Molecular simulation of translational and rotational diffusion of Janus nanoparticles at liquid interfaces

Rezvantalab, Hossein; Drazer, Germán; Shojaei-Zadeh, Shahab

doi:10.1063/1.4904549

Cited by 29 publications

(21 citation statements)

References 79 publications

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“…Recent numerical studies have characterized the diffusion of homogneous 40 and Janus 19 nanoparticles adsorbed at fluid-fluid interface. On the other hand, very few theoretical predictions on the rotational motion even of bare spherical particles partially immersed in a liquid can be found in the literature.…”

Section: Interfacial Rotational Frictionsmentioning

confidence: 99%

“…These findings shed light on the fundamental aspects which need to be clarified in order to understand the behavior of Janus particles both in passive and active conditions. Therefore our results will serve as a starting point for more detailed numerical as well as experimental studies aiming at understanding how the activity of the particle breaks the equilibrium picture as well as how the details of fluid rearrangement close to particle surface 19 can modulate the overall transport properties of the particle.…”

Section: Introductionmentioning

confidence: 97%

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Wetting and orientation of catalytic Janus colloids at the surface of water

et al. 2016

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Janus colloidal particles show remarkable properties in terms of surface activity, self-assembly and wetting. Moreover they can perform autonomous motion if they can chemically react with the liquid in which they are immersed. In order to understand the self-propelled motion of catalytic Janus colloids at the air-water interface, wetting and the orientation of the catalytic surface are important properties to be investigated. Wetting plays a central role in active motion since it determines the contact between the fuel and the catalytic surface as well as the efficiency of the transduction of the chemical reaction into motion. Active motion is not expected to occur either when the catalytic face is completely out of the aqueous phase or when the Janus boundaries are parallel to the interfacial plane. The design of a Janus colloid possessing two hydrophilic faces is required to allow the catalytic face to react with the fuel (e.g. HO for platinum) in water and to permit some rotational freedom of the Janus colloid in order to generate propulsion parallel to the interfacial plane. Here, we discuss some theoretical aspects that should be accounted for when studying Janus colloids at the surface of water. The free energy of ideal Janus colloidal particles at the interface is modeled as a function of the immersion depth and the particle orientation. Analytical expressions of the energy profiles are established. Energetic aspects are then discussed in relation to the particle's ability to rotate at the interface. By introducing contact angle hysteresis we describe how the effects of contact line pinning modifies the scenario described in the ideal case. Experimental observations of the contact angle hysteresis of Janus colloids at the interface reveal the effect of pinning; and orientations of silica particles half covered with a platinum layer at the interface do not comply with the ideal scenarios. Experimental observations suggest that Janus colloids at the fluid interface behave as a kinetically driven system, where the contact line motion over the defects decorating the Janus faces rules the orientation and rotational diffusion of the particle.

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Section: Interfacial Rotational Frictionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 97%

Wetting and orientation of catalytic Janus colloids at the surface of water

et al. 2016

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“…The drag on smaller, nanometer-sized colloidal particles has been addressed theoretically, using molecular dynamics (MD) simulations of spherical rigid or structureless particles at an atomistically fluctuating liquid interface to calculate the surface diffusivity from the mean square displacement. For liquid/vapor interfaces in Lennard-Jones (LJ) systems [7][8][9], the expected increase of surface diffusivity with displacement into the vapor is found, and for a water/polydimethylsiloxane interface [10] the diffusivity was intermediate between the simulated bulk diffusion coefficients.…”

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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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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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“…113 Further extensive theoretical simulations and experimental observations indicate that the interfacial activity of the Janus NPs can vary depending on several parameters, including the shape, size, morphology, and distribution of the spatial domains, and that the Janus NPs show an enhanced interfacial activity compared to the corresponding homogeneous particles, regardless of the synthesis and interfacial activity characterization methods. [114][115][116][117][118][119][120][121][122][123][124][125][126][127][128] In analogy to the emulsification of fluid mixtures, Janus NPs are also expected to strongly attach to the interface inside polymer matrices, either in polymer blends or in block copolymers, which will be discussed in sections 3 and 4, respectively. A thorough study of the interfacial behavior of Janus NPs at the fluid-fluid interface is not only essential for further practical application of Janus NPs as solid stabilizers, but also helpful for fundamentally understanding how Janus NPs interact with polymeric interfaces, even though polymer interfaces cannot be simply understood as a fluid-fluid interface.…”

Section: Interfacial Properties Of Janus Nanoparticles At Fluid-fluidmentioning

confidence: 99%