Brownian diffusion of a partially wetted colloid

Boniello, Giuseppe; Blanc, Christophe; Fedorenko, D.; Medfai, Mayssa; Mbarek, Nadia Ben; Martin, Ivar; Gross, Michel; Stocco, Antonio; Nobili, Maurizio

doi:10.1038/nmat4348

Cited by 107 publications

(149 citation statements)

References 34 publications

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“…For spherical colloids approximately one µm in diameter at oil/water interfaces, measurements of the surface diffusivity and drag from the Stokes-Einstein equation are in agreement [14,15] with continuum theory. But for these same particles at an air/water interface or nanometer sized particles at an oil/water interface, the surface diffusivity is smaller than would have been predicted from the Stokes Einstein equation using the continuum surface drag, or, paradoxically decreases with crossing into the gas or less viscous phase, becoming smaller than the bulk diffusivity [16][17][18][19][20]. Several mechanisms have been proposed to explain the unexpectedly low surface diffusivities.…”

mentioning

confidence: 92%

“…Obviously, surfactant contaminants could give rise to viscous surface shear and Marangoni forces which increase the surface drag. However, in diffusion experiments in which tension measurements indicate a relatively clean interface, studies suggest that thermal fluctuations at the contact line can create forces on the particle [20]. In particular, the surface of colloids are typically not smooth, and the fluid interface can become pinned at heterogeneities causing hysteresis in the measurement of the contact angle [21].…”

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confidence: 99%

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Diffusivity and hydrodynamic drag of nanoparticles at a vapor-liquid interface

Koplik

Maldarelli

2017

Phys. Rev. Fluids

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Measurements of the surface diffusivity of colloidal spheres translating along a vapor/liquid interface show an unexpected decrease in diffusivity, or increase in surface drag (from the Stokes-Einstein relation) when the particles situate further into the vapor phase. However, direct measurements of the surface drag from the colloid velocity due to an external force find the expected decrease with deeper immersion into the vapor. The paradoxical drag increase observed in diffusion experiments has been attributed to the attachment of the fluid interface to heterogeneities on the colloid surface, which causes the interface, in response to thermal fluctuations, to either jump or remain pinned, creating added drag. We have performed molecular dynamics simulations of the diffusivity and force experiments for a nanoparticle with a rough surface at a vapor/liquid interface to examine the effect of contact line fluctuations. The drag calculated from both experiments agree and decrease as the particle positions further into the vapor. The surface drag is smaller than the bulk liquid drag due to the partial submersion into the liquid, and the finite thickness of the interfacial zone relative to the nanoparticle size. Contact line fluctuations do not give rise to an anomalous increase in drag.When a colloid particle breaches the interfacial zone between two adjoining immiscible fluid phases the interfacial energy of the particle changes, because the area of the fluid interface decreases and the contact areas of the particle with the bounding fluid phases are altered. For a particle which only partially wets both adjoining fluids, the change in interfacial energy is at a minimum when the colloid partially straddles both adjoining phases. For example, for a spherical colloid of radius R at a vapor/liquid interface of interfacial tension γ (Fig. 1a), the minimum free energy relative to the vapor phase is [1] ∆F = −πγR 2 (1 + cos θ) 2 where the contact angle θ is measured through the liquid. For particles of large enough size, ∆F can overwhelm the typical thermal energy k B T and the particle becomes trapped, as thermal fluctuations cannot dislodge the particle from the interface. Monolayers of strongly adsorbed colloids at the fluid interfaces of foams and emulsions provide steric barriers to coalescence of the dispersed phase, and find applications as foam and emulsion stabilizers (e.g. Pickering emulsions). Particle stabilized foams and emulsions are also used for the fabrication of colloid-based solid foams, gels, and bijels, and crystalline monolayers find applications as superhydrophobic or antireflection coatings, and templates for micro and nanostructured materials [2]. Central to these applications is the surface organization of the colloids, which is a balance between interparticle attractive and repulsive forces, external forces applied parallel to the interface, and the viscous resistance or surface drag due to the hydrodynamic motion of the colloids along the fluid surface. As we explain, the surface drag is not w...

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confidence: 92%

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confidence: 99%

Diffusivity and hydrodynamic drag of nanoparticles at a vapor-liquid interface

Koplik

Maldarelli

2017

Phys. Rev. Fluids

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“…Recent experimental measurements of the (in-plane) diffusivity of microparticles at interfaces by Boniello et al 31 revealed that hydrodynamic drag models cannot explain the dissipative forces experienced by a particle straddling a liquid-fluid interface at different contact angles. Boniello et al proposed that thermal motion of the liquid interface can induce a dominant contribution to (in-plane) dissipative forces that can be estimated via the fluctuation-dissipation theorem.…”

Section: Introductionmentioning

confidence: 99%

Colloidal particle adsorption at liquid interfaces: capillary driven dynamics and thermally activated kinetics

Rahmani¹,

Wang²,

Manoharan³

et al. 2016

Soft Matter

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The adsorption of single colloidal microparticles (0.5-1 µm radius) at a water-oil interface has been recently studied experimentally using digital holographic microscopy [Kaz et al., Nat. Mater., 2012, 11, 138-142]. An initially fast adsorption dynamics driven by capillary forces is followed by an unexpectedly slow relaxation to equilibrium that is logarithmic in time and can span hours or days. The slow relaxation kinetics has been attributed to the presence of surface "defects" with nanoscale dimensions (1-5 nm) that induce multiple metastable configurations of the contact line perimeter. A kinetic model considering thermally activated transitions between such metastable configurations has been proposed [Colosqui et al., Phys. Rev. Lett., 2013, 111, 028302] to predict both the relaxation rate and the crossover point to the slow logarithmic regime. However, the adsorption dynamics observed experimentally before the crossover point has remained unstudied. In this work, we propose a Langevin model that is able to describe the entire adsorption process of single colloidal particles by considering metastable states produced by surface defects and thermal motion of the particle and liquid interface. Invoking the fluctuation dissipation theorem, we introduce a drag term that considers significant dissipative forces induced by thermal fluctuations of the liquid interface. Langevin dynamics simulations based on the proposed adsorption model yield close agreement with experimental observations for different microparticles, capturing the crossover from (fast) capillary driven dynamics to (slow) thermally activated kinetics.

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“…Other work examines the effect of pinning on particle dynamics lateral to an interface. Recent experimental studies by Boniello et al [13] indicate that the lateral diffusion of colloidal particles at a fluid interface is likely slowed by transient pinning events. Sharifi-Mood and coworkers [14] showed that strong pinning can locally distort the interface around a colloidal particle, affecting how particles migrate on a curved surface.…”

Section: Introductionmentioning

confidence: 99%

Contact-line pinning controls how quickly colloidal particles equilibrate with liquid interfaces

et al. 2016

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Previous experiments have shown that spherical colloidal particles relax to equilibrium slowly after they adsorb to a liquid-liquid interface, despite the large interfacial energy gradient driving the adsorption. The slow relaxation has been explained in terms of transient pinning and depinning of the contact line on the surface of the particles. However, the nature of the pinning sites has not been investigated in detail. We use digital holographic microscopy to track a variety of colloidal spheres-inorganic and organic, charge-stabilized and sterically stabilized, aqueous and non-aqueous-as they breach liquid interfaces. We find that nearly all of these particles relax logarithmically in time over timescales much larger than those expected from viscous dissipation alone. By comparing our results to theoretical models of the pinning dynamics, we infer the area per defect to be on the order of a few square nanometers for each of the colloids we examine, whereas the energy per defect can vary from a few kT for non-aqueous and inorganic spheres to tens of kT for aqueous polymer particles. The results suggest that the likely pinning sites are topographical features inherent to colloidal particles-surface roughness in the case of silica particles and grafted polymer "hairs" in the case of polymer particles. We conclude that the slow relaxation must be taken into

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Brownian diffusion of a partially wetted colloid

Cited by 107 publications

References 34 publications

Diffusivity and hydrodynamic drag of nanoparticles at a vapor-liquid interface

Diffusivity and hydrodynamic drag of nanoparticles at a vapor-liquid interface

Colloidal particle adsorption at liquid interfaces: capillary driven dynamics and thermally activated kinetics

Contact-line pinning controls how quickly colloidal particles equilibrate with liquid interfaces

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