Universal contact-line dynamics at the nanoscale

Abstract

The relaxation dynamics of the contact angle between a viscous liquid and a smooth substrate is studied at the nanoscale. Through atomic force microscopy measurements of polystyrene nanostripes we simultaneously monitor both the temporal evolution of the liquid-air interface and the position of the contact line. The initial configuration exhibits high curvature gradients and a non-equilibrium contact angle that drive liquid flow. Both these conditions are relaxed to achieve the final state, leading to three successive regimes in time: (i) stationary contact line levelling; (ii) receding contact line dewetting; (iii) collapse of the two fronts. For the first regime, we reveal the existence of a self-similar evolution of the liquid interface, which is in excellent agreement with numerical calculations from a lubrication model. For different liquid viscosities and film thicknesses we provide evidence for a transition to dewetting featuring a universal critical contact angle and dimensionless time.

Solidification dynamics of an impacted drop

“…In fact, when trying to fit the best line for the data of figure 5(d), we found an angle slightly different but with no significant improvement when comparing with the θ * correlation, so that we have kept this value for the sake of simplicity. The fact that this dewetting angle θ * appears to be almost independent on the substrates, the liquids and the film thicknesses, while our experiments and those of Rivetti et al (2015) concern film with totally different thickness is a very interesting and intriguing result. It might shed light on an universal dewetting mechanism that should deserve specific investigations in the future.…”

“…The contact line relaxation starts at an angle given by the spreading dynamics (influenced by the drop impact velocity, the substrate wetting angle, and the drop size), and ends when θ reaches a critical value θ * enabling its depinning, required prelude to its receding motion. In the experiment of Rivetti et al (2015), this angle is θ * = 4.5 • ± 0.5 • and surprisingly appears to be independent of the liquid and of the film thickness.…”

“…It is thus expected that the thickness of the ice pancake h p is built by thermal conduction during this time. Consequently, we need first to characterize the duration of this regime, that we call stationary contact line (SCL) regime (Rivetti et al 2015). Figure 5 (a) presents the variation of the liquid film radius during an impact, plotted as a function of time, for three different substrate temperatures.…”

“…To explain the existence of this delay τ SCL between spreading and retraction when water freezes, we will use an argument of a previous work (Rivetti et al 2015) on the relaxation of a contact line pinned at the edge of a polymer film. Similarly, we assume here that the time τ SCL is due to the relaxation dynamics of the contact angle θ formed by the liquid film and the growing ice pancake, and pinned at its edge.…”

Morphological evolution of microscopic dewetting droplets with slip

“…A recent study has demonstrated that a pronounced global bump exists in the dewetting of very flat droplets (h 0 /R(0) ≈ 0.02) even though the slip length is very small (b ≈ 10 −5 ) (Edwards et al 2016). In such cases, the transient global bump itself can be treated as quasistatic, as is the case in dewetting rims of thin liquid films (Redon et al 1991;Snoeijer & Eggers 2010;Rivetti et al 2015).…”