Sessile droplets on deformable substrates (original) (raw)

Equilibrium of droplets on a deformable substrate: Influence of disjoining pressure

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2016

Equilibrium of liquid droplets on soft deformable substrates is investigated.. Disjoining pressure action in the vicinity of the apparent three phase contact line is taken into account. It is shown that the disjoining pressure action determines the substrate deformation. A simplified linear disjoining pressure isotherm and simple Winkler's model to account for the substrate deformation are used which allows to deduce an analytical solutions for both the liquid profile and substrate deformation. The apparent equilibrium contact angle that the liquid Highlights Deformable substrates Disjoining pressure Transition zone *Highlights (for review) Equilibrium of droplets on a deformable substrate: influence of disjoining pressure

Static and dynamic wetting of soft substrates

Current Opinion in Colloid & Interface Science, 2018

A survey of recent literature on wetting phenomena reveals that there is a fast-growing interest in wetting of soft or deformable substrates, due to its potential applications in many industrial, technical and biological processes. Unlike rigid substrates, a droplet deposited on a soft substrate deforms the substrate via a combination of the normal component of surface tension and the Laplace pressure, i.e. by capillary force and the action of disjoining pressure. In turn, the capillary and disjoining pressure-induced substrate deformation affects the wetting phenomena on the substrate. In this review, we summarize recent achievements on static and dynamic wetting of soft substrates and provide an outlook to future progress. In static wetting, theoretical, numerical and experimental investigations of capillary and disjoining pressure-induced substrate deformation are introduced, and corresponding effects on contact angle and contact angle hysteresis are discussed. In dynamic wetting, the influence of substrate stiffness on spontaneous wetting, droplet impact dynamics, and other types of forced wetting and dewetting is considered. Finally, other interesting capillarity-controlled phenomena occurring on soft and softlike substrates are briefly introduced.

Kinetics of Wetting and Spreading of Droplets over Various Substrates

Langmuir : the ACS journal of surfaces and colloids, 2017

There has been a substantial increase in the number of publications in the field of wetting and spreading since 2010. This increase in the rate of publications can be attributed to the broader application of wetting phenomena in new areas. It is impossible to review such a huge number of publications; that is, some topics in the field of wetting and spreading are selected to be discussed below. These topics are as follows: (i) Contact angle hysteresis on smooth homogeneous solid surfaces via disjoining/conjoining pressure. It is shown that the hysteresis contact angles can be calculated via disjoining/conjoining pressure. The theory indicates that the equilibrium contact angle is closer to a static receding contact angle than to a static advancing contact angle. (ii) The wetting of deformable substrates, which is caused by surface forces action in the vicinity of the apparent three-phase contact line, leading to a deformation on the substrate. (iii) The kinetics of wetting and sprea...

Deformation of an elastic substrate due to a resting sessile droplet

European Journal of Applied Mathematics, 2017

On a sufficiently soft substrate, a resting fluid droplet will cause significant deformation of the substrate. This deformation is driven by a combination of capillary forces at the contact line and the fluid pressure at the solid surface. These forces are balanced at the surface by the solid traction stress induced by the substrate deformation. Young's Law, which predicts the equilibrium contact angle of the droplet, also indicates an a priori radial force balance for rigid substrates, but not necessarily for soft substrates that deform under loading. It remains an open question whether the contact line transmits a non-zero force tangent to the substrate surface in addition to the conventional normal (vertical) force. We present an analytic Fourier transform solution technique that includes general interfacial energy conditions, which govern the contact angle of a 2D droplet. This includes evaluating the effect of gravity on the droplet shape in order to determine the correct f...

Driven large contact angle droplets on chemically heterogeneous substrates

EPL (Europhysics Letters), 2012

We study the depinning and subsequent motion of two-dimensional droplets with large contact angles that are driven by a body force on flat substrates decorated with a sinusoidal wettability pattern. To this end, we solve the Stokes equation employing a boundary element method. At the substrate a Navier slip condition and a spatially varying microscopic contact angle are imposed. Depending on the substrate properties, we observe a range of driving forces where resting and periodically moving droplets are found, even though inertial effects are neglected. This is possible in the considered overdamped regime because additional energy is stored in the non-equilibrium configuration of the droplet interfaces. Finally, we present the dependence of the driving at de-and repinning on wettability contrast and slip length, complemented by a bifurcation analysis of pinned-droplet configurations.

Viscous droplet impingement on soft substrates

2022

Viscous droplets impinging on soft substrates may exhibit several distinct behaviours including repeated bouncing, wetting, and hovering, i.e., spreading and retracting after impact without bouncing back or wetting. We experimentally study the conditions enabling these characteristic behaviours by systematically varying the substrate elasticity, impact velocity and the liquid viscosity. For each substrate elasticity, the transition to wetting is determined as the dependence of the Weber number We, which measures the droplet's kinetic energy against its surface energy, on the Ohnesorge number Oh, which compares viscosity to inertia and capillarity. We find that while We at the wetting transition monotonically decreases with Oh for relatively rigid substrates, it exhibits a counter-intuitive behaviour in which it first increases then gradually decreases for softer substrates. We experimentally determine the dependence of the maximum Weber number allowing non-wetting impacts on the...

How droplets dry on stretched soft substrates

Cornell University - arXiv, 2022

Liquid droplets usually wet smooth and homogeneous substrates isotropically. Recent research works have revealed that droplets sit, slide and spread anisotropically on uniaxially stretched soft substrates, showing an enhanced wettability and contact line mobility along the stretching direction. This phenomenon arises from the anisotropic deformation of the substrate below the contact line. Here, we investigate how the stretching of soft substrates affects droplets drying. We observe that water droplet evaporates with an elongated noncircular contact line on the stretched substrates and switches the elongation direction during evaporation. The contact line velocity and its temporal evolution depend on the orientation of the contact line relative to the stretching direction. On the substrate stretched by 250%, the contact line recedes about 20% of the droplet lifetime earlier along the stretching direction and faster than its perpendicular direction. When nanoparticles are added into the liquid, the circular deposition pattern, i.e., the so-called coffee-ring, becomes elongated along the direction perpendicular to the stretching direction. Particularly, such non-circular deposition pattern exhibits periodic height gradients along its rim. The finer structure of the pattern can be controlled by applying different stretching ratios to the soft substrate and thus are correlated to the anisotropic surface stresses near the contact line.

Elastocapillary deformations on partially-wetting substrates: rival contact-line models

Soft Matter, 2014

A partially-wetting liquid can deform the underlying elastic substrate upon which it rests. This situation requires the development of theoretical models to describe the wetting forces imparted by the drop onto the solid substrate, particularly those at the contactline. We construct a general solution using a displacement potential function for the elastic deformations within a finite elastic substrate associated with these wetting forces, and compare the results for several different contact-line models. Our work incorporates internal contributions to the surface stress from both liquid/solid Σ ls and solid/gas Σ sg solid surface tensions (surface stress), which results in a non-standard boundary-value problem that we solve using a dual integral equation. We compare our results to relevant experiments and conclude that the generalization of solid surface tension Σ ls = Σ sg is an essential feature in any model of partial-wetting. The comparisons also allow us to systematically eliminate some proposed contact-line models.

Morphing and vectoring impacting droplets by means of wettability-engineered surfaces

Scientific reports, 2014

Driven by its importance in nature and technology, droplet impact on solid surfaces has been studied for decades. To date, research on control of droplet impact outcome has focused on optimizing pre-impact parameters, e.g., droplet size and velocity. Here we follow a different, post-impact, surface engineering approach yielding controlled vectoring and morphing of droplets during and after impact. Surfaces with patterned domains of extreme wettability (high or low) are fabricated and implemented for controlling the impact process during and even after rebound--a previously neglected aspect of impact studies on non-wetting surfaces. For non-rebound cases, droplets can be morphed from spheres to complex shapes--without unwanted loss of liquid. The procedure relies on competition between surface tension and fluid inertial forces, and harnesses the naturally occurring contact-line pinning mechanisms at sharp wettability changes to create viable dry regions in the spread liquid volume. U...