Doctor of Philosophy
Department of Chemistry - Faculty of Science
Halldorsson, Jenny, Investigation of the factors influencing the wettability of conducting polymers for fluid control in microfluidic devices, PhD thesis, Department of Chemistry, University of Wollongong, 2007. http://ro.uow.edu.au/theses/31/
The advent of miniaturised analytical systems has the potential to revolutionise the way in which chemical and biological analyses are made, due to the benefits of reduced reagent consumption, increased sensitivity and decreased analysis times. With increasingly smaller device dimensions comes the need for more efficient methods of controlling fluid flow on the microscale. Electrokinetic techniques, such as electrophoresis and electroosmosis, are well suited for delivering analytes and reagents in microfluidic devices, however high driving voltages and power requirements limit the size to which the device may be miniaturised and thus limit the applicability of these flow control methods for portable, hand-held devices. In order to overcome this limitation, an elegant use of surface tension forces which dominate at the microscale is necessary. Conducting organic polymers, such as polythiophene and polypyrrole, have found widespread use in recent years due to their attractive mechanical properties and processability, in addition to their ability to be reversibly switched between oxidised (conducting) and reduced (insulating) forms. This redox switching may be accompanied by a change in polymer properties such as wettability and surface energy which may be altered dramatically upon external stimulation, commonly in the form of a small applied electrical potential. The effect of redox switching upon conducting polymer wettability for fluid control in microfluidic devices is the central theme which was explored in this thesis. In particular, the aims of this thesis were to characterise the wettability of conducting polymers and investigate the factors which influence it, as well as explore the use of conducting polymers for fluid control in simple, dynamically controlled microfluidic devices, based on the change in wetting properties upon in¬-situ electrochemical redox switching. The role of polymer oxidation state, film thickness, polymerisation substrate and the configuration of the electrochemical cell used for fluid control were considered. Goniometry and tensiometry were used to characterise polymer wetting properties, while microscopy techniques (scanning electron microscopy, atomic force microscopy and optical profilometry) were used to probe the morphology of polymer and understand the role of roughness on conducting polymer wettability and fluid movement. The electrochemical properties of polymers were characterised by cyclic voltammetry, while Raman spectroscopy was employed to gain insight into the role of water and film thickness in determining the oxidation state of polyterthiophene in Chapter 3. The insights gained during polymer wettability characterisations were extended to investigate surface tension-induced fluidic control using electrochemical cells in both channel-based and droplet-based configurations upon the application of a small voltage. The knowledge gained during the course of this study should form the basis for developing devices which will contribute to interesting solutions for improving flow control on the microscale.