Intermolecular And Surface Forces

Improving emulsion formation, stability and performance using mixed emulsifiers: A review.

Despite the enormous interest in superhydrophobicity for self-cleaning, a clear picture of contaminant removal is missing, in particular, on a single-particle level. Here, we monitor the removal of individual contaminant particles on the micrometer scale by confocal microscopy. We correlate this space- and time-resolved information with measurements of the friction force. The balance of capillary and adhesion force between the drop and the contamination on the substrate determines the friction force of drops during self-cleaning. These friction forces are in the range of micro-Newtons. We show that hydrophilic and hydrophobic particles hardly influence superhydrophobicity provided that the particle size exceeds the pore size or the thickness of the contamination falls below the height of the protrusions. These detailed insights into self-cleaning allow the rational design of superhydrophobic surfaces that resist contamination as demonstrated by outdoor environmental (>200 days) and industrial standardized contamination experiments.

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When and how self-cleaning of superhydrophobic surfaces works

Under the right process conditions, nanoparticles can cluster together to form defined, dispersed structures, which can be termed supraparticles. Controlling the size, shape, and morphology of such entities is a central step in various fields of science and technology, ranging from colloid chemistry and soft matter physics to powder technology and pharmaceutical and food sciences. These diverse scientific communities have been investigating formation processes and structure/property relations of such supraparticles under completely different boundary conditions. On the fundamental side, the field is driven by the desire to gain maximum control of the assembly structures using very defined and tailored colloidal building blocks, whereas more applied disciplines focus on optimizing the functional properties from rather ill-defined starting materials. With this review article, we aim to provide a connecting perspective by outlining fundamental principles that govern the formation and functionality of suprapa...

Supraparticles: Functionality from Uniform Structural Motifs

Approaches to self-assembly of colloidal monolayers: A guide for nanotechnologists.

Surface heterogeneities, including roughness, significantly affect the adsorption, motion and interactions of particles at fluid interfaces. However, a systematic experimental study, linking surface roughness to particle wettability at a microscopic level, is currently missing. Here we synthesize a library of all-silica microparticles with uniform surface chemistry, but tuneable surface roughness and study their spontaneous adsorption at oil-water interfaces. We demonstrate that surface roughness strongly pins the particles' contact lines and arrests their adsorption in long-lived metastable positions, and we directly measure the roughness-induced interface deformations around isolated particles. Pinning imparts tremendous contact angle hysteresis, which can practically invert the particle wettability for sufficient roughness, irrespective of their chemical nature. As a unique consequence, the same rough particles stabilize both water-in-oil and oil-in-water emulsions depending on the phase they are initially dispersed in. These results both shed light on fundamental phenomena concerning particle adsorption at fluid interfaces and indicate future design rules for particle-based emulsifiers.

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Universal emulsion stabilization from the arrested adsorption of rough particles at liquid-liquid interfaces.

https://www.dce.uni-sofia.bg/files/publications/2012/2012-14-SA-KD-EB-PK-KA-Micelle-Charge-Aggregation-Number.pdf

Determination of the aggregation number and charge of ionic surfactant micelles from the stepwise thinning of foam films

The synergistic growth of giant wormlike micelles in ternary mixed solutions composed of an anionic surfactant (sodium laurylethersulfate, SLES), a zwitterionic surfactant (cocamidopropyl betaine, CAPB), and octanoic acid (HC8) is studied. Rheological data and their analysis in terms of Cole–Cole plots and micellar characteristic times are presented, and the micellar structures behind the observed rheological behavior are revealed by cryo-TEM micrographs. The surfactant composition is fixed near the maximal micelle size of the binary SLES + CAPB system, whereas the concentration of HC8 is varied. At a given HC8 concentration, the viscosity of the ternary micellar solutions exhibits a very high and sharp peak. Polarized-light optical microscopy indicates that all investigated solutions are isotropic rather than liquid-crystalline. The cryo-TEM imaging shows complex phase behavior: wormlike micelles to the left of the peak, giant entangled wormlike micelles at the peak, and long wormlike micelles coexisting...

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Synergistic Growth of Giant Wormlike Micelles in Ternary Mixed Surfactant Solutions: Effect of Octanoic Acid.

Microparticle adsorption and self-assembly at fluid interfaces are strongly affected by the particle three-phase contact angle θ. On the single-particle level, θ can be determined by several techniques, including colloidal-probe AFM, the gel-trapping technique (GTT) and the freeze-fracture shadow-casting (FreSCa) method. While GTT and FreSCa provide contact angle distributions measured over many particles, colloidal-probe AFM measures the wettability of an individual (specified) particle attached onto an AFM cantilever. In this paper, we extract θ for smooth microparticles through the analysis of force–distance curves upon particle approach and retraction from the fluid interface. From each retraction curve, we determine: (i) the maximal force, Fmax; (ii) the detachment distance, Dmax; and (iii) the work for quasistatic detachment, W. To relate Fmax, Dmax and W to θ, we developed a detailed theoretical model based on the capillary theory of flotation. The model was validated in three different ways. First, the contact angles, evaluated from Fmax, Dmax and W, are all close in value and were used to calculate the entire force–distance curves upon particle retraction without any adjustable parameters. Second, the model was successfully applied to predict the experimental force–distance curve of a truncated sphere, whose cut is positioned below the point of particle detachment from the interface. Third, our theory was confirmed by the excellent agreement between the particle contact angles obtained from the colloidal-probe AFM data and the ensemble-average contact angles measured by both GTT and FreSCa. Additionally, we devised a very accurate closed-form expression for W (representing the energy barrier for particle detachment), thus extending previous results in the literature.

Particle detachment from fluid interfaces: theory vs. experiments

Here, we have investigated the synergistic growth of long wormlike micelles and their transformation into disklike micelles, which occurs in three-component solutions composed of sodium lauryl ether sulfate (SLES; anionic), cocamidopropyl betaine (CAPB; zwitterionic), and dodecanoic acid (HC12; nonionic). The solution rheology is characterized in terms of zero-shear viscosities and characteristic times for micellar breaking and reptation. Furthermore, the microstructure evolution, leading to the observed rheological behavior, is revealed by cryo-transmission electron microscopy (TEM) micrographs. In all cases, the CAPB-to-SLES ratio is fixed, whereas the fatty acid concentration is varied. At a certain HC12 concentration, the solution viscosity passes through a maximum. The cryo-TEM imaging indicates that wormlike micelles appear before the peak, grow further up to the peak, and finally transform into disklike aggregates (a very rare micellar structure) after the peak. The transformation of worms into dis...

Svetoslav E. Anachkov

Papers

Universal emulsion stabilization from the arrested adsorption of rough particles at liquid-liquid interfaces.

Determination of the aggregation number and charge of ionic surfactant micelles from the stepwise thinning of foam films

Synergistic Growth of Giant Wormlike Micelles in Ternary Mixed Surfactant Solutions: Effect of Octanoic Acid.

Particle detachment from fluid interfaces: theory vs. experiments

Viscosity Peak due to Shape Transition from Wormlike to Disklike Micelles: Effect of Dodecanoic Acid