Evidence of ElectroOsmotic Flow in Nano-Membrane Devices
Fluorescent beads were employed to assess electroosmotic flow in various devices. The beads were 200 nm in size and were negatively charged (on account of being carboxylated polystyrene). These beads were suspended in a 0.2 mM K2HPO4 buffer. They were also incubated in 1% BSA solution in an attempt to reduce their surface charge, though this was mostly ineffective (only a slight reduction in charge response versus un-incubated beads was observed).
The first device tested consisted of two glass slides held apart by vacuum grease to create a 32 um channel. This device exhibited minimal EO, with the bead’s behavior being dominated by their charge rather than by nearby flow.
The following video shows the performance of beads initially under no applied voltage, followed by a +50V potential to the right.
The strength of the observed EO increased slightly when 20 um channel was tested, though the majority of beads still travelled in the direction favored by their charge.
The following video shows the performance of beads initially under no applied voltage, followed by a +50V potential to the left.
The final device tested contained both a 100 um glass on glass channel as well as a 22 nm silicon oxide membrane (12% porosity). This device exhibited relatively strong EO flow, apparently able to overpower the bead’s charge attraction. Similar devices without intact membranes did not exhibit such a strong EO response, implying that the membrane is primarily responsible for the flow.
The following video shows part of the device which initially is under a +50V potential to the left, then a +50V potential to the right, then no potential.
Nano-porous membrane flow (+50L,+50R,0)
ElectroOsmotic behavior can again be observed in the next video, where there is initially no potential, then +50V out of the hole, then zero potential, followed by +50V into the hole.
Nano-porous membrane flow (0,+50 out, 0, +50 in)
To study the effect of ionic strength on EO, a small amount of 3M KCl was added to each port of the device. The following video shows the result of a +50V potential to the right (after a period of zero potential).
Nano-porous membrane flow (0,+50R)
The flow has clearly become less responsive to voltage after the addition of KCl, and after a while longer EO was entirely replaced by charge preference. A qualitative study of the device afterwards indicates that the membrane has become covered in salt crystals and has wrinkled in places. It also appears more permeable than before (though not as permeable as a broken membrane). A separate experiment in the lab indicated that applying high voltage to KCl solution can result in hypochlorite formation which in turn raises the solution pH to around 11 and can apparently etch away membranes. It seems likely that the membrane in the device has had its pores expanded by this solution, which in turn resulted in the observed decrease in EO strength and responsiveness.