Nanoengineering surfaces is a concept that comes up at multiple occasions within these research projects. I’m fascinated by the idea of constructing something that is nanoscopic in at least one dimension that can have macroscopic effects on the operation of a technology. In all these projects, the idea is to use nanoengineered surfaces as components in lab-on-a-chip systems.
One appealing idea is the concept of “smart” nanochannels, i.e. channels that would respond to stimuli such as variation in temperature, pH, or concentrations of specific chemicals. The most obvious surface modification that could have the potential to respond to such signals is grafting (or otherwise associating) polymer chains to interfaces. In fact, it is charged polyelectrolyte brushes that have the greatest potential as “smart brushes”. Because of the long-range electrostatic coupling between chains and free counterions, the structure of polyelectrolyte brushes depends sensitively on many factors, which could be used to switch the brush from one state to another.
Swelling and deswelling by an external voltage, applied normal to the grafting surface has been experimentally demonstrated and suggests a convenient and quick switch that acts on both the polyelectrolyte brush and also the counterions suspended in the solvent. The extension/collapse transition could act as a stimuli responsive gate, limiting the passage of flowing fluid or ions and so has potential in the design of functional and controllable MEMS devices.
With our collaborators Dr.Pai-Yi Hsiao and his student Yu-Fan Ho from the National Tsing Hua University, we have found some very interesting results from studying this system. More to come once we know where our manuscript stands. For now you can read more about the electrohydrodynamics of soft surfaces in our review on electrophoresis.