Colloids have been a very useful model for description of particle interactions. With their different shapes and sizes scientists have been able to study phase behavior of particles of those shapes. Some have been studied by their magnetic dipole moment to study spin interactions. Some have been studied for their autonomous motion simply caused by a gradient of a property, namely, a phoretic mobility.
There are various phoretic mobilities that induce interesting behavior of the particles:
1) Electrophoresis – Motion caused by an electric potential gradient. Electrophoresis of particles near a boundary have applications in microfluidics. The velocity of the particle increases when it nears the wall. The governind equation for electrophoretic motion for a cylindrical particle is the Laplace equation in terms of the electric potential (Del squared operator of the potential = 0).
2) Motion caused by magnetic field gradient. The particles exhibit a magnetic moment, as previously mentioned, that depend on the intensity of the magnetic field, Bpol. When a second, driving field is imposed then the magnetic field gradient is created and induces particle motion
3) Diffusiophoresis – Motion caused by concentration gradient. Some particles are modified to have an active catalytic side. Usually the modification comes from a metal deposition of Platinum (Pt) and Palladium (Pd). The reason is that when there is contact between the metals and hydrogen peroxide, the metals catalyze the decomposition of hydrogen peroxide into water and oxygen.
These mechanisms have been studied because they can be used to make micro- and nanoscale motors. This is the long-term interest of the phoretic behavior of colloidal particles, so to speak.
1. Erber, A., Zientara, M., Baraban, L., Kreidler, C., Leiderer, P. “Various Driving Mechanisms for Generating Motion of Colloidal Particles” Journal of Physics / Condensed Matter, (20), 2008, 404215
2. House, D. L. and Luo, H. “Electrophoretic mobility of a colloidal cylinder between two parallel walls” Engineering Analysis with Boundary Elements, 34, (2010), 471–476
Luis D. Ruiz Santiago