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.

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.

Abstract The effective dilatational and shear viscosities of a particle-laden fluid interface are computed in the dilute limit under the assumption of an asymptotically vanishing viscosity ratio between the two fluids. Spherical particles with a given contact angle of the fluid interface at the particle surface are considered. A planar fluid interface and a small Reynolds number are assumed. The theoretical analysis is based on a domain perturbation expansion in the deviation of the contact angle from $90^{\circ }$ up to the second order. The resulting effective dilatational viscosity shows a stronger dependence on the contact angle than the effective shear viscosity, and its magnitude is larger for all contact angles. The limits of validity of the theory are discussed.

The electrophoretic mobility of a microparticle with a small potential attached to a fluid interface is studied analytically in the limit of two fluids with vanishing viscosity and permittivity ratios under the assumption of a weak applied electric field. The analysis is based on perturbation expansions in the capillary number, the electric capillary number, and in the deviation of the contact angle on the particle surface from 90 degrees. For the case of a very thin electric double layer (EDL), the Smoluchowski limit for the electrophoretic mobility is recovered. For arbitrary values of the EDL thickness, a spherical particle is considered. In that case the electrophoretic mobility is a function of the EDL thickness and the contact angle at the particle surface. Hydrophobic particles have a higher mobility than hydrophilic ones. The deformation of the fluid interface is shown to be separable into a dynamic and static part. The motion of the particle (dynamic part) does not contribute to the interfacial deformation in the case of a small capillary number, even for arbitrary EDL thickness. The static interfacial deformation due to electric stresses, known as electrodipping, is examined for spherical particles and shown to be independent of the first-order perturbation in the contact angle. The limits of validity of our theory and possible applications are discussed.

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.