Microfluidic devices for manipulating fluids are widespread and finding uses in many scientific and industrial contexts. Their design often requires unusual geometries and the interplay of multiple physical effects such as pressure gradients, electrokinetics, and capillarity. These circumstances lead to interesting variants of well-studied fluid dynamical problems and some new fluid responses. We provide an overview of flows in microdevices with focus on electrokinetics, mixing and dispersion, and multiphase flows. We highlight topics important for the description of the fluid dynamics: driving forces, geometry, and the chemical characteristics of surfaces.

Engineering flows in small devices

We survey progress over the past 25 years in the development of microscale devices for pumping fluids. We attempt to provide both a reference for micropump researchers and a resource for those outside the field who wish to identify the best micropump for a particular application. Reciprocating displacement micropumps have been the subject of extensive research in both academia and the private sector and have been produced with a wide range of actuators, valve configurations and materials. Aperiodic displacement micropumps based on mechanisms such as localized phase change have been shown to be suitable for specialized applications. Electroosmotic micropumps exhibit favorable scaling and are promising for a variety of applications requiring high flow rates and pressures. Dynamic micropumps based on electrohydrodynamic and magnetohydrodynamic effects have also been developed. Much progress has been made, but with micropumps suitable for important applications still not available, this remains a fertile area for future research.

https://microfluidics.stanford.edu/Publications/Micropumps_Cooling/Laser%20Review%20of%20Micropumps%20in%20JMM.pdf

A review of micropumps

Development of theories of inhomogeneous fluids, J.S. Rowlinson statistical mechanical sum rules, J.R. Henderson density functionals in the theory of non-uniform fluids, R. Evans integral equation theories for inhomogeneous fluids, Douglas Henderson inhomogeneous two-dimensional plasmas, B. Jancovici statistical mechanics of electrolytes at interfaces, L. Blum and Douglas Henderson wetting experiments, Carl Franck fluids between walls and in pores, Marcelo Lozada-Cassou freezing, A.D.J. Haymet nucleation, David W. Oxtoby liquid crystals, J.F. Marko nature of microemulsion, K.A. Dawson and Christopher J. Mundy kinetic theory of strongly inhomogeneous fluids, H. Ted Davis.

Fundamentals of Inhomogeneous Fluids

Electroosmotic flow (EOF) micropumps which use electroosmosis to transport liquids have been fabricated and used to achieve pressures in excess of 20 atm and flow rates of 3.6 μl/min for 2 kV applied potentials. These pumps use deionized water as working fluids in order to reduce the ion current of the pump during operation and increase thermodynamic efficiency. EOF pumps are fabricated by packing the 3.5 μm diameter non-porous silica particles into 500–700 μm diameter fused-silica capillaries and by using a silicate frit fabrication process to hold the particles in place. The devices have no moving parts and can operate as both open (high flow rate) and closed (high pressure) systems. Pressure versus flow rate performance data are presented and combined with measurements of physical dimensions, dry and wet weight, and ion current to calculate the pump structure porosity, tortuosity, effective pore radius, and zeta potential.

Fabrication and characterization of electroosmotic micropumps

SUMMARY
Ion channels are proteins which catalyse rapid, passive, electrogenic uniport of ions through pores spanning an otherwise poorly permeable lipid bilayer. Among other processes, fluxes through ion channels are responsible for action potentials – large, transient changes in membrane potential which have been known of in plants for over 100 years. Much disparate information on ion channels in plant cells has accumulated over the past few years. In an attempt to synthesize these data, the properties of at least 18 different ion channels are collated in this review. Channels are initially classified according to ion selectivity (Ca2+, Cl−, K+ and H+); then gating characteristics (i.e. control of opening and closing), unitary conductance and pharmacology are used to distinguish further different sub-types of channels. To provide a background for this overview, the fundamental properties which define ion channels in animal cells, namely conduction, selectivity and gating, are described. Appropriate techniques for the study of ion channels are also assessed. The review concludes with a discussion on the role of ion channels in plant cells, although any comment on functions beyond turgor regulation and general statements about signalling remains largely speculative. The study of ion channels in plant cells is still at an early stage and it is hoped that this review will provide a framework upon which further work in both algae and vascular plants can be based.

Tansley Review No. 21 Plant ion channels: whole‐cell and single channel studies

Electrokinetic flow in a narrow cylindrical capillary

The first member of the Born-Green-Yvon hierarchy of integral equations is solved numerically for a system of charged hard spheres near a charged wall under two alternative closures. The first is analogous to the superposition approximation and yields results resembling those obtained from other integral equations. The second closure employs electroneutrality to approximate the dependence of the pair correlation function upon distances from the wall. This improved theory gives qualitatively different results which are in general agreement with the modified Gouy-Chapman theory over a wide range of charge densities on the wall.

The electrical double layer in the Born-Green-Yvon equation

The Poisson–Boltzmann equation for two large charged spheres immersed in an ionic solution with either constant surface charge density or constant surface potential is solved numerically. The repul...

Thomas L. Croxton

Papers

Electrokinetic flow in a narrow cylindrical capillary

The electrical double layer in the Born-Green-Yvon equation

The interaction of two charged spheres in the Poisson-Boltzmann equation

Numerical solution of the Born-Green-Yvon equation for the restricted primitive model of ionic solutions

Measurement of intracellular chloride activity in mouse liver slices with microelectrodes