Control and Measurement of the Phase Behavior of Aqueous Solutions Using Microfluidics
Soft Matter Keywords
Microfluidics, surface tension, phase diagram, permeation.
The purpose of this study was to demonstrate the development of a microfluidic device called the Phase Chip. It was shown that the Phase Chip is able to control the concentration of solutes in aqueous solutions of nanoliter volumes by controlling the permeation of water through poly(dimethylsiloxane) (PDMS), which is also the material used to construct the device. The device uses surface tension forces to guide the solution droplets through the chip's channels and wells. The chip is able do "formulate drops, generate drops, mix drops, store drops in specific sites, vary the amount of water in the drops and thereby regulate the concentration of solutes." Some of the advantages that this device offers are the ability to control the exact composition of the solution drops, the lack of cross-contamination between drops, and the transportation of drops without loss of volume. Some of the applications of this novel device are measuring the phase diagrams of water and polymer or salt mixtures and enhancing protein crystallization rates.
Surface Tension Guides Drops
The drops are forced to flow in an oil-water interface through channels that are 100 um wide by 40 um high. The wells are constructed next to the channels and are also connected to them; however the wells are much wider and deeper than the channel. The difference in geometry between the wells and channels causes the surface area and interfacial energy of the drop to decrease, i.e. the drop's surface area and interfacial energy are reduced when the drop transfers to the well. The drop inside the well feels less confined than when it is inside the channel so the surface tension force will drive the drop from the channel and into the well. The drops fill the wells in sequence and if surfactant is added to the solution, a drop traveling through the channel will not mix its contents with a drop inside a well.
The concentration is controlled by having a module in the chip (labeled (d) in the figure on the right) that has a thin membrane permeable to water but impermeable to salts, polymers or proteins. Dry air or a salt solution are introduced through the reservoir if a higher solute concentration is desired because the dry air or salt solution create a chemical gradient and force water to permeate through this PDMS membrane. In contrast, if a more diluted solute concentration is desired, then pure water is introduced into the reservoir and the chemical gradient is reversed.
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