detailed discussion of this instability) also increases, with approximately 18 seen at C, 20 at C and 21–24 seen at C.…”

“…This is likely due to the more rapid development of a concentration gradient when the droplet touches the substrate as ethanol evaporates more vigorously at the higher temperatures. Mamalis et al (2018) also saw an increase in the spreading exponents with substrate temperature in their experiments with self-rewetting droplets. Additionally, when the temperature is increased, the number of fingers produced at the contact line (see figure 7 and § 4.3 for a detailed discussion of this instability) also increases, with approximately 18 seen at T w = 30 • C, 20 at T w = 50 • C and 21-24 seen at T w = 70 • C. The finger length, which we define as the distance from the apparent contact line of the bulk droplet to the apex of the extended finger, also increases with substrate temperature as a higher evaporation rate drives the instability.…”

“…Immediately after depositions, the droplets spread to their maximum radius. The very initial stages are dominated by inertial spreading, similar to pure and other binary mixture droplets (Winkels et al 2012;Mamalis, Koutsos & Sefiane 2018). Table 3 gives the spreading coefficients, n (where R ∝ t n ), for each linear regime and their corresponding breakpoints in time, b, to the next linear regime.…”

Interaction of two non-coalescing bubbles rising in a non-isothermal self-rewetting fluid

“…As we investigate the thermo-capillary driven flow of a millimetre size bubble, one can also easily achieve the value of Reynolds number (Re = 10) and the range of the Weber number considered in the present study. Recently, Mamalis et al [36] have measured the surface tension of solutions of water + 1-butanol 5 % volume and water + 1-pentanol 2 % volume and showed that surface tensions of these fluids exhibit parabolic dependencies with temperature (see Figure 4 of Ref. [36]).…”

Unsteady motion of a long bubble or droplet in a self-rewetting system

“…These studies show that the behaviour of a bubble or droplet in or of a self-rewetting fluid can be qualitatively different from that for an ordinary fluid. Specifically, the theoretical studies by Karapetsas et al [35] of the motion of a droplet on a heated substrate, and by Tripathi et al [36] of the rise of a bubble in a vertical channel, as well as the experimental studies by Shanahan and Sefiane [37] of the motion of a bubble in a mean flow, by Mamalis, Koutsos and Sefiane [38] of the motion of a droplet on a heated inclined substrate, by Mamalis, Koutsos and Sefiane [39] of the rise of a bubble in a vertical micro-channel, and by Mamalis, Koutsos and Sefiane [40] of the spreading of a droplet on a heated substrate, all demonstrate this. In the present work we seek to bring further insight into this problem by formulating and analysing a theoretical model for the unsteady motion of a long bubble or droplet in a self-rewetting system in a non-uniformly heated tube.…”