Wetting and dewetting processes in the axial retraction of liquid filaments

Abstract

We study the hydrodynamic mechanisms involved in the motion of the contact line formed at the end region of a liquid filament laying on a planar and horizontal substrate. Since the flow develops under partially wetting conditions, the tip of the filament recedes and forms a bulged region (head) that subsequently develops a neck region behind it. Later the neck breaks up leading to a separated drop, while the rest of the filament restarts the sequence. One main feature of this flow is that the whole dynamics and final drop shapes are strongly influenced by the hysteresis of the contact angle typical in most of the liquid-substrate systems. The time evolution till breakup is studied experimentally and pictured in terms of a hybrid wettability theory which involves the Cox-Voinov hydrodynamic approach combined with the molecular kinetic theory developed by Blake. The parameters of this theory are determined for our liquid-substrate system (silicone oil-coated glass). The experimental results of the retracting filament are described in terms of a simple heuristic model and compared with numerical simulations of the full Navier-Stokes equations. This study is of special interest in the context of pulsed laser-induced dewetting.

Drop pattern resulting from the breakup of a bidimensional grid of liquid filaments

“…Note that this condition introduces a high nonlinearity to the problem, since the solution itself, namely the velocity field at z = 0, yields the corresponding contact angle. Since we are using the same type of fluid (PDMS) and substrate as in [13], the values of the coefficients Γ, , and v 0 are equal to those given in that reference, i.e. Γ = 95.4553, = 0.0008302a = 1.24 × 10 −4 cm, and v 0 = 6.2121 × 10 −7 cm/s.…”

Contact-angle-hysteresis effects on a drop sitting on an incline plane

“…2, which uses the drop volume V as a varying parameter. Moreover, the values of the angles limiting the pinning and depinning regions are coincident with those in [24], where the same kind of substrate and fluid were used.…”

“…The same kind of substrate and PDMS was previously used in [24], where the the static contact angle, θ, of a single sessile drop with circular footprint on a horizontal substrate was measured. In that work, we were able to study the hysteresis cycle of θ by injecting and withdrawing fluid with a needle in contact with the top of the drop.…”

“…Unlike the experiments in [24], the goniometer is now mounted on a tilted base with a variable inclination angle, α. Since the goal here is to analyze the deformations of the drop shape due to changes in α, the substrate is inclined in successive steps of ∆α = 2.5 • , and the cycle 0 Fig.…”

“…2, where ∆r indicates the displacement of the equilibrium contact line from the initial situation, marked by the point O where ∆r = 0 (see also Fig. 2b in [24]). The entrance to the cycle is shown by the path OA along which the drop volume increases while θ remains constant.…”