My research interests include many aspects of liquid-solid interface science, such as hydrophobicity, molecular mechanisms of lubrication, electrode interfaces with novel electrolytes, and bio-interfaces. In my laboratory we carry out experiments to detect how particles or surfaces interact across liquids. The aim is to reach a fundamental and molecular level picture of the physics whilst also selecting test-cases which have practical implications and applications. Over the past few years my research group has become particularly excited about a new class of liquids: ionic liquids. Some details of our current research in that area are below.
Ionic liquids at surfaces
Ionic liquids are pure salts which are in the liquid state under ambient conditions. They have a remarkable combination of physical properties: they do not evaporate, they are ion conducting, and they can solvate both polar and non-polar substances. The details of their characteristics can be ‘tuned’ for particular applications, an important example being their use as electrolytes in energy storage devices. The molecular arrangement in ionic liquids – both in the bulk and at surfaces (e.g. electrode surfaces) – is entirely different to dilute aqueous electrolytes or molecular liquids (Figure 1).
Figure 1 – schematic diagram showing the vastly different situations of dilute electrolyte solutions and ionic liquids at charged surfaces.
In the Perkin group we are studying the liquid structure and dynamics in ionic liquids at surfaces and confined to thin films. We find that the delicate balance between different molecular interactions in the liquid – including electrostatic, van der Waals, ‘ion-phobic’ and hydrogen bonding – leads to remarkable liquid structures. Examples of intricate ion arrangements include alternating cation layers and anion layers (Figure 2 left) and toe-to-toe bilayers (Figure 2 right) when confined between two charged surfaces.
Figure 2 – Interaction force as a function of film thickness between mica surfaces across two different ionic liquids. The oscillations in force indicate layered structures in the liquid film; some examples for different film thickness are show in the inset schematic diagrams.
The Surface Force Balance
In the Perkin group we employ a Surface Force Balance (SFB) to detect the interaction force as a function of distance between atomically smooth and macroscopic surfaces across films of liquid. Sub-molecular resolution in film thickness is achieved using white light interferometry (“FECO” – Figure 3). The SFB also detects lateral (e.g. friction) forces with extremely high resolution and at well-defined film thickness. Other surface characterisation methods and tools used in our work include contact angle goniometry, atomic force microscopy (AFM), surface tensiometry.
Figure 3 – diagram of the white light interferometry used in the SFB, including a photograph of the resulting FECO interference fringes which are used to calculate the film thickness.
Joining the Perkin group
Prospective part-II students, DPhil students, and post-docs are welcome to contact us by email to discuss possible projects and arrange a visit.