Colloidal crystal

The optical properties of colloidal particles that are used as tracers in microrheology experiments or as model “atoms” in real-space studies of phase behavior must find a difficult balance: the particles must be nearly index-matched in the suspension medium so that they do not scatter too strongly, but they must also be individually visible under optical or confocal microscopy so that their positions can be tracked to high precision (nanometers).

New types of micrometer-scale particles with engineered scattering properties were synthesized. [1] These particles are composed of small fluorescent polystyrene cores and large crosslinked poly(N-isopropylacrylamide) (polyNIPAM) shells. Immersed in water at ambient temperature, the shell of these particles is swollen, inducing that optical properties (absorbed, scattered light) were controlled only by the small polystyrene core limiting interference effects between scattered waves from different particles. Raising the temperature to transition temperature leads to a decrease of the thermosensitive shell volume inducing strong variations in the optical properties of the core-shell colloids.

To illustrate that these particles can be used as model colloids for real-space studies, we prepared colloidal crystals of the core-shell particles through a depletion process. Self-assembly of these colloids as characterized by confocal microscopy are studied.

[1] A. Perro, G. Meng, J. Fung and V. N. Manoharan, Langmuir, 25 (19), 11295-11298 (2009).

[2] G. Meng, V. N. Manoharan and A. Perro, Soft Matter, 13, 6293-6296 (2017).