The
stability of water droplets on striped surfaces exposing regions
of different wettability is studied experimentally, numerically, and
based on a scaling model. Different values of the stripe widths and
different contact angle contrasts between the hydrophilic and hydrophobic
stripes are considered. The boundary between the contact angle contrasts
leaving the droplets intact and those leading to droplet breakup is
computed numerically. The minimum contrast for which breakup occurs
increases with increasing hydrophobic contact angle. The existence
of an unstable and a stable regime is confirmed experimentally. In
the unstable regime, when approching droplet breakup, a configuration
with two liquid fingers on the hydrophilic stripes connected by a
capillary bridge on the hydrophobic stripe is found. For decreasing
volumes, the width of this capillary bridge decreases until a critical
value is reached at which the droplet breaks up. The critical width
depends on the ratio of the hydrophilic and the hydrophobic stripe
width. A simple scaling model is presented with which the critical
width can be predicted. According to the model, the droplet becomes
unstable when the increasing Laplace pressure inside the bridge can
no longer be balanced by the pressure inside the liquid fingers on
the hydrophilic stripes.
The instability of the interface between a dielectric and a conducting liquid, excited by a spatially homogeneous interface-normal time-periodic electric field, is studied based on experiments and theory. Special attention is paid to the spatial structure of the excited Faraday waves. The dominant modes of the instability are extracted using high-speed imaging in combination with an algorithm evaluating light refraction at the liquid–liquid interface. The influence of the liquid viscosities on the critical voltage corresponding to the onset of instability and on the dominant wavelength is studied. Overall, good agreement with theoretical predictions that are based on viscous fluids in an infinite domain is demonstrated. Depending on the relative influence of the domain boundary, the patterns exhibit either discrete modes corresponding to surface harmonics or boundary-independent patterns. The agreement between experiments and theory confirms that the electrostatically forced Faraday instability is sufficiently well understood, which may pave the way to control electrostatically driven instabilities. Last but not least, the analogies to classical Faraday instabilities may enable new approaches to study effects that have so far only been observed for mechanical forcing.
Significance
Imagine you spill your drink and miss some spots when cleaning up. The next morning you notice that the stains look quite different on different surfaces. What has happened? In droplets of liquid mixtures, the components evaporate at different rates, which leads to gradients in concentration and surface tension. These gradients can cause, for example, so-called Marangoni flows, which in turn affect the evaporation process. To better understand evaporation-induced liquid flows, the concentration gradients have to be measured without disturbing the liquid. Marker molecules might be surface-active or even may affect the evaporation process. We report here on marker-free and contactless measurements of concentrations by spatially resolved Raman and NMR spectroscopy in evaporating binary droplets.
Real world surfaces can often be modeled as a collection of edges, corners, dents or spikes of varying roundness. These features exhibit individual spontaneous wetting behaviors comprising pinned contact lines, rivulets or cusps. If occurring in proximity to one another, as is often the case in applications, these wetting properties interact, resulting in an overall changed wetting pattern on the surface. Hence, there is considerable interest in understanding when, and to what extent, interactions occur, and how wetting then deviates from the wetting of isolated surface features.The present study addresses these questions by experimentally and theoretically studying the capillary interaction of sharp-edged 90 • (outer) and 270 • (inner) corners in proximity to one another. It is shown that the spontaneous wetting at the convex outer corner is influenced by the concave inner corner even when they are separated by a distance of several times the capillary length, while the wetting of the inner corner takes place unaffected by the outer corner, except when the separating distance is much 1 arXiv:1908.01221v1 [physics.flu-dyn] 3 Aug 2019 smaller than the capillary length. The final contact line shape at the inner corner is measured and theoretically modelled for contact angles up to 90 • .
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.