1. Micro-surface potential measurements: The repulsive forcesbetween polyelectrolyte brushes in presence of multivalentcounterionsMatthew V. Tirrell, University of California-Santa Barbara, DMR 0710521 (Materials World Network) We demonstrate a new way to determine weak repulsive forces operative between colloidal particlesby measuring the rate of slow coagulation. The rate of slow coagulation is directly related to thecompetition of the repulsion with thermal motion. Since the thermal forces are weak, measurementsof the coagulation rate by dynamic light scattering can lead to precise information on repulsive potentials having a magnitudeof just a few kT. We demonstrate this novel method by studying colloidal spherical polyelectrolytebrush particles in aqueous solution containing trivalent La3+counterions. The particlesconsist of a monodisperse polystyrene core of 121 nm radius from which linear sodium poly(styrenesulfonate) chains are densely grafted (contour length: 48 nm). We determine the rate ofcoagulation by time-resolved simultaneous static and dynamic light scattering in presence of LaCl3(0.2 mM to 150 mM). Direct measurements of the repulsive force between macroscopic brush layersdemonstrate that the potential is decaying exponentially with distance. This is in good agreementwith a simple theoretical treatment from which the effective surface potential, Ψ0, can be determined.The good agreement of data obtained by the novel microscopic method with direct macroscopicmeasurements underscores the general validity of our approach. Christian Schneider (1), Arben Jusufi (2), Robert Farina (3), Feng Li (3),Philip Pincus (3), Matthew Tirrell (3), Matthias Ballauff (1)(1) Physikalische Chemie I, University of Bayreuth, 95440 Bayreuth, Germany (2) Department of Chemical Engineering, Princeton University, Princeton, NJ 08544, USA and (3) Departments of Chemical Engineering and Materials, Materials Research Laboratory, University of California-Santa Barbara, Santa Barbara, CA 93106 USA Langmuir, in press (2008).

2. Micro-surface potential measurements: The repulsive forcesbetween polyelectrolyte brushes in presence of multivalentcounterionsMatthew V. Tirrell, University of California-Santa Barbara, DMR 0710521 (Materials World Network) The principal broad impact of this work in the first year has been the effective international cooperation between Bayreuth and Santa Barbara. Christian Schneider spent six weeks in Santa Barbara in the fall of 2007, working on surface force measurement with Rob Farina. Ballauff has visited Santa Barbara three times in the last year; Tirrell visited with Ballauff at an international meeting in Mainz in September of 2007. Out of this excellent and frequent contact came this novel measurement idea and its rapid demonstration.