Capillarity
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Mechanics of fluid interfaces
Fluid Mechanics Surfactants CapillarityThis material treats the mechanics that arises from the presence of surface tension of liquid interfaces. Liquid interface dynamics, especially due to the action of surface tension, has always been and remains one of the most interesting topics in mechanics. Even the discovery of surface tension or the first measurement of surface tension was not trivial. Today, as certain aspects of our world are miniaturized due to the emergence of “nanotechnology”, effects of surface tension are more imminently noticeable.
Cheerios effect
Cheerios Effect Capillarity Fluid MechanicsPour milk into a bowl, drop in some Cheerios (the breakfast cereal), and notice how they clump together. This attraction is mediated by gravity and surface tension. Each cheerio makes a little depression in the milk-air interface, and the neighbouring one just falls into it.
Capillary interactions between nearby interfacial objects
We develop a general method to study the capillary interactions between objects of arbitrary shape which float close to each other on an interface, a regime in which multipole expansion is not useful. The force is represented as a power series in the small distance between the objects, of which the leading order is finite. For objects with size a much larger than the capillary length lc, the force scales as (a/lc)1/2 and the prefactor depends on the mean radius of curvature R at the closest points.
Short-time dynamics of partial wetting
When a liquid drop contacts a wettable surface, the liquid spreads over the solid to minimize the total surface energy. The first moments of spreading tend to be rapid. For example, a millimeter-sized water droplet will wet an area having the same diameter as the drop within a millisecond. For perfectly wetting systems, this spreading is inertially dominated. Here we identify that even in the presence of a contact line, the initial wetting is dominated by inertia rather than viscosity.
An experimental study of the coalescence between a drop and an interface in Newtonian and polymeric liquids
When a water drop falls onto an oil-water interface, the drop usually rests for some time before merging with the water underneath the interface. We report experiments on this process using water- and oil-based Newtonian liquids and polymer solutions, with an emphasis on the non-Newtonian effects. We deduce that the drop surface is immobilized by contaminants pre-existing in the fluids, and find that the rest time scales with the matrix viscosity for Newtonian fluids.
Partial coalescence between a drop and a liquid-liquid interface
This Letter reports experimental results for partial coalescence when a drop merges with an interface. We find an intermediate range of drop sizes in which the merger is not complete but a daughter drop is left behind. This phenomenon is governed primarily by inertia and interfacial tension, and three regimes can be further delineated depending on the roles of viscosity and gravity. Scaling relationships are developed for the drop size ratio and the coalescence time.