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

1: Magnetic Particles: Preparation, Properties and Applications: M. Ozaki. 2: Maghemite (gamma-Fe2O3): A Versatile Magnetic Colloidal Material C.J. Serna, M.P. Morales. 3: Dynamics of Adsorption and Oxidation of Organic Molecules on Illuminated Titanium Dioxide Particles Immersed in Water M.A. Blesa, R.J. Candal, S.A. Bilmes. 4: Colloidal Aggregation in Two-Dimensions A. Moncho-Jorda, F. Martinez-Lopez, M.A. Cabrerizo-Vilchez, R. Hidalgo Alvarez, M. Quesada-PMerez. 5: Kinetics of Particle and Protein Adsorption Z. Adamczyk.

Surface and Colloid Science

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

The role of electroosmotic flow in transdermal iontophoresis

Electrokinetic flow in a narrow cylindrical capillary

A method for the determination of the kinetic order and energy of the atom formation process in electrothermal atomization atomic absorption spectrometry (ETA-AAS)

The equilibrium density profiles for the restricted primitive model (RPM) for electrolytes, which takes into account the ionic size, are used to calculate several electrokinetic quantities for a 1:1 ionic solution in a charged slit. The three‐point‐extension hypernetted‐chain–mean‐spherical equation is used to calculate the RPM density profiles. The results are compared to those obtained through the Poisson–Boltzmann equation for a point‐ion model. Important quantitative and qualitative differences are found for ultrafine slits. These differences, as well as the general behavior of the electrokinetic quantities, are explained in terms of the structure of the electrical double layer inside the slit.

Wilmer Olivares

Papers

Electrokinetic flow in a narrow cylindrical capillary

A method for the determination of the kinetic order and energy of the atom formation process in electrothermal atomization atomic absorption spectrometry (ETA-AAS)

Electrokinetic flow in ultrafine slits - ionic size effects

Interaction between electrical double layers

Electrothermal atomization of silver in graphite furnaces. Part 1. A two-precursor mechanism