 Professor J. Klein
Physical & Theoretical Chemistry Laboratory
Telephone: +44 (0) 1865-275401
Our group studies the physics and physical chemistry of soft matter,
including simple liquids, polymers and bio-macromolecules. In particular,
we study their behaviour and interactions at surfaces, interfaces
and in confined geometries. We employ a variety of experimental
techniques, with emphasis on direct ångstrom level measurements
of forces between atomically-smooth surfaces, and between adsorbed
or confined molecules. We have developed force-measuring methods
with resolution and sensitivity some 3 - 4 orders of magnitude better
(per molecule) than scanning probe techniques (e.g.AFM), and use
these to probe both dynamics and equilibrium properties. Some recent
projects are outlined below.
1. Liquids confined to nanometre-sized pores
The properties of liquids confined to gaps or pores of molecular
dimensions are very different to those in bulk, due to the different
balance between entropy and enthalpy in ultra-small volumes, and
to the effects of surface fields. This is especially important for
understanding lubrication phenomena and properties of thin films
or membranes. Using a surface-force balance (SFB) we probe directly
the liquid-to-solid transitions induced by increasing confinement,
and the solid-like properties of such constrained 'liquids'.
2. Wetting and dewetting
Liquid films on non-wetting substrates are common in areas from
detergency to microlithography and in devices where optically- or
electronically-active organic films are used. Such films may spontaneously
rupture and de-wet as a results of long-ranged van der Waals fields.
In the case of liquid mixtures phase separation may additionally
occur, modifying the film breakup and its final structure. Using
ellipsometry, computerised video-microscopy and nuclear-reaction
analysis we investigate the behaviour of model polymeric liquids
- where molecular time and spatial scales are conveniently enhanced
- and relate them to microscopic models.
3. Nanotribology
The study of friction at the nanometre scale (nanotribology) forms
an important part of our research. Using SFBs we investigate the
way in which polymer molecules can modify friction between surfaces
to which they are attached, of direct relevance to biolubrication
where (polymeric) articular cartilage surfaces rub past each other.
We particularly examine at the molecular level the role of configurational
entropy (and the resulting steric interactions) in reducing friction
between polymer layers, and the interactions between charged molecules
and of counterions which are ubiquitous in biological systems.
4Surfactants and boundary lubrication
Surfactants are ubiquitous
in both technological applications and in living
systems, where they act to compatibilise surfaces and also as
boundary lubricants. We use custom-synthesized surfactants to
examine, with unique sensitivity, the interactions
and particularly the mechanism of boundary lubrication
in aqueous systems, using at the same time neutron
reflectometry (with R.K. Thomas of the department) to determine
their molecular-level structure on the surfaces.
5. Chiral recognition in biomaterials
Recent evidence suggests that cell-surface interactions may be
controlled at the level of chiral recognition a cell may adhere
to a given surface but not to its enantiomorphic counterpart. To
get better insight into this we are studying, using ultra-sensitive
SFB's, the interactions of cell-coating biopolymers with different
chiral groups.
Selected recent publications
- 1. Klein, J. and Kumacheva, E., 'Simple liquids confined
to molecularly thin layers. I: Confinement-induced liquid-to-solid
phase transitions', J. Chem. Phys. 108, 6996 - 7009 (1998)
- Raviv,
U., Laurat, P. and Klein, J., ‘Fluidity of water
confined to subnanometre films’ – Nature, 413, 51-54
(2001)
- Raviv, U. and Klein, J., ‘Fluidity of bound hydration
layers’ – Science 297, 1540-1543 (2002)
- Tadmor, R.,
Janik, J., Fetters, L.J. and Klein, J., ‘Sliding
friction with polymer brushes’, Phys. Rev. Lett. 91 (11)
115503 (2003)
- Raviv, U., Giasson, S., Kampf, N., Gohy, J-F.,
Jerome, R. and Klein, J., ‘Lubrication by charged polymers’,
Nature, 425, 163-165 (2003)
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