Abstract
HypothesisRotational 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., 2015). For the rotational diffusions of partially wetted colloids, slowing down of the particle dynamics can be also expected. ExperimentsHere, 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. FindingsOur 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.
Highlights
A solid particle straddling an interface should not be seen as a geometrical object partially immersed in a liquid but rather as an intriguing partial wetting configuration
This configuration, should not be considered static since both the colloidal particle and the contact line are subjected to thermal agitation, which results in the particle Brownian motion and in the displacement of contact line back and forth around some equilibrium position [2]
As pointed out in Boniello et al for the slowing down of the translational diffusion of bare particles at the air-water interface, our experimental results cannot be described by current hydrodynamic models that assume a flat and static fluid interface [2]
Summary
A solid particle straddling an interface should not be seen as a geometrical object partially immersed in a liquid but rather as an intriguing partial wetting configuration. The fluid interface deformation due to gravity or other external fields is usually weak and the solid particle immersion is set by an equilibrium contact angle in analogy with the partial wetting of a sessile drop on a solid substrate [1]. This configuration, should not be considered static since both the colloidal particle and the contact line are subjected to thermal agitation, which results in the particle Brownian motion and in the displacement of contact line back and forth around some equilibrium position [2]. The roles of partial wetting dynamics and contact line pinning (neglected in the previous hydrodynamic models) have been pointed out to explain both the slow particle motion across the interface and the particle translational diffusion parallel to the interface [18,19][20][2]
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