We investigate experimentally and theoretically the stability and collapse of holes in liquid layers on bounded substrates with various wettabilities. It is shown that for a liquid layer with a thickness of the order of the capillary length, a stable hole exists when the hole diameter is bigger than a critical value $d_{c}$. Consequently, a further increase of the liquid volume causes the hole to collapse. It is found that$d_{c}$increases with the size of the container, but its dependence on the contact angle is very weak. The experimental results are compared with theory, and good agreement is obtained. Moreover, we present investigations of the dynamics of the hole and the evolution of the liquid film profile after the collapse. The diameter of the hole during collapse and the minimum thickness of the liquid film shortly after the collapse obey different power laws with time. Simple theoretical models are developed which indicate that the collapse of the hole is triggered by surface tension and the subsequent closure process results from inertia, whereas the growth of the liquid column after hole closure results from the balance between the capillary force and inertia. Corresponding scaling coefficients are determined.

We demonstrate theoretically and experimentally that injection of momentum in a region surrounding an object in microscale flow can yield both 'cloaking' conditions, where the flow field outside the cloaking region is unaffected by the object, and 'shielding' conditions, where the hydrodynamic forces on the object are eliminated. Using field-effect electro-osmosis as a mechanism for injection of momentum, we present a theoretical framework and analytical solutions for a range of geometrical shapes, validate these both numerically and experimentally, and demonstrate the ability to dynamically switch between the different states.

Liquids are traditionally handled and stored in solid vessels. Solid walls are not functional, adaptive, or self‐repairing, and are difficult to remove and re‐form. Liquid walls can overcome these limitations, but cannot form free‐standing 3D walls. Herein, a liquid analogue of a well, termed a “liquid well” is introduced. Water tethered to a surface with hydrophobic–hydrophilic core–shell patterns forms stable liquid walls capable of containing another immiscible fluid, similar to fluid confinement by solid walls. Liquid wells with different liquids, volumes, and shapes are prepared and investigated by confocal and Raman microscopy. The confinement of various low‐surface‐tension liquids (LSTLs) on surfaces by liquid wells can compete with or be complementary to existing confinement strategies using perfluorinated surfaces, for example, in terms of the shape and height of the confined LSTLs. Liquid wells show unique properties arising from their liquid aggregate state: they are self‐healing, dynamic, and functional, that is, not restricted to a passive confining role. Water walls can be easily removed and re‐formed, making them interesting as sacrificial templates. This is demonstrated in a process termed water‐templated polymerization (WTP). Numerical phase‐field model simulations are performed to scrutinize the conditions required for the formation of stable liquid wells.

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