Ciro Semprebon

Drops on functional fibers: from barrels to clamshells and back

Drops on cylindrical fibers are a familiar sight, for instance in the form of dew drops on spider webs. They can exist in two competing morphologies, a cylindrically symmetric barrel state completely engulfing the fiber and an asymmetric clamshell state, in which the drop touches the fiber only sideways. Despite their omnipresence and their practical relevance, e.g. for the adherence of drops to fibers in separation technology and filter materials, the physical mechanisms governing the stability of the two morphologies remained elusive. Using electrowetting-functionalized fibers we can tune the wettability of the fibers and thereby reversibly switch between the two states. This allows determination of the stability limits of both morphologies as a function of the two relevant control parameters, namely the contact angle and the liquid volume. While clamshells are found to prevail for large contact angles and small volumes, and barrels prevail for small angles and large volumes, there is also a wide range of intermediate parameter values, for which both morphologies are mechanically stable. Mapping out the energy landscape of the system by numerical minimization of the free energy we find that the barrel state is easily deformed by non-axisymmetric perturbations. Such perturbations facilitate the transition to the clamshell state and thereby the removal of drops from the fibers. From a general perspective, the demonstration of electrowetting-based reversible switching of liquid morphologies on fibers opens up opportunities for designing functional textiles and porous materials for various applications in detergency, filtering, and controlled absorption and release of liquids.

Morphological transitions of droplets wetting rectangular domains.

We report the results of comprehensive experiments and numerical calculations of interfacial morphologies of water confined to the hydrophilic top face of rectangular posts of width W = 500 μm and lengths between L = 5W and 30W. A continuous evolution of the interfacial shape from a homogeneous liquid filament to a bulged filament and back is observed during changes in the liquid volume. Above a certain threshold length of L* = 16.0W, the transition between the two morphologies is discontinuous and a bistability of interfacial shapes is observed in a certain interval of the reduced liquid volume V/W(3).

Advancing modes on regularly patterned substrates

In this article we study the static advancing contact angle θ+a of a liquid interface in contact with a microstructured substrate equipped with a periodic array of circular posts of height H, diameter W and center to center distance D. Assuming a homogeneous material contact angle θo and gravity to be negligible, we numerically minimize the interfacial energy of an asymptotically plane liquid interface aligned with a row of posts for a fixed value of the apparent contact angle θa. A number of branches of mechanically stable interfacial morphologies are observed and classified by the topology of the liquid interface and the three phase contact line. Increasing θa in small steps, we determined the static advancing contact angle θ+a as the apparent (asymptotic) contact angle, above which no mechanically stable interfacial configuration exists. Specific types of advancing modes can be assigned to certain regions of the control parameter θo, aspect ratio h = H/W, and line fraction w = W/D. A rich spectrum of advancing modes is found in the region of material contact angles θo between 45 and 55° where interfacial instabilities due to liquid coalescence and contact line depinning compete.

Ternary free-energy lattice Boltzmann model with tunable surface tensions and contact angles

We present a ternary free-energy lattice Boltzmann model. The distinguishing feature of our model is that we are able to analytically derive and independently vary all fluid-fluid surface tensions and the solid surface contact angles. We carry out a number of benchmark tests: (i) double emulsions and liquid lenses to validate the surface tensions, (ii) ternary fluids in contact with a square well to compare the contact angles against analytical predictions, and (iii) ternary phase separation to verify that the multicomponent fluid dynamics is accurately captured. Additionally we also describe how the model presented here can be extended to include an arbitrary number of fluid components.

Role of contact-angle hysteresis for fluid transport in wet granular matter

The stability of sand castles is determined by the structure of wet granulates. Experimental data on the size distribution of fluid pockets are ambiguous with regard to their origin. We discovered that contact-angle hysteresis plays a fundamental role in the equilibrium distribution of bridge volumes, and not geometrical disorder as commonly conjectured. This has substantial consequences on the mechanical properties of wet granular beds, including a history-dependent rheology and lowered strength. Our findings are obtained using a model in which the Laplace pressures, bridge volumes, and contact angles are dynamical variables associated with the contact points. While accounting for contact line pinning, we track the temporal evolution of each bridge. We observe a crossover to a power-law decay of the variance of capillary pressures at late times and a saturation of the variance of bridge volumes to a finite value connected to contact line pinning. Large-scale simulations of liquid transport in the bridge network reveal that the equilibration dynamics at early times is well described by a mean-field model. The spread of final bridge volumes can be directly related to the magnitude of contact-angle hysteresis.

Filling transitions on rough surfaces: Inadequacy of Gaussian surface models.

We present numerical studies of wetting on various topographic substrates, including random topographies. We find good agreement with recent predictions based on an analytical interface-displacement-type theory, except that we find critical end points within the physical parameter range. As predicted, Gaussian random surfaces are found to behave qualitatively different from non-Gaussian topographies. This shows that Gaussian random processes as models for rough surfaces must be used with great care, if at all, in the context of wetting phenomena.

Morphological transitions of water channels Induced by vertical vibrations

We report the results of comprehensive experiments and numerical calculations of interfacial morphologies of water confined to the hydrophilic top face of rectangular posts subjected to vertical vibrations. In response to mechanical driving, an initially flat liquid channel is collected into a liquid bulge that forms in the center of the rectangular post if the acceleration exceeds a certain threshold. The bulge morphology persists after the driving is switched off, in agreement with the morphological bistability of static interfacial shapes on posts with large length-to-width ratios. In a narrow frequency band, the channel does not decay into a bulge at any acceleration amplitude, but displays irregular capillary waves and sloshing instead. On short posts, however, a liquid bulge can be dynamically sustained through vertical vibrations but quickly decays into a homogeneous channel after the external driving is stopped. To explain the dynamic bulging of the liquid interface, we propose an effective lifting force pulling on the drops slowly moving center of mass in the presence of fast oscillation modes.

Statics and dynamics of liquid barrels in wedge geometries

We present a theoretical study of the statics and dynamics of a partially wetting liquid droplet, of equilibrium contact angle

Cluster-Assembled Nanostructured Titanium Oxide Films with Tailored Wettability

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Pinning and wicking in regular pillar arrays

, confined in a solid wedge geometry of opening angle