Research Guides

Professor Susan Perkin

We are an experimental research group exploring the physics and physical chemistry of liquids and soft matter, in particular at interfaces and confined to thin films.

Ionic liquids and concentrated electrolytes

Electrolytes are fluids containing mobile charges. Much of the natural world is made up of electrolyte: the sea, animals and plants are all made up of largely of electrolyte.  Electrolytes are also of critical importance technologically, for example in batteries and supercapacitors. Classical physical chemistry of electrolytes explains the behavior of electrolytes at low ion-concentration, however a current challenge is to extend our knowledge to reach the high-concentration regime.  A particularly fascinating class of high-concentration electrolytes are the ionic liquids: salts which are liquid under ambient conditions despite containing no solvent.  Our experimental approach is to measure surface forces across ionic liquids and other concentrated electrolytes, allowing us to determine properties such as screening length, near-surface ordering, electrode-electrolyte capacitance, charge regulation and other bulk and surface properties of the electrolytes.

Nano-confined liquids

The properties of fluids confined to nanoscale films or pores can diverge dramatically compared to the bulk fluid. This has important and wide-ranging consequences; from the role of trapped water and ions in biological complexes to the cycling of ions in and out nanoporous electrodes in batteries.  We investigate this using a Surface Force Balance (SFB) to look directly at the structure and dynamic properties of fluids confined to films of just a few molecular layers in thickness.

Molecular mechanisms of friction and lubrication

Understanding friction and lubrication at the molecular level is important for many applications, ranging from the design of artificial joints (aqueous lubrication) to micro- and nano-fluidic devices. The classical laws of friction do not necessarily transfer directly to the molecular scale, so well-characterised model experiments are necessary for determining the relevant rules for energy dissipation at the nanoscale.   To this end, we carry out experimental measurements of the lateral force during shear of nano-confined fluid or soft matter (oils, water, electrolytes, polymers, surfactants etc) with well controlled film thickness, applied load, shear rate, etc.  Current work involves designing ways to externally switch or control friction.

Innovating methods and instrumentation

We use a custom-built Surface Force Balance (SFB, also called Surface Force Apparatus) for high resolution measurements of optical, electrical, and mechamical properties of liquid films confined between two smooth solid surfaces. White light multiple-beam interferometry is used to determine the film thickness and optical characteristics.  We have an ongoing programme of research developing new methodologies for the SFB in order to apply controlled electrical potential to the surfaces (using graphene or gold electrodes in place of the standard mica substrates); methods for applying electric fields across the liquid film; and methods for analysing dynamic forces such as viscous drainage and boundary slip/stick.


Smith, A.M.; Lee, A.A.; Perkin S.
The Electrostatic Screening Length in Concentrated Electrolytes Increases with Concentration
J. Phys. Chem. Lett., 2016, 7, 2157-2163.

Lee, A., Perez-Martinez, C., Smith, A., Perkin, S.
Scaling analysis of the screening length in concentrated electrolytes
Phys. Rev. Lett. 2017, 119, 026002

Lhermerout, R. and Perkin, S.
Nanoconfined ionic liquids: Disentangling electrostatic and viscous forces
Physical Review Fluids  2018, 3, 014201

Smith, A., Lee, A., Perkin, S.
Switching the Structural Force in Ionic Liquid-Solvent Mixtures by Varying Composition
Phys. Rev. Lett. 2017, 118, 096002

Smith, A. M.; Lovelock, K. R. J.; Gosvami, N. N.; Licence, P.; Dolan, A.; Welton, T.; Perkin, S.
Monolayer to Bilayer Structural Transition in Confined Pyrrolidinium-Based Ionic Liquids.
J Phys Chem Lett 2013, 4, 378-382

Smith, A. M.; Lovelock, K. R. J.; Gosvami, N. N.; Welton, T.; Perkin, S.
Quantized friction across ionic liquid thin films.
Phys Chem Chem Phys 2013, 15, 15317-15320.

Smith, A. M..; Parkes, M. A.; Perkin, S.
Molecular Friction Mechanisms Across Nanofilms of a Bilayer-forming Ionic Liquid.
J. Phys. Chem. Lett. 2014, 5, 4032-4037

Britton, J. et al.
A Graphene Surface Force Balance.
Langmuir 201430, 11485-11492.

Balabajew et al.
Contact-free calibration of an asymmetric multi-layer interferometer for the surface force balance
Review of Scientific Instruments 2017, 88, 123903

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