Gel-controlled droplet spreading

Abstract

Spreading and stationary droplets of a thermally responsive fluid on a heated surface are studied. The fluid undergoes a reversible gel formation at elevated temperature. The spatio-temporal pattern of gel formation within the droplet is examined using an experimental method based on spectral domain optical coherence tomography and time varying speckle patterns. Two stages of gel formation can be distinguished: first, a thin crust appears starting at the contact line. Second, a gel layer appears above the heated plate and then expands upward. We attribute the first stage of gel formation to solvent evaporation and heating through the air and the second to thermal conduction through the fluid from the base. Gel formation at the contact line is likely responsible for the arrest of spreading droplets, but was not detectable with our experimental protocol at the time of contact line arrest, suggesting that this arose over a microscopic length scale. Overall, substrate heating provides an effective way to control the final shape of droplets of thermo-responsive fluids.

Drop spreading and gelation of thermoresponsive polymers

“…For both mechanisms discussed above the solidification is assumed to be initiated at the contact line. This is recently confirmed explicitly for slowly spreading Pluronic F127 [9]. In these experiments the temporal evolution of the gelation front inside the drop was resolved, displaying several interesting features such as the formation of a thin crust over the free surface.…”

“…The rate of evaporation even diverges for contact angles below 90 • [21][22][23][24]. The role of solvent evaporation for gel formation in Pluronic was also invoked in [9]. For all these reasons we experimentally verified the possible influence of %RH on the spreading dynamics and final radius.…”

“…Therefore, we opted for solutions of Pluronic F127, which is a thermoresponsive co-polymer that undergoes a cross-linking gelation transition above a given temperature (without latent heat). The same system was recently used to study drop impact [8] and the gelation process during spreading [9]. Here we focus on the detailed spreading dynamics, the parameters that control the gelation-induced contact line arrest and the role of relative humidity.…”

Viscoplastic water entry

“…A few studies have previously investigated the impact and spreading of viscoplastic droplets on a solid surface. It is shown that yield stress can prohibit the recoil of the droplet, retard the deformation after the drop has reached the maximum shape, and prevent splashing (German & Bertola 2009;Luu & Forterre 2009;Jalaal & Stoeber 2014;Blackwell et al 2015;Jalaal, Balmforth & Stoeber 2015;Jalaal 2016;Jalaal et al 2018).…”

Static and Dynamic Optical Analysis of Micro Wrinkle Formation on a Liquid Surface

“…OCT imaging techniques have also been increasingly developed and applied across a broad a range of different disciplines, for example, the non-invasive analysis of paintings for art history and conservation [24], and profiling surface membranes for reconfigurable optics applications [25]. OCT developments and applications which involve imaging liquids and liquid surfaces have included micro-rheology studies of simple and complex fluids [26], analysing capillary waves in biological fluids [27,28], droplet imaging [29] and interface imaging to study surface wettability [30], gelation [31], and evaporation [32].…”