Many-Body Electrostatic Forces Between Colloidal Particles at Vanishing Ionic Strength
Jason W. Merrill, Sunil K. Sainis, and Eric R. Dufresne
Physical Review Letters 103 (2009) 138301
wiki entry by Emily Russell, Fall 2010
The article can be found here.
This paper reports a striking demonstration that effective pair potentials do not tell the full story in colloidal systems, and that furthermore, a constant surface potential is a better model than a constant surface charge, at least in some cases. The authors are able to observe many-body effects on the forces in systems of small numbers of colloidal particles, and use a simple Poisson-Boltzmann model to predict these effects from the pair potentials.
The experiments were carried out on 600nm PMMA colloids in nonpolar hexadecane as the solvent. NaAOT, a surfactant, was added, which forms reverse micelles, increasing the particle charge, and decreasing the screening length.
The particles were positioned using optical tweezers. Three configurations were studied: pairs of particles; an equilateral triangle of particles (each pair of which had been measured previously); and a hexagonal arrangement of seven particles. The particles were released and tracked to determine their drift velocities; individual drift velocities were converted to the velocity of the breathing mode of the system, and the force on this breathing mode determined by <math> f = k_B T v_d / D </math>. (With a moment's thought, it is not obvious that this equation should apply for many-body modes; this is addressed in an earlier paper by the group, Statistics of Particle Trajectories at Short Time-Intervals Reveal fN-Scale Colloidal Forces.)
The forces observed in isolated particle pairs are fit to obtain the surface potential and screening length. These parameters are then input to the linearized Poisson-Boltzmann equation, which is numerically solved with constant-potential boundary conditions to predict the forces in the triangular and hexagonal situations, taking into account the many-body effects. Forces were also predicted assuming only pairwise interactions, and these two predictions were compared to the measured forces.
The results are very nicely summed up in the main figure of the paper, Fig. 1.
At short screening length (high surfactant concentrations), the pairwise and many-body predictions were quite similar; for the hexagonal configuration, a small deviation was observed from the pairwise prediction. The dramatic results are obvious in the measurements at long screening length (low surfactant concentration), where the measurements are fit very well by the calculations taking into account many-body effects, the forces being substantially smaller in the triangular and hexagonal configurations than a naive pairwise calculation would predict. The authors emphasize that the potential and screening length obtained from the fits to the pair data are the only parameters input to the calculations for the triangular and hexagonal configurations.
It turns out that it is not the non-linearity of the Poisson-Boltzmann equation itself which makes many-body effects important in this system; indeed, the authors find that the linearized PB equation is sufficient to predict the forces. Instead, the surface charges of the particles change depending on the presence of other particles in order to maintain a constant surface charge. The pairwise prediction would be appropriate only if the surface charges themselves remained constant.