Fluctuations in channel transport across multiple lengthscales

Channel transport is an important physical process, with relevance to diverse fields from cell biology to single molecule sensors, catalysts and filters. Given the ubiquitous nature of channel transport phenomena, achieving a deep understanding of their governing dynamics is ever more desirable. Experimental model systems provide powerful platforms to probe these dynamics. This thesis presents experimental investigations into the fundamental phenomena governing transport through confining channels, in particular the role of fluctuations in such processes. In sensing fields, fluctuations, or noise, are considered undesirable, however careful study of their spectral composition can yield deep insight into underlying physical mechanisms. To achieve a broad understanding over multiple length scales, two experimental models are exploited: the first centred on probing currents of ions through nanoscale glass pores and the second, the flow of colloidal particles through a microfluidic channel. Studies in nanopores begin with an investigation into the intrinsic fluctuations in the ionic current flowing through the pore. A careful study of the 1/f noise present at low frequencies shows that this regime is dominated by fluctuations in the surface current rather than bulk current. Building on this, fluctuations associated with polymer adsorption in the pores are considered. It is found that adsorption generates a characteristic change in current fluctuations which depends sensitively on experimental conditions. Careful analysis of the change in fluctuations yields information on the underlying shape of the adsorption potential on the scale of a single polymer. Next, to probe fluctuations in observable currents of particles, colloidal flow through microfluidic channels is studied using optical microscopy. By using colloids which sediment into a quasi-2D monolayer, the entire transport process can be visualised in detail. To characterise the system, the capture dynamics in two different microfluidic geometries is probed. The importance of long range behaviour to the capture process is demonstrated by showing that drastically different capture rates can be achieved even when conditions within the channel itself are held constant. Fluctuations in the colloidal currents through microchannels are then studied. We start with a model from classical electronics for shot noise in a system with a single characteristic transit time, which is then modified to account for a distribution of transit times, arising from the distribution of particle speeds within the channel. Excellent agreement is found between the spectra of fluctuations for measured currents and those predicted by the modified model, with the particle speed distribution representing the key input into the model. By presenting a series of studies in multiple experimental model systems, this work represents an advancement in the understanding of channel transport fluctuations, and demonstrates the untapped potential of fluctuations analysis as a method of probing physical processes at the micro- and nano-scale. Characterising these systems lays the groundwork for further studies of fluctuating transport in the future, opening the door to more detailed models of molecular transport, cellular processes, and all confined motion on the sub-micrometre scale.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2022

Author dc:creator

• Knowles, Stuart

Advisor dc:contributor.advisor

• Thorneywork, Alice

• Fluctuations
• Nanopores
• Microfluidics
• Colloids
• Polymers
• Nanofluidics
• Noise

Licence

• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.90222

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/342803