Particles in thin liquid films and at interfaces

Pressure threshold for inhibition of dense granular film opening

Controlling the stability of a granular film is essential in a wide range of industrial applications, from aerated building materials to recovering ore by flotation and treating wastewater. We therefore carry out experiments of granular film opening where particles of hundred of micrometers above random close packing zip the two interfaces of a soap film which liquid pressure is controlled. We create a hole at the center of this dense granular film and, surprisingly, we observe that the opening is not always inhibited. Different behaviours are identified: total bursting of the granular film, intermittent opening and jammed state for which the hole does not evolve. The liquid pressure drives the transition from one opening behaviour to another. Lower is the liquid pressure, more jammed is the system. The critical pressure transition scales as the surface tension over the particle size until the finite size of the granular film is only few tens of the particle size. Ultimately we evidence that spontaneous hole in thin film between particle do not lead to the granular film failure.

Collecting differently sized particles on water surface by maneuvering pedal waves on the foot of the water snail Pomacea canaliculata

The water snail (Pomacea canaliculata) features both broad dietary scope and high feeding rate, possibly making it one of the most successful mollusc invaders globally. The water snail can collect differently sized food particles on the air-water interface by generating pedal waves on the foot surface, such as granular duckweed and animal carrion in wild aquatic environment. While foraging by pedal waves, the biomechanical challenges by manipulating the differently sized particles might be overcome by mediating the waveform for optimized energy costs, the physics and the function of which have not been uncovered yet. Herein, we study the two-dimensional topographic features of the snail foot in the pedal wave feeding pattern by a laser sensor. Feeding on differently sized particles, the average wavelength of the pedal waves remains nearly constant, whereas the amplitude increases while transporting larger particles. We develop a hydrodynamical model and discover that applying a larger amplitude to transport the particle can shorten the transport time, facilitating the food transport rate. However, to maintain a relatively constant mass uptake rate, the water snail exhibits a flexible strategy of collecting larger particles with increased amplitudes. This work demonstrates that the water snail implements a strategy by generating varying waveforms on the foot surface to take up the floating food at an optimized feeding rate. This feeding fashion may open up a new way for developing bio-inspired solid waste collectors at the gas-liquid interface.

Pickering foams and parameters influencing their characteristics

Pickering foams are available in many applications and have been continually gaining interest in the last two decades. Pickering foams are multifaceted, and their characteristics are highly dependent on many factors, such as particle size, charge, hydrophobicity and concentration as well as the charge and concentration of surfactants and salts available in the system. A literature review of these individual studies at first might seem confusing and somewhat contradictory, particularly in multi-component systems with particles and surfactants with different charges in the presence of salts. This paper provides a comprehensive overview of particle-stabilized foams, also known as Pickering foams and froths. Underlying mechanisms of foam stabilization by particles with different morphology, surface chemistry, size and type are reviewed and clarified. This paper also outlines the role of salts and different factors such as pH, temperature and gas type on Pickering foams. Further, we highlight recent developments in Pickering foams in different applications such as food, mining, oil and gas, and wastewater treatment industries, where Pickering foams are abundant. We conclude this overview by presenting important research avenues based on the gaps identified here. The focus of this review is limited to Pickering foams of surfactants with added salts and does not include studies on polymers, proteins, or other macromolecules.

Spontaneous Elastocapillary Winding of Thin Elastic Fibers in Contact with Bubbles

We study the elastocapillary interaction between flexible microfibers in contact with bubbles trapped at the surface of a liquid bath. Microfibers placed on top of bubbles are found to migrate to and wrap into a coil around the perimeter of the bubble for certain bubble-fiber size combinations. The wrapping process is spontaneous: the coil spins atop the bubble, thereby drawing in excess fiber floating on the bath. A two-dimensional microfiber coil emerges which increases the lifetime of the bubbles. A simple model incorporating surface and bending energies captures the spontaneous winding process.

Interfacial Behavior of Particle-Laden Bubbles under Asymmetric Shear Flow

The behavior of moving bubbles has mostly been studied in an axisymmetric flow field. To extend the knowledge to practical conditions, we investigate the interfacial and hydrodynamic properties of bubbles under asymmetric shear forces. Experiments are performed with a buoyant bubble at the tip of a capillary placed in a defined shear flow in the presence of surfactants, nanoparticles, and glass beads. The response of the interface to the surrounding asymmetric flow is measured under successive reduction of the surface area. Profile analysis tensiometry is utilized to investigate the dynamic surface tension and the surface rheology of the surfactant- and nanoparticle-laden interfaces. Microscopic particle image and tracking velocimetry are used to study the bulk flow and the interfacial mobility of the buoyant bubble. According to our results, the rotational component of the shear flow provokes an interfacial flow, which redistributes the adsorbed surfactants and particles at the interface. In the presence of NPSCs, a contiguous network of particles forms at the interface through densification of surface structures. We show that this interconnected nanoparticle network eventually stops the interfacial flow and decreases the mobility of the glass beads at the interface. The immobilization of the interface is characterized by a dimensionless number, defined as the ratio of the interfacial elasticity to bulk shear forces. This number provides an estimate of the interfacial forces required to impose interfacial immobility at a defined flow field. Our findings can serve as a basis to formulate boundary conditions for refined modeling and to predict the hydrodynamics of bubbles and droplets.

Effect of selected monovalent salts on surfactant stabilized foams

Surfactant-stabilized foams have been at the centre of scientific research for over a century due to their ubiquitous applications in different industries. Many of these applications involve inorganic salts either due to their natural presence (e.g. use of seawater in froth floatation) or their addition (e.g. in cosmetics) to manipulate foam characteristics for the best outcomes. This paper provides a clear understanding of the effect of salts on surfactant-stabilized foams through a critical literature survey of this topic. Available literature shows a double effect of salts (LiCl, NaCl and KCl) on foam characteristics in the presence of surfactants. To elucidate the underlying mechanisms of the stabilizing effect of salts on foams, the effect of salts on surfactant-free thin liquid films is first discussed, followed by a discussion on the effect of salts on surfactant-stabilized foams with the focus on anionic surfactants. We discuss two distinctive salt concentrations, salt transition concentration in surfactant-free solutions and salt critical concentration in surfactant-laden systems to explain their effects. Using the available data in literature supported by dedicated experiments, we demonstrate the destabilizing effect of salts on foams at and above their critical concentrations in the presence of anionic surfactants. This effect is attributed to retarding the adsorption of the surfactant molecules at the interface due to the formation of nano and micro-scale aggregates.

Capillary interactions between soft capsules protruding through thin fluid films

When a suspension dries, the suspending fluid evaporates, leaving behind a dry film composed of the suspended particles. During the final stages of drying, the height of the fluid film on the substrate drops below the particle size, inducing local interface deformations that lead to strong capillary interactions among the particles. Although capillary interactions between rigid particles are well studied, much is still to be understood about the behaviour of soft particles and the role of their softness during the final stages of film drying. Here, we use our recently-introduced numerical method that couples a fluid described using the lattice Boltzmann approach to a finite element description of deformable objects to investigate the drying process of a film with suspended soft particles. Our measured menisci deformations and lateral capillary forces, which agree well with previous theoretical and experimental works in case of rigid particles, show that the deformations become smaller with increasing particle softness, resulting in weaker lateral interaction forces. At large interparticle distances, the force approaches that of rigid particles. Finally, we investigate the time dependent formation of particle clusters at the late stages of the film drying.

Equilibrium Orientation and Adsorption of an Ellipsoidal Janus Particle at a Fluid–Fluid Interface

We investigate the equilibrium orientation and adsorption process of a single, ellipsoidal Janus particle at a fluid–fluid interface. The particle surface comprises equally sized parts that are hydrophobic or hydrophilic. We present free energy models to predict the equilibrium orientation and compare the theoretical predictions with lattice Boltzmann simulations. We find that the deformation of the fluid interface strongly influences the equilibrium orientation of the Janus ellipsoid. The adsorption process of the Janus ellipsoid can lead to different final orientations determined by the interplay of particle aspect ratio and particle wettablity contrast.

Impact of Surface Amphiphilicity on the Interfacial Behavior of Janus Particle Layers under Compression

Langmuir monolayers of silica/gold Janus particles with two different degrees of amphiphilicity have been examined to study the significance of particle surface amphiphilicity on the structure and mechanical properties of the interfacial layers. The response of the layers to the applied compression provides insight into the nature and strength of the interparticle interactions. Different collapse modes observed for the interfacial layers are linked to the amphiphilicity of Janus particles and their configuration at the interface. Molecular dynamics simulations on nanoparticles with similar contact angles provide insight on the arrangement of particles at the interface and support our conclusion that the interfacial configuration and collapse of anisotropic particles at the air/water interface are controlled by particle amphiphilicity.

Equilibrium configurations and capillary interactions of Janus dumbbells and spherocylinders at fluid-fluid interfaces

We numerically investigate the adsorption of a variety of Janus particles (dumbbells, elongated dumbbells and spherocylinders) at a fluid-fluid interface by using a numerical method that takes into account the interfacial deformations. We first determine the equilibrium configuration of a single adsorbed particle, and we find that the overall shape of the induced deformation field has a strong hexapolar mode while non-Janus particles of the same shape do not induce any interfacial deformation. We then calculate the capillary interactions between two Janus spherocylinders adsorbed at an interface. The hexapolar deformation field induces capillary attractions for laterally aligned Janus spherocylinders and repulsions for laterally anti-aligned ones. We also experimentally synthesize micrometer-sized charged Janus dumbbells and let them adsorb at a water-decane interface. After several hours we observe the formation of aggregates of dumbbells predominantly induced by interactions that appear to be capillary in nature. Our Janus dumbbells attach laterally and are all aligned, as predicted by our numerical calculations.

Self-assembly of cubic colloidal particles at fluid-fluid interfaces by hexapolar capillary interactions

Colloidal particles adsorbed at fluid-fluid interfaces can self-assemble, thanks to capillary interactions, into 2D ordered structures. Recently, it has been predicted by theoretical and numerical calculations [G. Soligno et al., Phys. Rev. Lett., 2016, 116, 258001] that cubes with smooth edges adsorbed at a flat fluid-fluid interface generate hexapolar capillary deformations that cause the particles to self-assemble into honeycomb and hexagonal lattices, at equilibrium and for Young's contact angle π/2. Here we extend these results. Firstly, we show that capillary interactions induced by hexapolar deformations can drive the particles at the interface to form also thermodynamically-stable square lattices, in addition to honeycomb and hexagonal lattices. Then, we study the effects of tuning the particle shape on the particle self-assembly at the interface, considering, respectively, smooth-edge cubes, sharp-edge cubes, slightly truncated-edge cubes, and highly truncated-edge cubes. In our calculations, both capillary and hard-particle interactions are taken into account. We show that such variations in the particle shape significantly affect both qualitatively and quantitatively the self-assembly of the particles at the interface, and we sum up our results in the form of temperature-density phase diagrams. For example, using typical experimental parameters, our results show that only 4-to-5 nm sized sharp-edge and smooth-edge cubes can self-assemble into a honeycomb lattice, while slightly and highly truncated-edge cubes can form a honeycomb lattice only if they have a 8-to-12 and 10-to-16 nm size, respectively, for the same experimental parameters. Also, our results show that the capillarity-induced square lattice phase is stable only for the smooth-edge and truncated-edge cubes, but not for the sharp-edge cubes.

Nano- and microparticles at fluid and biological interfaces

Systems with interfaces are abundant in both technological applications and biology. While a fluid interface separates two fluids, membranes separate the inside of vesicles from the outside, the interior of biological cells from the environment, and compartmentalize cells into organelles. The physical properties of interfaces are characterized by interface tension, those of membranes are characterized by bending and stretching elasticity. Amphiphilic molecules like surfactants that are added to a system with two immiscible fluids decrease the interface tension and induce a bending rigidity. Lipid bilayer membranes of vesicles can be stretched or compressed by osmotic pressure; in biological cells, also the presence of a cytoskeleton can induce membrane tension. If the thickness of the interface or the membrane is small compared with its lateral extension, both can be described using two-dimensional mathematical surfaces embedded in three-dimensional space. We review recent work on the interaction of particles with interfaces and membranes. This can be micrometer-sized particles at interfaces that stabilise emulsions or form colloidosomes, as well as typically nanometer-sized particles at membranes, such as viruses, parasites, and engineered drug delivery systems. In both cases, we first discuss the interaction of single particles with interfaces and membranes, e.g. particles in external fields, non-spherical particles, and particles at curved interfaces, followed by interface-mediated interaction between two particles, many-particle interactions, interface and membrane curvature-induced phenomena, and applications.

Chemical physics of electroactive materials: concluding remarks

It is an honour to be charged with providing the concluding remarks for a Faraday Discussion. As many have remarked before, it is nonetheless a prodigious task, and what follows is necessarily a personal, and probably perverse, view of a watershed event in the Chemical Physics of Electroactive materials. The spirit of the conference was captured in a single sentence during the meeting itself."It is the nexus between rheology, electrochemistry, colloid science and energy storage". The current scientific climate is increasingly dominated by a limited number of global challenges, and there is thus a tendency for research to resemble a football match played by 6 year olds, where everyone on the field chases the (funding) ball instead of playing to their "discipline". It is thus reassuring to see how the application of rigorous chemical physics is leading to ingenious new solutions for both energy storage and harvesting, via, for example, nanoactuation, electrowetting, ionic materials and nanoplasmonics. In fact, the same language of chemical physics allows seamless transition between applications as diverse as mechano-electric energy generation, active moisture transport and plasmonic shutters - even the origins of life were addressed in the context of electro-autocatalysis!

Shape-Induced Deformation, Capillary Bridging, and Self-Assembly of Cuboids at the Fluid-Fluid Interface

The controlled assembly of anisotropic particles through shape-induced interface deformations is shown to be a potential route for the fabrication of novel functional materials. In this article, the shape-induced interface deformation, capillary bridging, and directed self-assembly of cuboidal-shaped hematite particles at fluid-fluid interfaces are reported. The multipolar nature of the interface distortions is directly visualized using high-resolution scanning electron microscopy and 3D optical surface profiling. The nature of the interface deformations around cuboidal particles vary from monopolar to octupolar types depending on their orientation and position with respect to the interface. The deformations are of either hexapolar or octupolar type in the face-up orientation, quadrupolar or monopolar type in the edge-up orientation, and monopolar type in the vertex-up orientation. The particles adsorbed at the interface interact through the interface deformations, forming capillary bridges that lead to isolated assemblies of two or more particles. The arrangement of particles in any assembly is such that the condition for capillary attraction is satisfied, that is, in accordance with predictions based on the nature of interface deformations. At sufficient particle concentrations, these isolated structures interact to form a percolating network of cuboids. Furthermore, the difference in the nature of the assembly structures formed at the air-water interface and in the bulk water phase indicates that the interfacial assembly of these particles is controlled by the capillary interactions.

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

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.

Detailed Structural and Mechanical Response of Wet Foam to the Settling Particle

Liquid foam, as a complex fluid, provides an observable prototype for studying a discrete fluid system. In this work, a numerical study on the settling behavior of a round particle in wet polydisperse foam has been conducted on the bubble scale. The local and nonuniform distribution of bubble pressure, as well as the localized plastic events, is presented. It shows a foam region of higher pressure in front of the settling particle due to the extrusion deformation of the bubbles applied by the particle. Additionally, the forces exerted on the particle by the disordered wet foam are measured during the sedimentation. It exhibits in particular a power-law dependence of the drag force caused by the bubble as a function of the foam quality. Moreover, sedimentation experiments are demonstrated to verify this power-law relation. The evolution of the components of drag force is demonstrated when a plastic event occurs in front of the settling particle. The result shows that both the contributions of the pulling force of foam films and the bubble pressure to the drag force decrease in that case. Likewise, the variation of both these contributions to the drag force is illustrated as well when a bubble in the wake detaches from the particle. These results assist in understanding the mesoscopic response of wet foam to a settling particle.

Interfacial sciences in unconventional petroleum production: from fundamentals to applications

With the ever increasing demand for energy to meet the needs of growth in population and improvement in the living standards, in particular in developing countries, the abundant unconventional oil reserves (about 70% of total world oil), such as heavy oil, oil/tar sands and shale oil, are playing an increasingly important role in securing global energy supply.