Home

Two-Particle Dynamics on an Electrode in ac Electric Fields

Junhyung Kim

(Advisors: John L. Anderson and Stephen Garoff)

Our goal is to investigate mechanisms of aggregating or separating particles deposited on an electrode in ac fields. Negatively charged particles sitting on an electrode either aggregate or separate in ac fields depending on frequencies. At lower frequencies such as 50Hz, 100Hz, and 200Hz, two particles aggregate make a doublet. At higher frequencies over 1,000Hz, two particles separate from each other.
 

Aggregation of particles at 100Hz

t = 0 sec t = 20 sec t = 50 sec
 

Separation of particles at 1,000Hz
t = 0 sec t = 60 sec t = 120 sec
 

Approach

Relative motions of pairs of particles (9.7mm in diameter) deposited on an electrode in ac fields at various frequencies are tracked by optical microscopy and the trajectories of the pairs are determined by automated image analysis.

Interesting features of the aggregation at lower frequencies

First, when the center-to-center separation of a pair is initially 6 particle radii or more apart, a transient "incubation" period of tens of seconds is observed before the particles begin to move toward each other. Second, the two particles never come into contact, rather at long times the pair maintains a stationary gap between them equal to about one-half the particle radius. This stationary gap between particles is also observed for the aggregation of clusters of three or more particles. Finally, the rate of approach for a pair of particles as well as its direction decrease as the frequency increase.

Trajectories of pairs of particles at 100Hz Frequency dependence
References

1 "Two-Particle Dynamics on an Electrode in ac Electric Fields", J. Kim, S. A. Guelcher, S. Garoff, and J. L. Anderson, Advances in Colloid and Interface Science (2001, In press)