Quantifying the approach and retreat of objects from a solid interface using electrochemical techniques

Abstract

This thesis investigates the approach and retreat of conducting and non-conducting micro-spheres to a solid interface. In some cases the interface was an electrode which was able to detect the approach and contact between the sphere and the interface. Three different electrodes, platinum, carbon fibre and gold, were used to explore the interaction between the spheres and the electrode. It was shown that once contact between the surfaces had been made, the conducting spheres produced a different electrode response compared to the insulating spheres.
Several techniques were used to understand the nature of the interaction between the micro-spheres and the electrode surfaces. A bespoke impedance technique was employed which used a fixed frequency to obtain values for the capacitance and uncompensated resistace. This technique allowed for near real-time data capture and enabled the capacitance and uncompensated resistance of dynamic environments to be investigated. This technique was used, in conjunction with imaging, to investigate both conducting and non-conducting micro-spheres as they approached the electrode either under the effects of gravity or when held by a support. The experiments showed that the interaction between the sphere and the surface was complex. It also revealed the presence of paracitic vibrations within the building.
Electrochemical impedance spectroscopy and cyclic voltammetry were employed to investigate stationary spheres at defined distances from the electrode. These measurements were instrumental in understanding the changes observed in the uncompensated resistance and capacitance recorded in a dynamic envrionment using the fixed-frequency technique.
Simulations of the primary and secondary current distribution were performed based on the experimental set-up; COMSOL was used to implement the simulations. The simulations were able to separate the effect of the spheres and the effect of the support capillary to the uncompensated resistance and capacitance of the electrode. It was shown that the conducting spheres lowered the uncompensated resistance at the electrode interface and increased the observed capacitance, though in the physical experiments this was often masked by the support holding the sphere.
Non-electroactive surfaces were also investigated using the fixed frequency technique; in these cases the surfaces used were glass, PTFE and stainless steel and the object was to determine the adhesion between the micro-sphere and the selected surface. In this case the sphere was lifted from a surface multiple times using a glass pore and a pressure rig. A two-electrode system whereby the electrodes were electronically separated by the glass pore was employed. When the sphere was held in the pore mouth the flow of current between the two electrodes was reduced, this reduction triggered the system to release the sphere and begin the measurement again. This enabled multiple autonomous measurements of the lift required to pull the sphere from the desired surface.

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Identifiers

ORCID for Laura Powell: orcid.org/0000-0001-6214-6206

ORCID for Peter Birkin: orcid.org/0000-0002-6656-4074

ORCID for Guy Denuault: orcid.org/0000-0002-8630-9492

ORCID for Philip Bartlett: orcid.org/0000-0002-7300-6900

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