Measurements of dispersion forces between colloidal latex particles with the atomic force microscope and comparison with Lifshitz theory

Abstract

Interaction forces between carboxylate colloidal latex particles of about 2 μm in diameter immersed in aqueous solutions of monovalent salts were measured with the colloidal probe technique, which is based on the atomic force microscope. We have systematically varied the ionic strength, the type of salt, and also the surface charge densities of the particles through changes in the solution pH. Based on these measurements, we have accurately measured the dispersion forces acting between the particles and estimated the apparent Hamaker constant to be (2.0 ± 0.5) × 10(-21) J at a separation distance of about 10 nm. This value is basically independent of the salt concentration and the type of salt. Good agreement with Lifshitz theory is found when roughness effects are taken into account. The combination of retardation and roughness effects reduces the value of the apparent Hamaker constant and its ionic strength dependence with respect to the case of ideally smooth surfaces.

Dispersion forces acting between silica particles across water: influence of nanoscale roughness

“…One approach relies on the Derjaguin approximation, and averaging the effect of roughness over a distribution of pillars. 8,35 This approach predicts that roughness does substantially reduce the strength of dispersion forces. This effect can be simply understood since intervening voids within the rough surface remain present at contact.…”

“…8,17,18 The situation is particularly disturbing for the relevant case of silica (SiO 2 ) interacting across water. For this system, some experiments suggest dispersion forces that are comparable to (or even stronger than) the ones predicted theoretically.…”

“…[5][6][7] The presence of ionic charges may reduce the magnitude of dispersion forces, even though this effect is relatively minor. 1,4,8 Dispersion forces control particle aggregation, particle deposition, or flotation phenomena. 1,2,9,10 More recently, they received renewed interest in the development of microelectromechanical and nanoscale systems, or low friction devices.…”

Orthokinetic aggregation of charged colloidal particles in the presence of repulsive double layer force: A trajectory analysis with the solution of non-linear Poisson–Boltzmann equation

“…In our consideration based on the comparison between experimental and theoretical rates, we may consider that the van der Waals attraction is increasingly screened by the charge of ions with increasing salt concentration. Such screening effect on the van der Waals attraction can be included in the calculation [24,47]. A study using the multi-particle colloidal probe technique [47] reported that such effect is not large as much as that we observed in coagulation kinetics.…”

Interactions between charged particles with bathing multivalent counterions: experiments vs. dressed ion theory

“…Note that the screening of the zero Matsubara term has only a limited effect on the effective Hamaker constant. The difference between the effective Hamaker constant for polystyrene particles across water at the low salt (1 mM) and high salt (1 M) is less than 20 % at relevant separation distances [73,74].…”