Solid walls become increasingly important when miniaturizing fluidic circuitry to the micron scale or smaller. 1 They limit achievable flow-rates due to friction and high pressure drop, and are plagued by fouling 2 . Approaches to reduce the wall interactions have been explored using hydrophobic coatings 3,4 , liquid-infused porous surfaces [4][5][6] , nanoparticle surfactant jamming 7 , changing the surface electronic structure 8 , electrowetting 9,10 , surface tension pinning 11,12 , and atomically flat channels 13 . An interesting idea is to avoid the solid walls altogether. Droplet microfluidics achieves this, but requires continuous flow of both the liquid transported inside the droplets and the outer carrier liquid 14 . We demonstrate a new approach, where wall-less aqueous liquid channels are stabilised by a quadrupolar magnetic field that acts on a surrounding immiscible magnetic liquid. This creates self-healing, uncloggable, and near-frictionless liquidin-liquid microfluidic channels that can be deformed and even closed in real time without ever touching a solid wall. Basic fluidic operations including valving, mixing, and 'magnetostaltic' pumping can be achieved by moving permanent magnets having no physical contact with the channel. This wall-less approach is compatible with conventional microfluidics, while opening unique prospects for implementing nanofluidics without excessively high pressures.Magnetic forces have been used to avoid contact with the walls of a device by levitation of particles or live cells in suspension 15 , and a first attempt to make wall-less microfluidic channels resulted in continuous 'magnetic antitubes' of water surrounded by an aqueous paramagnetic salt solution 16 using

We study here the microscopic deformations of elastic lamellae constituting a superhydrophobic substrate under different wetting conditions of a sessile droplet using electrowetting. The deformation profiles of the lamellae are experimentally evaluated using confocal microscopy. These experimental results are then explained using a variational principle formalism within the framework of linear elasticity. We show that the local deformation profile of a lamella is mainly controlled by the net horizontal component of the capillary forces acting on its top due to the pinned droplet contact line. We also discuss the indirect role of electrowetting in dictating the deformation characteristics of the elastic lamellae. One important conclusion is that the small deflection assumption, which is frequently used in the literature, fails to provide a quantitative description of the experimental results; a full solution of the non-linear governing equation is necessary to describe the experimentally obtained deflection profiles.

The frictional forces of a viscous liquid flow are a major energy loss issue and severely limit microfluidics practical use. Reducing this drag by more than a few tens of percent remain elusive. Here, we show how cylindrical liquid−in−liquid flow leads to drag reduction of 60−99% for sub-mm and mm-sized channels, regardless of whether the viscosity of the transported liquid is larger or smaller than that of the confining one. In contrast to lubrication or sheath flow, we do not require a continuous flow of the confining lubricant, here made of a ferrofluid held in place by magnetic forces. In a laminar flow model with appropriate boundary conditions, we introduce a modified Reynolds number with a scaling that depends on geometrical factors and viscosity ratio of the two liquids. It explains our whole range of data and reveals the key design parameters for optimizing the drag reduction values. Our approach promises a new route for microfluidics designs with pressure gradient reduced by orders of magnitude.

No abstract

No abstract