We are an experimental research group exploring a wide variety of physical phenomena involving liquids and soft matter at interfaces and confined to thin films. Our particular expertise is with the surface force balance (SFB) with which we study mechanical, optical, electrical and dynamic properties of films confined between two smooth surfaces. In the SFB, the confining surfaces can be insulators (e.g. atomcally smooth mica crystals) or electrodes. Confined liquids arise in many natural and technological scenarios (from the extracellular matrix to the interior of a supercapacitor electrode…).  Material properties are often drastically altered in confined geometry compared to in the bulk. Understanding and controlling these phenomena provide us with many fascinating challenges. The outcomes are of direct relevance in many fields of application from energy storage to bio-materials to engineering fluids

Illustration of the arrangement of ions in an ionic fluid confined between symmetrically charged walls. The near-surface layer can over-screen the charge, resulting in an oscillation in charge density in the direction perpendicular to the walls persisting for several molecular diameters.


Perkin, S.; Albrecht, T.; Klein, J.
Layering and shear properties of an ionic liquid, 1-ethyl-3-methylimidazolium ethylsulfate, confined to nano-films between mica surfaces.
Phys Chem Chem Phys 2010, 12, 1243-1247.

Perkin, S.; Crowhurst, L.; Niedermeyer, H.; Welton, T.; Smith, A. M.; Gosvami, N. N.
Self-assembly in the electrical double layer of ionic liquids.
Chem Commun 2011, 47, 6572-6574.

Perkin, S.
Ionic liquids in confined geometries.
Phys Chem Chem Phys 2012, 14, 5052-5062.

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.; Lee A.; Perkin S.      Switching the Structural Force in Ionic Liquid-Solvent Mixtures by Varying Composition 

Phys. Rev. Lett. 2017, 118, 096002


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.  In technology electrolytes are also important, with a key example being the finely-tuned electrolytes used in batteries and supercapacitors.

Classical physical chemistry of electrolytes explains well 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.

Right: A typical profile of the force between charged plates (here mica) across a highly concentrated electrolyte (here the ionic liquid [C4C1Pyrr][NTf2]). An exponentially decaying force is apparent to several 10’s of nanometers, equivalent to several 10’s of ion diameters. The inset shows an expanded view of the oscillatory structural force region on a linear scale.


Cartoon depiction of a highly concentrated electrolyte: positive ions, negative ions and solvent molecules are shown with such high ion density that solvent cannot fully surrounded ('solvate') the ions.


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.M.; Perkin S.
Underscreening in concentrated electrolytes
Faraday Discuss., 2017, 119, 239-259

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


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.

Diagram of the friction measurement: fluid is confined between two smooth surfaces and the thickness of the liquid is measured (using interferometry) with accuracy and precision better than one molecular diameter. The load is applied in the normal direction and the lateral velocity are controlled. The resulting lateral force is measured via the deflection of a spring attached to the lower surface.


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

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.

Example of the  lateral motion applied to one surface (top) and the measured friction force transmitted through the fluid film and measured as a lateral displacement of the bottom surface (bottom).  In this example the surfaces move together until sufficient lateral force is applied for the film to yield, after which there is a stick-slip motion. When the applied motion reverses direction the response also reverses. 



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 thin 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.

SEM image of few-layer graphene deposited onto copper by chemical vapour deposition. Few-layer graphene and single-layer graphene samples are transferred onto optical lenses for force measurements and electrical measurements using the SFB.


Perkin, S. et al.
Forces between mica surfaces, prepared in different ways, across aqueous and nonaqueous liquids confined to molecularly thin films.
Langmuir 2006, 22, 6142-6152.

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

van Engers et al.
Direct Measurement of the Surface Energy of Graphene
Nano Letters  2017, 17, 3815-3821

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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