Scanning Probe Characterization of Localized pH Changes on a Sapphire Surface in the Presence of an Applied Field
- PDF / 245,926 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 6 Downloads / 187 Views
A3.1.1
Scanning Probe Characterization of Localized pH Changes on a Sapphire Surface in the Presence of an Applied Field Joseph W. Bullard, Ryan J. Kershner, and Michael J. Cima Department of Materials Science and Engineering Massachusetts Institute of Technology 77 Massachusetts Avenue, Room 12-011 Cambridge, MA 02139 ABSTRACT Single crystal sapphire substrates were lithographically patterned with a system of parallel platinum electrodes, which were used to manipulate 1.58µm silica particles inplane, in the presence of an aqueous solution. Observation of the motion of these particles revealed the adhesion of some of them to the sapphire surface near the platinum working electrode, even in the range of pH where the zeta potentials of silica and sapphire are of the same sign. This phenomenon suggests the existence of localized differences in pH, attributable to the presence of potential determining ions produced in the faradaic processes occurring at the electrodes during the electrophoretic manipulation of silica particles. Atomic force microscopy (AFM) was used to corroborate this hypothesis, measuring the forces between a silica particle and a sapphire substrate in the presence of an applied field. The resultant force-distance curves demonstrate a change in the interaction forces between particle and substrate as a function of distance from the electrode. Variations in this interaction correspond to localized differences in the zeta potential of the substrate, which, in turn, are related to localized differences in pH. Quantification of these spatial variations in pH as a function of time yields further information about the diffusion of these faradaically produced potential determining ions across the substrate. INTRODUCTION The electrophoretic manipulation of microscale colloids is a promising technology for the assembly of a variety of microdevices. In recent years, for example, microwires, optical waveguides, and miniaturized biosensors have all been fabricated electrophoretically [1-3]. The development of such applications builds from wellestablished uses of electrophoretic deposition (EPD) in large-scale industrial processes, such as the formation of geometrically complex ceramic parts and the application of protective coatings to metallic objects [4,5]. Numerous studies outlining the principles governing EPD have been performed over the years. Verwey and Overbeek present the first complete study of the subject in their 1940 paper [6], suggesting a mechanism for the deposition of material at the working electrode characteristic of sedimentation. Sakar and Nicholson [7] outline the kinetics of deposition processes and, along with work by Hunter [8], detail the physics of the formation of a charged surface on colloidal particles, as well as the motion of such particles in the presence of an applied field. A vast majority of EPD studies, with the work of Sakar and Nicholson being a notable exception, rely on ex situ data for analysis,
A3.1.2
studying features of the deposited material to quantify EPD charac
Data Loading...
