Control over colloidal crystallization by shear and electric fields

Abstract

We used shear flow and an electric field to control colloidal crystallization. The structures were examined in situ with confocal microscopy. For experiments under shear, a new parallel plate shear cell was designed. It had a zero-velocity plane that was stationary with respect to the microscope. The plates were microscopy slides of a few square centimeters. They both had a maximum travel of 1 cm and for the systems that we study an oscillatory shear with such a large amplitude can be regarded as a continuous shear. The gap width was variable between 20- 200 m. The plates were parallel within 1-2 m. The flow profile of a dilute suspension was linear, but that of a crystallizing dispersion appeared to be non-linear. At the walls a sliding layer structure formed of which the local viscosity was 1.5 times smaller than that of the liquid-like structure in the middle of the gap. At low shear rates hexagonal layers of a crystal showed a zigzag motion. We also examined shear melting and crystallization. Shear melting appeared to be a local process in which the local order sometimes increased and sometimes decreased while the average order decreased. Shear induced crystallization occurred uniformly over the sample and the order increased monotonously. By spin coating colloidal crystalline films were fabricated that consisted of randomly stacked hexagonal layers. The structures could be made permanent by using a dispersing medium that could be polymerized. The interparticle spacing was not fixed, but probably depended subtly on the surface charge of the particles and the ionic strength of the medium. Different from crystals formed by sedimentation, spin coated crystals were crystalline to the top. This is one of the indications that crystal formation started at the air interface with a 2D layer that grew into a 3D multilayer structure while it was sedimenting. From these spin coated crystals freestanding colloidal crystalline films could be made. Without crack-formation either the particles or the matrix could be removed. Even deforming a freestanding crystal of silica particles in air by irradiating it with a MeV high-energy ion beam could be done without crack formation. In a high frequency alternating current electric field, particles formed strings along the field lines. By making a regular pattern on one of the electrodes of a parallel plate capacitor, a gradient in the electric field was established. Depending on the dielectric contrast between the particles and the medium the strings of particles formed either on top or in between this pattern. This gave control over structure formation along the gradient of the electric field strength. We showed that the electric field induced crystals can also be made permanent. If the matrix is elastomeric, the structure can be subsequently deformed by a shear. Combining two external fields at the same time will also be possible in our parallel plate shear cell. The setup can be adapted such that an electric field can be applied in addition to a shear flow. This opens up possibilities to make crystals with even different symmetries.