Controlling the pinning time of a receding contact line under forced wetting conditions

Precursor-film-driven ultra-early depinning of the three-phase contact line

HYPOTHESIS

Despite its importance in colloid and interface science, contact line pinning remains poorly understood, especially in the presence of a precursor film. We hypothesized that this is due to a lack of an experimental method capable of directly observing their physics at the nanoscale.

METHODS

Using coherence scanning interferometry, we visualized the three-dimensional behavior of contact lines with a precursor film near a nanogroove structure composed of flat terrace surfaces and steps with an inclination angle of 30° while achieving nanoscale vertical resolution.

FINDINGS

We found that even when the contact line is pinned at the edge of the step, the precursor film is not and advances beyond the edge. Furthermore, we discovered that the precursor film has two distinct effects on contact line motion. Specifically, the precursor film facilitates depinning when the contact line descends the step - a contact angle change was 0.9°, only 3.0% of the value predicted by a classical theory of contact angle at a solid edge. This ultra-early depinning is attributed to the formation of a new liquid film past the edge, driven by the progression of the precursor film that overcomes the pinning effect. In contrast, when the contact line ascends the step, the precursor film acts as a resistance to movement due to steric interaction.

Unsteady wetting of soft solids

HYPOTHESIS

Spreading of liquids on soft solids often occurs intermittently, i.e., the liquid's wetting front switches between sticking and slipping. Studies of this so-called stick-slip wetting on soft solids mostly are confined within quasi-static or forced spreading conditions. In these situations, because the sticking duration is set much larger than the viscoelastic relaxation time of the solid, a ridge is persistently and fully developed at the wetting front as the soft solid yields to the liquid's surface tension. The sticking duration and spreading velocity, therefore, were shown to have little impact to the contact angle change required for stick-to-slip transitions. For unsteady wetting of soft solids, a commonly encountered but largely unexplored situation, we hypothesize that the stick-to-slip transition is controlled not only by a combination of sticking duration and the spreading velocity, but also by an increasing depinning threshold caused by the growing ridge at the wetting front.

EXPERIMENT

We performed unsteady wetting experiment on soft solids by letting water droplets spread freely on soft solid surfaces of various stiffness. We capture both the stick-slip spreading behavior and growing wetting ridges using synchronous high-speed imaging and high-speed interferometry. Recorded data of liquid spreading and solid deforming at the wetting front were analyzed to shed light on the relation between stick-slip characteristics and the growing wetting ridge.

FINDINGS

We find that intermittent wetting on a soft solid surface results from a competition between three key factors: liquid inertia, capillary force change during sticking, and growing pinning force caused by the solid's viscoelastic response. We theoretically formulate their quantitative contributions to predict how stick-to-slip transitions occur, i.e., how the contact angle change and sticking duration depend on the liquid's spreading velocity and the solid's viscoelastic characteristics. This provides a mechanistic understanding and methods to control unsteady wetting phenomena in diverse applications, from tissue engineering and fabrication of flexible electronics to biomedicine.

How droplets pin on solid surfaces

HYPOTHESIS

When a droplet starts sliding on a solid surface, the droplet-solid friction force develops in a manner comparable to the solid-solid friction force, showing a static regime and a kinetic regime. Today, the kinetic friction force that acts on a sliding droplet is well-characterized. But the mechanism underlying the static friction force is still less understood. Here we hypothesize that we can further draw an analogy between the detailed droplet-solid and solid-solid friction law, i.e., the static friction force is contact area dependent.

METHODS

We deconstruct a complex surface defect into three primary surface defects (atomic structure, topographical defect, and chemical heterogeneity). Using large-scale Molecular Dynamics simulations, we study the mechanisms of droplet-solid static friction forces induced by primary surface defects.

FINDINGS

Three element-wise static friction forces related to primary surface defects are revealed and the corresponding mechanisms for the static friction force are disclosed. We find that the static friction force induced by chemical heterogeneity is contact line length dependent, while the static friction force induced by atomic structure and topographical defect is contact area dependent. Moreover, the latter causes energy dissipation and leads to a wiggle movement of the droplet during the static-kinetic friction transition.

Roles of energy dissipation and asymmetric wettability in spontaneous imbibition dynamics in a nanochannel

HYPOTHESIS

The imbibition dynamics is controlled by energy dissipation mechanisms and influenced by asymmetric wettability in a nanochannel. We hypothesize that the imbibition dynamics can be described by a combined model of the Lucas-Washburn equation and the Cox-Voinov law considering velocity-dependent contact angles.

METHODS

Molecular dynamics simulations are utilized to investigate the imbibition dynamics. A wide range of wetting conditions is achieved via adjusting the liquid-solid interaction parameters, and the spontaneous imbibition processes are quantified and compared.

FINDINGS

The critical condition for the occurrence of spontaneous imbibition is analyzed from a surface energy perspective. The analyses of energy conversion and dissipation indicate that the viscous dissipation is dominant during spontaneous imbibition. The classical Lucas-Washburn equation is modified with the Cox-Voinov law considering the effect of the dynamic contact angle and an effective equilibrium contact angle. We show that the proposed theory well captures the imbibition dynamics embodied in the growth of imbibition length as well as the transient interface shape and velocity for both the symmetric and asymmetric wetting conditions. In nanochannels with asymmetric wettability, the imbibition length difference between the sidewalls and interface oscillations increases with wetting disparity. Our findings deepen the understanding of imbibition dynamics on the nanoscale, and provide a theoretical reference for relevant applications.

A generalized examination of capillary force balance at contact line: On rough surfaces or in two-liquid systems

We investigate the capillary force balance at the contact line on rough solid surfaces and in two-liquid systems. Our results confirm that solid-liquid interactions perpendicular to the interface have a significant influence on the lateral component of the capillary force exerted on the contact line. Surface roughness of the solid substrate reduces the mobility of liquid and alters how the perpendicular solid-liquid interactions transfer into a force acting parallel to the interface. A quantitative relation between surface roughness and the transfer strategy is proposed. Moreover, when a liquid is in coexistence with another immiscible liquid on a solid, the capillary forces exerted on liquids of both sides are involved in our theoretical model. The contact angle can be predicted by calculating three interfacial tensions. These arguments are then verified by molecular dynamics simulations. Our findings set up the generalized theoretical framework for the capillary force balance at the contact line and broaden its application in more realistic scenarios.