Particle Shape Anisotropy in Pickering Emulsions: Cubes and Peanuts

Folter, Julius W. J. de; Hutter, Eline M.; Castillo, Sonja I. R.; Klop, Kira E.; Philipse, Albert P.; Kegel, Willem K.

doi:10.1021/la402427q

Cited by 129 publications

(77 citation statements)

References 45 publications

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“…For cubes with cos θ ¼ 0.3, which induce negligible deformations, we do not expect capillary interactions, in agreement with experiments of cubes with cos θ ≈ 0.3 and L ≈ 1 μm [46]. Cubes with such a contact angle tend to assemble into tetragonal, possibly closedpacked structures [47].…”

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confidence: 87%

Self-Assembly of Cubes into 2D Hexagonal and Honeycomb Lattices by Hexapolar Capillary Interactions

2016

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Particles adsorbed at a fluid-fluid interface induce capillary deformations that determine their orientations and generate mutual capillary interactions which drive them to assemble into 2D ordered structures. We numerically calculate, by energy minimization, the capillary deformations induced by adsorbed cubes for various Young's contact angles. First, we show that capillarity is crucial not only for quantitative, but also for qualitative predictions of equilibrium configurations of a single cube. For a Young's contact angle close to 90°, we show that a single-adsorbed cube generates a hexapolar interface deformation with three rises and three depressions. Thanks to the threefold symmetry of this hexapole, strongly directional capillary interactions drive the cubes to self-assemble into hexagonal or graphenelike honeycomb lattices. By a simple free-energy model, we predict a density-temperature phase diagram in which both the honeycomb and hexagonal lattice phases are present as stable states. DOI: 10.1103/PhysRevLett.116.258001 Over a century ago, it had already been observed that submm sized particles strongly adsorb at fluid-fluid interfaces [1,2]. Indeed, a fluid-fluid interface of area A and surface tension γ has a free energy cost γA, and particles can reduce A by adsorbing at the interface. The bonding potential is usually strong enough to allow stable monolayers of particles. Since a pioneering study by Pieranski [3], a lot of interest has been devoted to these quasi-2D systems, which have many applications, e.g., emulsions [4][5][6][7][8][9], coatings [10,11], optics [12], and new material development [13]. Because of the contact angle constraint imposed by Young's Law, an adsorbed particle in general induces deformations in the shape of the fluid-fluid interface. These so-called capillary deformations are responsible for capillary interactions between the adsorbed particles [14,15], which regulate the particle self-assembly at the interface [16][17][18]. These interactions can be tuned by varying, e.g., the particle shape and chemistry [10,[19][20][21], or the curvature of the fluidfluid interface [22][23][24]. Very recent experiments [25][26][27] have shown that adsorbed nanocubes with truncated corners can assemble into graphenelike honeycomb and hexagonal lattices. The origin of these structures is unknown, although ligand adsorption and van der Waals forces between specific facets of the truncated cubes have been suggested [25]. In this Letter, however, we show that generic cubes with homogeneous surface properties generate hexapolar capillary deformations which are largely responsible for the observed structures. Cubes of other materials or dimensions could, therefore, form similar structures.A primary step for understanding adsorbedparticle systems is the study of an isolated particle at a macroscopically flat fluid-fluid interface. Important issues are the equilibrium configuration of the particle at the interface [28][29][30][31] and the adsorption energy [32-34] which depend on the particl...

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confidence: 87%

Self-Assembly of Cubes into 2D Hexagonal and Honeycomb Lattices by Hexapolar Capillary Interactions

2016

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“…The close packing leads in this way to the formation of a colloidal armor, thus leading to more foam stabilization. [39][40][41][42][43] Even for particles with similar wettability and size, ellipsoidal particles were shown to produce more stable interfaces than spherical particles. For all concentrations of ChN used, the experiments show that the bubble size and the bubble size distribution did not change within the time span of the experiments (1 h), indicating that they were fairly stable against disproportionation and coalescence (data not shown).…”

Section: View Article Onlinementioning

confidence: 99%

Aqueous foams stabilized by chitin nanocrystals

et al. 2015

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The aim of the present study was to explore the potential use of chitin nanocrystals, as colloidal rod-like particles, to stabilize aqueous foams. Chitin nanocrystals (ChN) were prepared by acid hydrolysis of crude chitin and foams were generated mainly by sonicating the respective dispersions. The foamability of the chitin nanocrystals was evaluated and the resulting foams were assessed for their stability, in terms of foam volume reduction and serum release patterns, during storage. Additionally, the samples were studied with light scattering and optical microscopy in order to explore the bubble size distribution and morphology of the foam. Nanocrystal concentration and charge density was varied to alter the packing of the crystals at the interface. At low concentrations of ChNs, foams were stable against coalescence and disproportionation for a period of three hours, whereas at higher concentrations, the foams were stable for several days. The enhanced stability of foams prepared with ChNs, compared to surfactant-stabilized foams, can be mainly attributed to the irreversible adsorption of the ChNs at the air-water interface, thereby providing Pickering stabilization. Both foam volume and stability of the foam were increased with an increase in ChNs concentration, and at pH values around the chitin's pKa (pH 7.0). Under these conditions, the ChNs show minimal electrostatic repulsion and therefore a higher packing of the nanocrystals is promoted. Moreover, decreased electrostatic repulsion enhances network formation between the ChNs in the aqueous films, thereby providing additional stability by gel formation. Overall, ChNs were proven to be effective in stabilizing foams, and may be useful in the design of Pickering-stabilized food grade foams.

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“…Capillary interactions between particles due to interfacial deformation, which can be significant [38], are not taken into account in the model. This is perhaps the biggest simplification of the model, although it is somewhat validated by the notable absence of the effect in experimental work for a variety of anisotropic colloids [43]. However, great care must be taken when applying the present model to highly complex particle morphologies in which capillary interactions can be strong and may direct particle orientation.…”

Section: Thermodynamics Of Emulsificationmentioning

confidence: 80%

“…The effect of capillarity on deforming the interface is also ignored. This assumption is somewhat validated by the negligible interfacial distortion observed in experimental measurements on anisotropic particles on the order of 100 nm [43] but a more complete description including this effect would be of interest for improving the accuracy of the model. We also ignore the effects of line tension since the magnitude of this force is negligible for smooth particles with a characteristic size exceeding the order of 10 nm [19].…”

Section: Limitations Of the Modelmentioning

confidence: 91%