Rotational diffusion of partially wetted colloids at fluid interfaces

Abstract

Hypothesis Rotational Brownian diffusions of colloidal particles at a fluid interface play important roles in particle self-assembly and in surface microrheology. Recent experiments on translational Brownian motion of spherical particles at the air-water interface show a significant slowing down of the translational diffusion with respect to the hydrodynamic predictions [Boniello et al. Nat. Mat. 14 (2015) 908-11]. For the rotational diffusions of partially wetted colloids, slowing down of the particle dynamics can be also expected. Experiments Here, the rotational dynamics of Janus colloids at the air-water interface have been experimentally investigated using optical microscopy. Bright field and fluorescent microscopies have been used to measure the in-plane and out-of-plane particle rotational diffusions exploiting the Janus geometry of the colloids we fabricated. Findings Our results show a severe slowing down of the rotational diffusion Dr, connected to the contact line motion and wetting-dewetting dynamics occurring on particle regions located at opposite liquid wedges. A slowing down of the particle rotational diffusion about an axis parallel to the interfacial normal Dr,|| was also observed. Contact line fluctuations due to partial wetting dynamics lead to a rotational line friction that we have modelled in order to describe our results.

Mesoporous Silica Colloids: Wetting, Surface Diffusion, and Cationic Surfactant Adsorption

“…43 Usually a large difference is also measured between surface macro-and micro-rheology ( S = 10 10 …10 9 Pa.s.m), 44 which could be due to additional dissipations or some boundary conditions not included in the analysis of the results. 45,46 Recent experiments using noncontact microrheology, with completely wetted particles as probes, report  S 10 8 …10 7 Pa.s.m, 45 which agree with our results for CTA + Br   For CTA + OH  instead our results  S  5.10 10 Pa.s.m are comparable to passive surface microrheology using partially wetted particles as probe particles. 44 Given that the liquid-gas interfacial tensions are similar for CTA + Br   and CTA + OH  , one possible explanation of the difference in D S between the two surfactant solutions is related to the surfactant dynamics occurring at the porous particle surface.…”

“…This article is dedicated to the memory of Giancarlo Stocco (1941Stocco ( -2019. We acknowledge Labex Chemisyst (ANR-10-LABX-05-01) and the University of Montpellier for financial supports.…”

Motion of micro- and nano- particles interacting with a fluid interface

“…Note that as already stated before, a no slip boundary condition on the solid particle suppresses the rolling rotational motion, ⊥ = ∞. Finite values of ⊥ were calculated by O'Neill et al [53], who modelled rotational drags considering a finite slip length on the particle surface, which leads to ⊥ of the same order of magnitude as the bulk value for micrometric or nanometric particles [60]. Only the symmetric case = 90° at a free surface has been described by hydrodynamic models so far, and we are not aware of any predictions of ∥ and ⊥ as a function of .…”

“…In particular, vertical fluctuations are more relevant for smaller colloids (nanoparticles), when the drag induced by interface fluctuations at the contact line can overcome the known viscous drag. Expressions for the rotational drags due fluctuations of the interface at the contact line were also given by Stocco et al to explain the rotational diffusions of Janus microparticles at the liquidgas interface [60].…”

“…Rotational drag coefficients ∥ and ⊥ = ⊥ ∞ were also measured for Janus spherical particles, ≈ 65° at the waterair interface: ∥ = 2.4-12 and ⊥ = 14-87, which cannot be described by the hydrodynamic models cited before [60].…”