Colloidal behaviour in a confined cylindrical cell

The interactions of colloidal particles in a confined cylindrical cell have been investigated using a combination of video microscopy, real-time digital imaging and still micrographs. Confinement of colloidal latex particles of diameter 4.5 μm in cylindrical glass capillaries of diameter 25 μm resulted in their alignment along the centre of the cell with a clear particle-free zone along the wall. This tendency was observed over a range of ionic strengths from deionised water to 0.1 M NaCl, but was greatest at low concentrations of electrolyte. The dimensions of the particle-free zone were orders of magnitude greater than the Debye lengths of the solutions. Particles of diameter 3.0 μm also exhibited alignment, but not 1.2 μm particles, indicating that there is a fine balance between the forces promoting alignment and kinetic effects. The particles along the centre of the capillary formed dynamic chains in which an average of ∼3 particles were in contact at any time. The rate of movement of the particles increased with decreasing salt content. These observations are consistent with asymmetries in electrostatic or hydrodynamic interactions induced by the cell wall, or possibly by a combination of such effects.