79 research outputs found
Cusp-shaped Elastic Creases and Furrows
The surfaces of growing biological tissues, swelling gels, and compressed
rubbers do not remain smooth, but frequently exhibit highly localized inward
folds. We reveal the morphology of this surface folding in a novel experimental
setup, which permits to deform the surface of a soft gel in a controlled
fashion. The interface first forms a sharp furrow, whose tip size decreases
rapidly with deformation. Above a critical deformation, the furrow bifurcates
to an inward folded crease of vanishing tip size. We show experimentally and
numerically that both creases and furrows exhibit a universal cusp-shape, whose
width scales like at a distance from the tip. We provide a
similarity theory that captures the singular profiles before and after the
self-folding bifurcation, and derive the length of the fold from large
deformation elasticity.Comment: 5 pages, 4 figure
Droplets move over viscoelastic substrates by surfing a ridge
Liquid drops on soft solids generate strong deformations below the contact
line, resulting from a balance of capillary and elastic forces. The movement of
these drops may cause strong, potentially singular dissipation in the soft
solid. Here we show that a drop on a soft substrate moves by surfing a ridge:
the initially flat solid surface is deformed into a sharp ridge whose
orientation angle depends on the contact line velocity. We measure this angle
for water on a silicone gel and develop a theory based on the substrate
rheology. We quantitatively recover the dynamic contact angle and provide a
mechanism for stick-slip motion when a drop is forced strongly: the contact
line depins and slides down the wetting ridge, forming a new one after a
transient. We anticipate that our theory will have implications in problems
such as self-organization of cell tissues or the design of capillarity-based
microrheometers.Comment: 9 pages, 5 figure
Liquid drops attract or repel by the inverted Cheerios effect
European Union Grant CIG 618335. S.K. acknowledges financial support from NWO through VIDI Grant 11304. A.P. and J.H.S. acknowledge financial support from European Research Council Consolidator Grant 616918
Brazil
Biological and Soft Matter Physic
Printing wet-on-wet: attraction and repulsion of drops on a viscous film
Wet-on-wet printing is frequently used in inkjet printing for graphical and
industrial applications, where substrates can be coated with a thin liquid film
prior to ink drop deposition. Two drops placed close together are expected to
interact via deformations of the thin viscous film, but the nature of these
capillary interactions is unknown. Here we show that the interaction can be
attractive or repulsive depending on the distance separating the two drops. The
distance at which the interaction changes from attraction to repulsion is found
to depend on the thickness of the film, and increases over time. We reveal the
origin of the non-monotonic interactions, which lies in the appearance of a
visco-capillary wave on the thin film induced by the drops. Using the thin-film
equation we identify the scaling law for the spreading of the waves, and
demonstrate that this governs the range over which interaction is observed.Comment: 5 pages, 5 figure
Reversal of Solvent Migration in Poroelastic Folds
Polymer networks and biological tissues are often swollen by a solvent such that their properties emerge from a coupling between swelling and elastic stress. This poroelastic coupling becomes particularly intricate in wetting, adhesion, and creasing, for which sharp folds appear that can even lead to phase separation. Here, we resolve the singular nature of poroelastic surface folds and determine the solvent distribution in the vicinity of the fold tip. Surprisingly, two opposite scenarios emerge depending on the angle of the fold. In obtuse folds such as creases, it is found that the solvent is completely expelled near the crease tip, according to a nontrivial spatial distribution. For wetting ridges with acute fold angles, the solvent migration is reversed as compared to creasing, and the degree of swelling is maximal at the fold tip. We discuss how our poroelastic fold analysis offers an explanation for phase separation, fracture, and contact angle hysteresis.</p
On the singular nature of the elastocapillary ridge
The functionality of soft interfaces is crucial to many applications in
biology and surface science. Recent studies have used liquid drops to probe the
surface mechanics of elastomeric networks. Experiments suggest an intricate
surface elasticity, also known as the Shuttleworth effect, where surface
tension is not constant but depends on substrate deformation. However,
interpretations have remained controversial due to singular elastic
deformations, induced exactly at the point where the droplet pulls the network.
Here we reveal the nature of the elastocapillary singularity on a hyperelastic
substrate with various constitutive relations for the interfacial energy.
First, we finely resolve the vicinity of the singularity using goal-adaptive
finite element simulations. This confirms the universal validity, also at large
elastic deformations, of the previously disputed Neumann's law for the contact
angles. Subsequently, we derive exact solutions of nonlinear elasticity that
describe the singularity analytically. These solutions are in perfect agreement
with numerics, and show that the stretch at the contact line, as previously
measured experimentally, consistently points to a strong Shuttleworth effect.
Finally, using Noether's theorem we provide a quantitative link between wetting
hysteresis and Eshelby-like forces, and thereby offer a complete framework for
soft wetting in the presence of the Shuttleworth effect.Comment: 17 Pages, 7 figure
Droplet actuation induced by coalescence: experimental evidences and phenomenological modeling
This paper considers the interaction between two droplets placed on a
substrate in immediate vicinity. We show here that when the two droplets are of
different fluids and especially when one of the droplet is highly volatile, a
wealth of fascinating phenomena can be observed. In particular, the interaction
may result in the actuation of the droplet system, i.e. its displacement over a
finite length. In order to control this displacement, we consider droplets
confined on a hydrophilic stripe created by plasma-treating a PDMS substrate.
This controlled actuation opens up unexplored opportunities in the field of
microfluidics. In order to explain the observed actuation phenomenon, we
propose a simple phenomenological model based on Newton's second law and a
simple balance between the driving force arising from surface energy gradients
and the viscous resistive force. This simple model is able to reproduce
qualitatively and quantitatively the observed droplet dynamics
The value of a fading tracer
Tracer particles are the workhorse of the fluid dynamicist for visualizing flow in transparent liquids. Thus a tracer becomes useless if its signal disappears, which frequently happens in practice, for instance due to bleaching. The opposite occurs in a recent work by Kim & Stone (J. Fluid Mech., vol. 850, 2018, pp. 769–783): the fading signal of a dissolving particle may reveal the local composition in a mixture. Such information is highly valuable in the study of evaporating droplets. In virtually all realistic cases, droplets consist of multiple components, ranging from trace impurities to engineered cocktails, which, for instance, generate a desired deposit pattern for a printing process. Different components typically evaporate at different rates, which causes inhomogeneities in droplet composition. Determining the latter is one of the main challenges in the field
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