Collective dynamics and dimensional effects of phase formation in the "aerosil - polystyrene latex" system
Purpose. In addition to characterizing the optical, electronic, mechanical, and catalytic properties of individual nanoparticles, much attention is paid to the development of methods for assembling nanoparticles into large ordered or disordered superstructures. These assembly methods are based on many different types of interparticle interactions (Van der Waals, magnetic, electrostatic, molecular dipole, covalent and hydrogen bonds). Recently, the drip method has been used to study early structural formation in colloidal systems. When particles interact in a drying drop, depletion forces must be taken into account. In this paper a model experiment has been carried out to study the effect of depletion forces on phase formation during the drying process of a drop. Methods and methodology. Colloidal suspensions of Aerosil at a concentration of 0.1 mg / ml with a particle size of 100 nm and polystyrene latex with a particle size of 20 nm at a concentration of 10 mg / ml were used as starting materials. Homogeneous colloidal suspensions of a given concentration were prepared using an ultrasonic disperser UZG-13. The particle sizes of the resulting suspensions were controlled by light scattering on a Photocor mini particle size meter. A comparative analysis of the drying process of a droplet with the initial components and with their mixture has been conducted in dynamic mode. The experiments were carried out in standard conditions. A digital optical microscope Bresser Advance ID was used to control the drying dynamics. The morphology and identification of the drying products have been carried out by a set of methods, including IR spectroscopy - Bruker VERTEX 70, scanning microscopy - Jeol JSM-6380LV and transmission microscopy - LIBRA 120 PLUS. Results. When a droplet of a mixture of aerosil and latex was dried, there was observed the formation and rapid growth of a new phase of microscopic sizes up to ten microns in a matter of tens of seconds. The color of the solution changes sharply from transparent light blue to bright blue. The formation of a new phase is localized in the central region of the drop. According to the data of IR spectroscopy and of electron and transmission microscopy, the resulting phase is crystalline SiO<sub>2</sub>. To interpret the obtained results, a computational experiment was carried out in a statistical model system of rigid non-interacting spheres in the Broun motion approximation. In the simulation the spatial redistribution of large particles in the presence of small particles is observed, leading to the occurrence of the thickenings. Phase formation is interpreted as the result of the action of the nonequilibrium depletion force under the conditions of the hydrodynamic instability of a drying drop. Conclusions. In the conditions of a model experiment on phase formation during drying of a drop of a non-interacting particles colloidal solution in the aerosil-polystyrene latex system, the formation and rapid growth of a new phase of crystalline SiO<sub>2</sub> has been detected. The phase formation process is accompanied by a sharp change in the color of the solution from light blue to blue. The crystallite size varies from ten nanometers to ten microns. A diffraction pattern of the new phase has been found indicating its crystalline nature.