M. G. Nikolaides, A. R. Bausch, M. F. Hsu, A. D. Dinsmore, M.P. Brenner, C. Gay, and D. A. Weitz. 2002. “
Electric-field-induced capillary attraction between like-charged particles at liquid interfaces.” Nature, 420, 6913, Pp. 299–301.
AbstractNanometre- and micrometre-sized charged particles at aqueous interfaces are typically stabilized by a repulsive Coulomb interaction. If one of the phases forming the interface is a nonpolar substance ( such as air or oil) that cannot sustain a charge, the particles will exhibit long-ranged dipolar repulsion(1); if the interface area is confined, mutual repulsion between the particles can induce ordering(2) and even crystallization(3,4). However, particle ordering has also been observed in the absence of area confinement(5), suggesting that like-charged particles at interfaces can also experience attractive interactions(6). Interface deformations are known to cause capillary forces that attract neighbouring particles to each other, but a satisfying explanation for the origin of such distortions remains outstanding(7,8). Here we present quantitative measurements of attractive interactions between colloidal particles at an oil - water interface and show that the attraction can be explained by capillary forces that arise from a distortion of the interface shape that is due to electrostatic stresses caused by the particles' dipolar field. This explanation, which is consistent with all reports on interfacial particle ordering so far, also suggests that the attractive interactions might be controllable: by tuning the polarity of one of the interfacial fluids, it should be possible to adjust the electrostaticstresses of the system and hence the interparticle attractions.
Shang-You Tee, P.\hspace0.167emJ. Mucha, Luca Cipelletti, S. Manley, M.\hspace0.167emP. Brenner, P.\hspace0.167emN. Segre, and D.\hspace0.167emA. Weitz. 2002. “
Nonuniversal Velocity Fluctuations of Sedimenting Particles.” Physical Review Letters, 89, 5.
AbstractVelocity fluctuations in sedimentation are studied to investigate the origin of a hypothesized universal scale [P. N. Segre, E. Herbolzheimer, and P. M. Chaikin, Phys. Rev. Lett. 79, 2574 (1997)]. Our experiments show that fluctuations decay continuously in time for sufficiently thick cells, never reaching steady state. Simulations and scaling arguments suggest that the decay arises from increasing vertical stratification of particle concentration due to spreading of the sediment front. The results suggest that the velocity fluctuations in sedimentation depend sensitively on cell geometry.