3D real space measurements on colloidal silica rods in a gravitational field

Abstract

The phase behaviour of rod-like colloidal particles with aspect ratios ( L/D ) of 4.7, 5.4 and 5.8 was studied in 3D real space using fluorescent silica particles and confocal microscopy. Full positional and orientational coordinates of an equilibrium sedimentation profile of L/D = 5 . 4 particles was obtained, allowing access to 3D density and order parameter profiles. From the density profile, the equation of state was calculated as well as an in-situ estimation of the grav- itational length based on the barometric region. Fewer phases and lower coexistence densities were observed than were predicted by computer simulations of hard sphereocylinders at this aspect ratio. This is ascribed to the polydispersity ( ∼ 10% in length and diameter) of the sys- tem and the electrical double layer around the particles resulting in a change in their effective dimensions. Attempts were made to correct for these charge effects resulting in some qualitative agreement with simulations in terms of the observed phases and coexistence densities. Evidence was found for the result predicted by theory and simulations that the nematic order parameter profile trails the density profile when entering the isotropic phase, with good magnitude agree- ment. Preliminary results on L/D = 5 . 8 particles with lower polydispersity suggest that the nematic phase may be observable in this system.