Liquid flow and control without solid walls

Abstract

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

Liquid‐Based Adaptive Structural Materials

“…circuits on surfaces without the functionalization of particles. Meanwhile, magnetofluids, liquid with magnetic particles, could directionally migrate and quickly reconfigure the topography under magnetic fields, and thus are helpful in manipulating ferrofluid droplets without contact, [18] and in creating passages in channels to conduct flow control, [19] etc. Microfluidics concerns the manipulation of small volumes of fluid samples through the network of channels that have dimensions less than tens and hundreds of micrometers.…”

“…To realize various flow control functions such as valving, pumping, splitting, and merging, Hermans et al guided aqueous liquid channels with immiscible magnetic liquid, both of which are stabilized by a quadrupolar magnetic field in 2020. [19] Such fluidic channels featured with liquid-liquid interfaces while being supported by magnetic particles show near-frictionless, self-healing, antifouling, and non-clogging properties. It is demonstrated that the magnetostaltic pumping method provides a new platform to transport delicate liquids such as whole human blood with no need for high pressure, and could also be applied for microfluidic circuitry.…”

First online X‐ray fluorescence characterization of liquid‐liquid extraction in microfluidics

“…Recently, Dunne et al developed a flow technique based on magnetic flow freeing themselves from the excessive hydrostatic pressure stemming from wall friction. [17] Nonetheless, such microfluidic networks with increasing complexity and length still result in higher hydrostatic pressures, which, in turn, could weaken the stability of the flow rate and the oil/water interface, when both fluids are in direct contact. The situation becomes increasingly challenging with a developing viscosity difference between the two phases.…”

Liquid Wells as Self‐Healing, Functional Analogues to Solid Vessels

“…[ 27,28 ] Other solid‐wall‐free strategies to confine liquids include embedding of water into a matrix of viscous poly(dimethylsiloxane) oil, [ 29,30 ] water–oil emulsions in microfluidic devices, [ 31,32 ] and aqueous channels held by immiscible magnetic liquid barriers. [ 33 ] With these strategies, however, only water can be confined. Here we demonstrate that on small scales the mechanical stability of water confined on a hydrophilic surface can be high enough to act as a solid‐like wall, capable of containing another liquid.…”