DLVO forces in thin liquid films beyond the conventional DLVO theory

Dynamics of Rising Bubbles and Their Impact with Viscoelastic Fluid Interfaces

Bubble dynamics plays a significant role in a wide range of industrial fields, such as food, pharmacy and chemical engineering. The physicochemical properties of complex fluids can greatly affect the speed with which bubbles rise, and the lifetime of bubbles, which in turn can affect the efficiency of food and drug manufacturing and also sewage purification. Therefore, it is of great scientific and practical significance to study the influence mechanism of nanoparticles and surfactants on bubble rising and impact in a complex fluid interface. This paper selects a mixed dispersion liquid of nanoparticles (SiO₂) and a surfactant (SDS) as the objects of the study, observes in real-time the entire processes of bubbles rising, impact at the gas-liquid interface, and rupture, and analyzes the dynamic mechanism of bubble impact in a complex fluid interface. By analyzing the morphological changes of the rising bubbles, the rising velocity and the lifetime of the bubbles, it is found that the surfactant molecules are distributed in the ultrapure water liquid pool and the liquid film surrounding the bubbles. Such distribution can reduce the viscoelasticity between bubbles and the liquid surface, and lower the surface tension of the liquid, which can reduce the rising velocity of bubbles, delay the drainage process of bubbles on a liquid surface, and enhance the lifetime of bubbles. If the liquid surface is covered with nanoparticles, a reticulate structure will be formed on the bubble liquid film, which can inhibit bubble discharge and prolong bubble lifetime. In addition, the influence of such a reticulate structure on liquid surface tension is limited and its function is far smaller than a surfactant.

Thermodynamic Stability of Volatile Droplets and Thin Films Governed by Disjoining Pressure in Open and Closed Containers

Distributed thin films of water and their coexistence with droplets are investigated using a capillary description based on a thermodynamic fundamental relation for the film Helmholtz energy, derived from disjoining pressure isotherms and an accurate equation of state. Gas-film and film-solid interfacial tensions are derived, and the latter has a dependence on film thickness. The resulting energy functionals from the capillary description are discretized, and stationary states are identified. The thermodynamic stability of configurations with thin films in systems that are closed (canonical ensemble) or connected to a particle reservoir (grand canonical ensemble) is evaluated by considering the eigenvalues of the corresponding Hessian matrices. The conventional stability criterion from the literature states that thin flat films are stable when the derivative of the disjoining pressure with respect to the film thickness is negative. This criterion is found to apply only in open systems. A closer inspection of the eigenvectors of the negative eigenvalues shows that condensation/evaporation destabilizes the film in an open system. In closed systems, thin films can be stable even though the disjoining pressure derivative is positive, and their stability is governed by mechanical instabilities of a similar kind to those responsible for spinodal dewetting in nonvolatile systems. The films are stabilized when their thickness and disjoining pressure derivative are such that the minimum unstable wavelength is larger than the container diameter. Droplets in coexistence with thin films are found to be unstable for all considered examples in open systems. In closed systems, they are found to be stable under certain conditions. The unstable droplets in both open and closed systems are saddle points in their respective energy landscapes. In the closed system, they represent the activation barrier for the transition between a stable film and a stable droplet. In the open system, the unstable droplets represent the activation barrier for the transition from a film into a bulk liquid phase. Thin films are found to be the equilibrium configuration up to a certain value of the total density in a closed system. Beyond this value, there is a morphological phase transition to stable droplet configurations.

Superspreading and Drying of Trisiloxane-Laden Quantum Dot Nanofluids on Hydrophobic Surfaces

Nanofluids hold promise for a wide range of areas of industry. However, understanding the wetting behavior and deposition formation in the course of drying and spreading of nanofluids, particularly containing surfactants, is still poor. In this paper, the evaporation dynamics of quantum dot-based nanofluids and evaporation-driven self-assembly in nanocolloidal suspensions on hexamethyldisilazane-, polystyrene-, and polypropylene-coated hydrophobic surfaces have been studied experimentally. Moreover, for the very first time, we make a step toward understanding the wetting dynamics of superspreader surfactant-laden nanofluids. It was revealed that drying of surfactant-free quantum dot nanofluids in contrast to pure liquids undergoes not three but four evaporation modes including last additional pinning mode when the contact angle decreases while the triple contact line is pinned by the nanocrystals. In contrast to previous studies, it was found out that addition of nanoparticles to aqueous surfactant solutions leads to deterioration of the spreading rate and to formation of a double coffee ring. For all surfaces examined, superspreading in the presence and absence of quantum dot nanoparticles takes place. Despite the formation of coffee rings on all substrates, they have different morphologies. In particular, the knot-like structures are incorporated into the ring on hexamethyldisilazane- and polystyrene-coated surfaces.

Van der Waals forces in free and wetting liquid films

Van der Waals interactions induced by fluctuations of electromagnetic field bear universal nature and act between individual atoms, condensed particles or bodies of any type. Continuously growing interest to theoretical understanding as well as to precise evaluation of van der Waals forces is caused by their fundamental role in many physical, chemical, and biological processes. In this paper, we scrutinize progress in the studies of van der Waals forces, related to recent active development of Coupled Dipole Method (CDM) for the analysis of the behavior and properties of nanosized systems. The application of CDM for the analysis of thin liquid films allowed achieving substantial progress in understanding the behavior of free and wetting films. It was shown that both the macroscopic properties, such as excess free energy and Hamaker constants and the local microscopic parameters, such as polarizabilities, can be successfully calculated based only on properties of individual molecules. The impact of lateral film confinement on the specific excess free energy and the film stability was elucidated, and effect of spatial constraints on the spectrum of vibrational states for liquid film and the underlying substrate was analyzed. It was shown that van der Waals interactions between molecules represent the universal mechanism for dynamic structuring and formation of boundary layers and that the CDM allows self-consistently calculating the properties of these layers in both solid and liquid phases.

Simultaneous measurements of thin film thickness using total internal reflection fluorescence microscopy and disjoining pressure using Scheludko cell

This work describes a method for measuring the thin film thickness using total internal reflection fluorescence microscopy, with the use of evanescent wave illumination. The thin liquid film was formed in a hole drilled at the center of a porous plate, which is used for measurement of the disjoining pressure by using the Scheludko cell method. The aim of simultaneous and in situ measurements of thin film thickness and disjoining pressure is to obtain the relationship between them, which is critical for explicitly depicting the thin film profile that determines the interfacial mass and heat fluxes in the thin film region near the triple line. This method can overcome the drawbacks of the optical methods that are insufficient for measuring the thickness of a thin film with curvature. The influence of structural forces formed by tracer nanoparticles seeded in the thin liquid film on the relationship was analyzed. The obtained expression for disjoining pressure vs thin film thickness provides a basis for analyzing the formation, evolution, and stability of the thin liquid film, which is the dominant mechanism of controlling the mesoscopic structure in many transport processes.

Gold nanoparticle-based colorimetric biosensors

Gold nanoparticles (AuNPs) provide excellent platforms for the development of colorimetric biosensors as they can be easily functionalised, displaying different colours depending on their size, shape and state of aggregation. In the last decade, a variety of biosensors have been developed to exploit the extent of colour changes as nano-particles (NPs) either aggregate or disperse, in the presence of analytes. Of critical importance to the design of these methods is that the behaviour of the systems has to be reproducible and predictable. Much has been accomplished in understanding the interactions between a variety of substrates and AuNPs, and how these interactions can be harnessed as colorimetric reporters in biosensors. However, despite these developments, only a few biosensors have been used in practice for the detection of analytes in biological samples. The transition from proof of concept to market biosensors requires extensive long-term reliability and shelf life testing, and modification of protocols and design features to make them safe and easy to use by the population at large. Developments in the next decade will see the adoption of user friendly biosensors for point-of-care and medical diagnosis as innovations are brought to improve the analytical performances and usability of the current designs. This review discusses the mechanisms, strategies, recent advances and perspectives for the use of AuNPs as colorimetric biosensors.

Image Charge Effects in the Wetting Behavior of Alkanes on Water with Accounting for Water Solubility

Different types of surface forces, acting in the films of pentane, hexane, and heptane on water are discussed. It is shown that an important contribution to the surface forces originates from the solubility of water in alkanes. The equations for the distribution of electric potential inside the film are derived within the Debye-Hückel approximation, taking into account the polarization of the film boundaries by discrete charges at water-alkane interface and by the dipoles of water molecules dissolved in the film. On the basis of above equations we estimate the image charge contribution to the surface forces, excess free energy, isotherms of water adsorption in alkane film, and the total isotherms of disjoining pressure in alkane film. The results indicate the essential influence of water/alkane interface charging on the disjoining pressure in alkane films, and the wettability of water surface by different alkanes is discussed.

Stepwise collapse of highly overlapping electrical double layers

When two charged surfaces and their accompanying double layers (EDL) approach each other in an electrolyte solution, the EDLs first begin to overlap and finally collapse under confinement. Precise surface force measurements reveal the underlying structural elements.

Image-charge forces in thin interlayers due to surface charges in electrolyte

The surface forces arising in wetting films of nonpolar liquids or in thin air interlayers between an electrolyte and a nonpolar medium in the case of discrete charging of the dielectric-electrolyte interface are considered. The contributions of polarization effects to the distribution of the electrostatic potential in the three contacting media were calculated. Within the Debye-Hückel approximation, the analytical solutions were derived for the disjoining pressure in thin films, for the case of either dilute or relatively concentrated electrolyte solutions in the aforementioned systems. Analysis of the analytical and numerical results demonstrated that for dilute solutions the contribution of image forces to the disjoining pressure may significantly exceed the van der Waals forces for films from a few to tens of nanometers thick.

Success and failure of colloidal approaches in adhesion of microorganisms to surfaces

Biofilms are communities of cells attached to surfaces, their contributions to biological process may be either a benefit or a threat depending on the microorganism involved and on the type of substrate and environment. Biofilm formation is a complex series of steps; due to the size of microorganisms, the initial phase of biofilm formation, the bacterial adhesion to the surface, has been studied and modeled using theories developed in colloidal science. In this review the application of approaches such as Derjaguin, Landau, Verwey, Overbeek (DLVO) theory and its extended version (xDLVO), to bacterial adhesion is described along with the suitability and applicability of such approaches to the investigation of the interface phenomena regulating cells adhesion. A further refinement of the xDLVO theory encompassing the brush model is also discussed. Finally, the evidences of phenomena neglected in colloidal approaches, such as surface heterogeneity and fluid flow, likely to be the source of failure are defined.

Possible origin of the inverse and direct Hofmeister series for lysozyme at low and high salt concentrations

Protein solubility studies below the isoelectric point exhibit a direct Hofmeister series at high salt concentrations and an inverse Hofmeister series at low salt concentrations. The efficiencies of different anions measured by salt concentrations needed to effect precipitation at fixed cations are the usual Hofmeister series (Cl(-) > NO(3)(-) > Br(-) > ClO(4)(-) > I(-) > SCN(-)). The sequence is reversed at low concentrations. This has been known for over a century. Reversal of the Hofmeister series is not peculiar to proteins. Its origin poses a key test for any theoretical model. Such specific ion effects in the cloud points of lysozyme suspensions have recently been revisited. Here, a model for lysozymes is considered that takes into account forces acting on ions that are missing from classical theory. It is shown that both direct and reverse Hofmeister effects can be predicted quantitatively. The attractive/repulsive force between two protein molecules was calculated. To do this, a modification of Poisson-Boltzmann theory is used that accounts for the effects of ion polarizabilities and ion sizes obtained from ab initio calculations. At low salt concentrations, the adsorption of the more polarizable anions is enhanced by ion-surface dispersion interactions. The increased adsorption screens the protein surface charge, thus reducing the surface forces to give an inverse Hofmeister series. At high concentrations, enhanced adsorption of the more polarizable counterions (anions) leads to an effective reversal in surface charge. Consequently, an increase in co-ion (cations) adsorption occurs, resulting in an increase in surface forces. It will be demonstrated that among the different contributions determining the predicted specific ion effect the entropic term due to anions is the main responsible for the Hofmeister sequence at low salt concentrations. Conversely, the entropic term due to cations determines the Hofmeister sequence at high salt concentrations. This behavior is a remarkable example of the charge-reversal phenomenon.

Wetting and surface forces

In this review we discuss the fundamental role of surface forces, with a particular emphasis on the effect of the disjoining pressure, in establishing the wetting regime in the three phase systems with both plane and curved geometry. The special attention is given to the conditions of the formation of wetting/adsorption liquid films on the surface of poorly wetted substrate and the possibility of their thermodynamic equilibrium with bulk liquid. The calculations of contact angles on the basis of the isotherms of disjoining pressure and the difference in wettability of flat and highly curved surfaces are discussed. Mechanisms of wetting hysteresis, related to the action of surface forces, are considered.

Charge heterogeneity of surfaces: mapping and effects on surface forces

The DLVO theory treats the total interaction force between two surfaces in a liquid medium as an arithmetic sum of two components: Lifshitz-van der Waals and electric double layer forces. Despite the success of the DLVO model developed for homogeneous surfaces, a vast majority of surfaces of particles and materials in technological systems are of a heterogeneous nature with a mosaic structure composed of microscopic and sub-microscopic domains of different surface characteristics. In such systems, the heterogeneity of the surface can be more important than the average surface character. Attractions can be stronger, by orders of magnitude, than would be expected from the classical mean-field DLVO model when area-averaged surface charge or potential is employed. Heterogeneity also introduces anisotropy of interactions into colloidal systems, vastly ignored in the past. To detect surface heterogeneities, analytical tools which provide accurate and spatially resolved information about material surface chemistry and potential - particularly at microscopic and sub-microscopic resolutions - are needed. Atomic force microscopy (AFM) offers the opportunity to locally probe not only changes in material surface characteristic but also charges of heterogeneous surfaces through measurements of force-distance curves in electrolyte solutions. Both diffuse-layer charge densities and potentials can be calculated by fitting the experimental data with a DLVO theoretical model. The surface charge characteristics of the heterogeneous substrate as recorded by AFM allow the charge variation to be mapped. Based on the obtained information, computer modeling and simulation can be performed to study the interactions among an ensemble of heterogeneous particles and their collective motions. In this paper, the diffuse-layer charge mapping by the AFM technique is briefly reviewed, and a new Diffuse Interface Field Approach to colloid modeling and simulation is briefly discussed.