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Research

Our Research

Colloidal suspensions and their interfaces are ubiquitous in nature and have attracted the interest of scientists for many centuries. Colloidal systems are highly interesting and relevant, as they find numerous applications in several industrial branches. In addition, colloids are widely accepted as a versatile model system for atoms and molecules as their rich phase behaviour is analogous to that of atomic and molecular systems. The typical colloidal length and time scales make it possible to directly observe colloidal particles in real-space and real-time using video and confocal microscopy. Advanced colloid chemistry techniques are available to tune the chemical and physical properties of the particles or even to develop completely new and unique colloidal model systems.

Optical Trapping

stirrer
A pentagon of three micron polystyrene particles are optically trapped, and formed into a colloidal stirrer, designed to introduce structural defects in dense 2D suspensions. Time-shared AOD spots are used to drive the rotation. (image by Michael Juniper)

An optical tweezer is a strongly focussed laser beam that can trap small objects, such as colloids, using the forces that are exerted by the light. The scattering forces push the particles down and the gradient forces pull the particle towards the center of the beam. Combining optical tweezers and colloidal systems facilitates the ability to control, manipulate and deform colloidal systems on the microscopic, i.e. single-particle level.

Further reading:

  • D. G. Grier, Nature 424, 810 (2003)
  • D. L. J. Vossen et al, Rev. Sci. Instrum. 78, 2960 (2004)
  • M. J. Lang, S. M. Block, Am. J. Phys. 71, 201 (2003)

Grain Boundaries

A 2D colloidal crystal, exhibiting grain boundaries and impurities.

The strength of materials is closely related to the grain size of the material. However, grain boundary stability is still far from understood. Using geometrical frustration, crystals which are rich in grain boundaries can be prepared. By studying the structural and dynamical behaviour of both colloidal single crystals and crystal imperfections insight will be gained into the relation between frustration and the stability of grain boundaries.

Magneto-rheological Fluids

Colloidal ferro-fluid.
A system of non-magnetic and super-paramagnetic particles embedded in a ferro-fluid. One set of particles acts as magnetic holes, the other as magnetic excesses. They act as sets of dipoles pointing in opposite direction resulting in novel structures distinct from the structures observed in a single component system.

Magneto-rheological fluids (MR fluids) are, in the simplest sense, suspensions whose flow properties respond to the application of a magnetic field. Using a colloidal suspension of particles with embedded magnetic material we study an example of this type of fluid.  When an external magnetic field is applied the domains in these particles align and a dipole moment is induced in the particles. The particles then proceed to attract each other, joining end to end and chains of them aggregating laterally.

Depending on the concentration and detail of the composition of the suspension, how the field is applied and the time scales over which you look different and fascinating structures and dynamics are observed. Macroscopic structures are dictated by the meso-scale and particle level mechanisms that form them. Linking and understanding of these 2 length scales is of great interested.

Further reading:

  • A.T.Skjeltorp, Journal of Applied Physics 57, 3285(1985).
  • M.Fermigier and A.P.Gast, Journal of Colloid and Interface Science 154, 522(1992).

Colloidal Synthesis

Well defined colloidal model systems are of utmost importance in the study of colloidal dispersions.  Using colloid chemistry the chemical and physical properties of the colloidal model system can be precisely adjusted to the physical experiments in mind.  Therefore, the synthesis and characterization of colloidal model systems plays a central role in our research.   A wide variety of chemical techniques are available to synthesize many different colloidal particles with very specific properties.

For example, we can use a variety of different materials, such as latex or silica.  Additionally, we can adjust particle shape–beyond simple microspheres–and fabricate rods, needles, and spherical caps.  Particles can be made to be magnetically responsive and also fluorescently labeled for easier imaging.  Also the specific interactions between colloidal particles can be controlled using surface chemistry.  To characterize the particles several techniques such as light scattering, various types of optical microscopy, and electron microscopy are used.

Further reading:

  • Alfons van Blaaderen, Colloids get complex, Nature (News and Views) 439, 545 (2006)
  • Roel P.A. Dullens, Colloidal hard spheres: Cooking and Looking, Soft Matter, 2, 805 (2006

Microfluidics

The field of microfluidics can be defined as the study of the behaviour and manipulation of fluids geometrically confined in artificial microsystems, with typical lengthscales ranging from 1 μm to 1 mm. The devices are fabricated from polydimethylsiloxane (PDMS) using soft lithographic techniques in our clean room.

Part of our research focuses on using microfluidic devices with microsyringe pumps to create highly monodisperse Pickering emulsions. We also use microfluidic devices to study the confinement effects on colloids, such as the fd virus.

Department of Chemistry, University of Oxford