Progress in study of the microscopic approach to determine the colloidal stability
All the characteristics of colloidal systems change remarkably in the transition from the dispersed to the aggregated state. Even within aggregated systems, the degree of aggregation varies tremendously. The question of how to determine the state of colloidal systems is of central importance to predicting and controlling the stability of colloidal suspensions. Traditionally, turbidity measurements, low angle light scattering, and dynamic light scattering are used to determine the coagulation rates and therefore the stability ratio. All these methods require information on the long-term, accumulated effects of the motions and interactions of huge numbers of particles; thus, we can reasonably call these a “macroscopic approach”.
Since the colloidal stability is essentially determined by motions and interactions of its component microscopic particles it would be ideal to study the stability at microscopic particle levels. However, Brownian motions make particle collisions take place at unpredictable times and locations; therefore one has no way to employ a microscope to investigate particle collisions and its outcomes. Prof. Zhiwei Sun and his colleagues of our institute, in cooperation with University of Science and Technology of China, have successfully developed a novel approach to the investigation of colloidal aggregation in experiments performed at microscopic particle levels. by means of artificially induced particle collisions with the aid of optical tweezers.. In this approach the optical tweezers were used to catch two particles and bring them together for a collision to take place in an area viewable by a microscope. Observing this “artificially induced collision,” it is possible to check how particle pairs interact with each other and what occurs after their release from the trap: whether they stick together or separate. And then the colloidal stability can statistically be evaluated based on a series of such tests according to a physical model calculation.
In contrast with commonly used approaches and Zeta potential measurement, this microscopic approach has the capability to treat not only electrostatically but also sterically stabilized systems. In addition, a visual understanding of the interaction between particles and of the aggregation phenomenon can be obtained. Furthermore, the modeling analysis and associated experiment provide useful information for gaining insight into the collision-reaction process of the two-particle system confined in an optical trap.
The papers associated with this work have been published in Journal of Chemical Physics (2005, 122, 184904 and 2003, 119, 2399.).