Rebound Suppression of a Droplet Impacting on a Supersolvophobic Surface by a Small Amount of Polymer Additives

Abstract

A small amount of polymer dissolved in a droplet suppresses droplet rebound when it impinges on a supersolvophobic surface. This work investigates impacting dynamics of a droplet of dilute polymer solution depending on the molecular weight and the concentration of the polymer by using multibody dissipative particle dynamics simulations. Either the long polymer or the high polymer concentration suppresses rebound of a droplet although its shear viscosity and the liquid–vapor surface tension are not different from those of a pure solvent droplet. We found a new mechanism of the anti-rebound in which the resistance is applied against the hopping motion, while behavior of the non-rebounding droplet at the earlier spreading and retraction stages is the same as for the rebounding droplets. Two polymer contributions to reducing the rebound tendency are quantitatively analyzed: the alteration of the substrate wettability by the polymer adsorption and the polymer elongation force.

Suppressing the Rebound of Impacting Droplets from Solvophobic Surfaces by Polymer Additives: Polymer Adsorption and Molecular Mechanisms

“…Recently, we published the, to our best knowledge, first such simulation of an impacting droplet of a dilute polymer solution aided by multi-body dissipative particle dynamics (MDPD). 25 We confirmed the rebound suppression after the retraction stage, namely, at the hopping stage, with a certain choice of material (simulation parameters). The polymer adsorbed on the surface plays two roles in the rebound suppression.…”

“…Several droplet impact experiments have been interpreted by the "slow-retraction" mechanism, in which the polymer deposited on the surface increases the friction between liquid and solid at phase 2, the retraction stage, i.e., polymer is suspected of slowing the horizontal motion of fluid, assuming that the surface is horizontally oriented. [14][15][16][17][18][19][20]33 On the other hand, our former study 25 and the experiment by Zang et al 21 found that the reduced rebound velocity can be explained by the resistance against the hopping motion. In this so-called "slow-hopping" mechanism, the polymer obstructs the vertical motion of the fluid.…”

“…Keeping the advantages of the DPD, the MDPD method allows to simulate phase coexistence for an one-component system by introducing a local-density-dependent attractive force, which provides droplet cohesion. More details of the MDPD method can be found in the SI, our former publication 25 and also in the original MDPD articles. [26][27][28][29]…”

“…The attraction amplitude between solvent and the surface is fixed to -10, which gives the static contact angle of the pure solvent droplet of 155°. (See our former study 25 for the calculation method) Depending on the attraction amplitude between the polymer and the surface beads (A P/W = −10, ... , -200), different anti-rebound mechanisms are achieved. Linear polymer is modeled with beads connected by harmonic springs with an equilibrium bond length of 0.65 and a force constant of 300.…”

“…Then, it cannot contribute anymore either to the friction of the contact line or to the resistance against the hopping. As the number of polymer beads adsorbed on the surface (n ads ) is the most relevant molecular determinant of the rebound tendency, 25 we calculate the number of surface-adsorbed beads, specifically, located inside the circular three-phase contact line (ñ ads ) in Figure 2(c). We thus disregard surface-adsorbed polymer outside the liquid-solid contact area.…”