Surface Rheology Research Articles

Breakup of thin liquid films with viscous interfaces

Thin liquid films are ubiquitous in nature and have many practical applications. From biological films to the curtain coating process, thin films are present in both large and small scales. Despite their importance, understanding the stability of these films remains a significant challenge due to the fluid–fluid interface that is free to deform, affected by interfacial tension and complex rheological behavior. Instabilities in thin films are often caused by van der Waals attractions, which can lead to the rupture of the layer. To investigate the rupture dynamics, numerical methods are commonly used, such as asymptotic derivations of the lubrication theory or interface tracking methods. In this paper, we present a computational study of the breakup dynamics of a stationary thin liquid sheet bounded by a passive gas with a viscous interface, using the arbitrary Lagrangian–Eulerian method and the Boussinesq–Scriven constitutive law to model the rheological behavior. Our results demonstrate that the stability of thin liquid films is influenced by both surface rheology and disjoining effects and that the viscous character of the interface can delay sheet breakup, leading to more stable films.

Polymeric Surfactants Bearing Divalent Carboxy Pendants for Stable Color Dispersions with High Redispersibility

Inkjet printing of ink requires dyes or pigments insoluble in water but uniformly dispersing as fine particles for a long period. In addition, redispersibility after solidification by solvent vaporization around the jetting heads is also important for performance. In this study, thus, a series of amphiphilic copolymers were prepared for a dispersant of inkjet using reversible addition fragmentation chain-transfer copolymerization of styrene (St), benzyl acrylate (BnA), and phenoxyethyl acrylate (PEA) as a hydrophobic monomer with acrylic acid (AA) and itaconic acid (IA) as a hydrophilic monomer. To understand the fundamental properties and potentials as a polymer dispersant, the surface parameters and rheology of the copolymers and their color dispersions in aqueous media were investigated. The aqueous solutions were typical Newtonian fluids, whereas the color dispersions were non-Newtonian fluids with shear-thinning properties and yield values ranging from 0.57 to 1.04 Pa. Although the surface parameters, such as surface tension (γ), molecular occupied area (Amin), and zeta potential (ζ), did not show clear correlation with polymer structures, their behaviors as a dispersant were significantly different. The color dispersants containing the copolymer of St or BnA as a hydrophobic monomer and IA and AMPS as hydrophilic monomers exhibited sufficient redispersion both for yellow and cyan inks, even after the aging test at 60 °C for 5 days, whereas other copolymers did not exhibit such superior performances. In addition, the color dispersions of the copolymer of St, IA, and AMPS (St-IA-AMPS) showed lower viscosity and yield value and became closer to Newtonian fluid compared with that of St, AA, and AMPS (St-AA-AMPS). Thus, St-IA-AMPS provided good redispersibility and avoided sedimentation. These results suggest that the most suitable dispersant for inkjet inks among the polymers investigated was the copolymer of St, IA, and AMPS.

Hydrodynamic aggregation of membrane inclusions due to non-Newtonian surface rheology

Biological membranes are self-assembled complex fluid interfaces that host proteins, molecular motors, and other macromolecules essential for cellular function. These membranes have a distinct in-plane fluid response with a surface viscosity that has been well characterized. The resulting quasi-two-dimensional fluid dynamical problem describes the motion of embedded proteins or particles. However, the viscous response of biological membranes is often non-Newtonian: in particular, the surface shear viscosity of phospholipids that comprise the membrane depends strongly on the surface pressure. We use the Lorentz reciprocal theorem to extract the effective long-ranged hydrodynamic interaction among membrane inclusions that arises due to such non-trivial rheology. We show that the corrective force that emerges ties back to the interplay between membrane flow and non-constant viscosity, which suggests a mechanism for biologically favorable protein aggregation within membranes. We quantify and describe the mechanism for such a large-scale concentration instability using a mean-field model. Finally, we employ numerical simulations to demonstrate the formation of hexatic crystals due to the effective hydrodynamic interactions within the membrane.

Stimuli-Responsive Langmuir Films Composed of Nanoparticles Decorated with Poly(N-isopropyl acrylamide) (PNIPAM) at the Air/Water Interface.

The nanotechnology shift from static toward stimuli-responsive systems is gaining momentum. We study adaptive and responsive Langmuir films at the air/water interface to facilitate the creation of two-dimensional (2D) complex systems. We verify the possibility of controlling the assembly of relatively large entities, i.e., nanoparticles with diameter around 90 nm, by inducing conformational changes within an about 5 nm poly(N-isopropyl acrylamide) (PNIPAM) capping layer. The system performs reversible switching between uniform and nonuniform states. The densely packed and uniform state is observed at a higher temperature, i.e., opposite to most phase transitions, where more ordered phases appear at lower temperatures. The induced nanoparticles' conformational changes result in different properties of the interfacial monolayer, including various types of aggregation. The analysis of surface pressure at different temperatures and upon temperature changes, surface potential measurements, surface rheology experiments, Brewster angle microscopy (BAM), and scanning electron microscopy (SEM) observations are accompanied by calculations to discuss the principles of the nanoparticles' self-assembly. Those findings provide guidelines for designing other adaptive 2D systems, such as programable membranes or optical interfacial devices.

Effects of Dispersion Medium on Surface Tension and Rheological Behavior of Pectin and Guar Gum Based Thickeners Used in Treatment of Dysphagia

Dysphagia is a complex condition as each individual may have unique swallowing difficulties and sensitivities. Experimental studies investigating the effects of dispersion media on thickeners can contribute to the development of customized thickening agents. Understanding how the dispersion environment affects the surface tension and rheological behavior of thickeners can lead to improved patient safety, comfort, and overall treatment efficacy by helping healthcare professionals make informed decisions when selecting appropriate thickening agents and optimizing their use in clinical practice. In this study, the effects of dispersion medium (water and lemonade) on the surface tension and rheology of thickened liquids prepared with pectin or guar gum-based thickening powders have been investigated. The rheological behaviors of the samples are determined at room temperature at the shear rate ranges of 10-100 s-1 using a rotary viscometer. Obtained data are modeled by Power Law and Herschel Bulkley. Both pectin and guar gum thickeners exhibit shear thinning behavior, meaning that their viscosity decreases under shear stress. The dispersion medium influences the degree of shear thinning observed. The low surface tension of this medium raises wetting and improves the dispersion of thickeners, resulting in faster dissolution and better mixing. This can be beneficial for the treatment of dysphagia as it allows for easier swallowing and better texture of thickened fluids.

Dried water-redispersible bacterial nanocellulose with sorbitol as capping agent

In view of the economic and technical limitations associated with diluted aqueous suspensions of cellulose nanofibrils, in the present contribution sorbitol was assayed as capping agent in the drying of bacterial nanocellulose (BNC) for the first time. The effect of different sorbitol:BNC mass ratios (1:1, 3:1 and 5:1), drying strategies (freeze-drying versus oven-drying), and redispersion methods (high-speed homogenization and mechanical stirring) on the water redispersibility of dried BNC was studied. Sedimentation, specific surface area and rheology assays all demonstrated the suitability of sorbitol for significantly limiting irreversible hydrogen bonding between adjacent cellulose nanofibrils during dehydration, leading to redispersed BNC suspensions with characteristics similar to those of never-dried ones. In the case of freeze-dried samples, results showed that the lowest sorbitol:BNC mass ratio assayed (i.e., 1:1) was enough to prevent irreversible hydrogen bonding between adjacent cellulose fibrils, whereas for oven-dried BNC samples full redispersion was achieved by mechanical stirring when sorbitol:BNC mass ratios of at least 3:1 were used. The feasibility of easily removing sorbitol from the dried product upon redispersion was also verified.

Dynamic interfacial properties and foam behavior of licorice root extract solutions

Licorice (Glycyrrhiza glabra) is a useful plant of the family Fabaceae, with sweet-tasting roots. The root extract of this plant is rich in saponins, so it can be considered a source of natural surfactants. This research provides some applicable information about the dynamic surface tension and foam behavior of aqueous solutions of licorice root extract (LRE). The pendant drop shape analysis was utilized to study the surface tension and dilational surface rheology of LRE at the water/air interface. The Bikerman type experiment was used to measure foamability and foam stability of aqueous LRE solutions. The equilibrium surface tensions reveal that the LRE contains surface-active components and is capable of reducing the surface tension by 25 mN/m at the critical aggregation concentration (CAC). The surface dilational visco-elasticity measurements proved that the adsorption layers are predominantly of elastic nature. Also the foamability and foam stability show a meaningful correlation with the dynamic surface properties. This study aims to contribute to the development of appropriate utilization of the benefits provided by a biosurfactant source in foam-related commercial applications.

Dynamic surface properties of carboxyfullerene solutions

Application of conventional methods of dilatational surface rheology in combination with optical techniques and atomic force microscopy allowed determination of the surface properties of a water-soluble fullerene derivative, carboxyfullerene C60(C(COOH)2)3. It has been shown that the surface activity of this derivative is higher than that of other soluble fullerene derivatives, for example, of fullerenols. The dynamic surface elasticity of carboxyfullerene solutions reaches up to values of about 180 mN/m. Brewster angle microscopy indicates a liquid-like nature of carboxyfullerene adsorption layers. The application of atomic force microscopy shows that these adsorption layers consist of numerous interconnected surface aggregates. The size and number of aggregates increase during adsorption. The compression of carboxyfullerene spread layers results in its partial dissolution. At the same time, the compression isotherms indicate a high stability of the layers of carboxyfullerene aggregates.

Competitive interfacial behavior in sodium caseinates-hydroxypropyl cellulose mixed systems

In this work, interactions between hydroxypropyl cellulose (HPC) and sodium caseinates (Na-cas) at the air-water interface were studied. The surface tension, surface rheology, and film compatibility of the mixed protein/polysaccharide system were investigated. Tensiometry experiments showed that HPC is more surface-active than proteins, and is preferentially adsorbed at the interface. This molecule is able to migrate to the interface faster than Na-cas despite its larger size. At low HPC concentration, proteins adsorb first but HPC being more surface-active will eventually substitute them at the interface. The use of the pendant drop tensiometer allowed the real-time monitoring of protein displacement; an immediate decrease in surface tension was observed when HPC was added to a Na-cas solution. The analysis of the compression isotherms of Na-cas/HPC mixed films revealed partial miscibility between the two species within the monolayer; the films were less expanded than expected from the ideal behavior of binary mixtures. The dilatational rheology experiments showed that the presence of a very small amount of HPC had a significant effect on the viscoelastic properties of the interface. This work reported for the first time the competitive adsorption between Na-cas and HPC at the A/W interface.

Interfacial Dynamics of Adsorption Layers as Supports for Biomedical Research and Diagnostics

The input of chemical and physical sciences to life sciences is increasingly important. Surface science as a complex multidisciplinary research area provides many relevant practical tools to support research in medicine. The tensiometry and surface rheology of human biological liquids as diagnostic tools have been very successfully applied. Additionally, for the characterization of pulmonary surfactants, this methodology is essential to deepen the insights into the functionality of the lungs and for the most efficient administration of certain drugs. Problems in ophthalmology can be addressed using surface science methods, such as the stability of the wetting films and the development of artificial tears. The serious problem of obesity is fast-developing in many industrial countries and must be better understood, while therapies for its treatment must also be developed. Finally, the application of fullerenes as a suitable system for detecting cancer in humans is discussed.

Effect of deacetylation degree and molecular weight on surface properties of chitosan obtained from biowastes

A surface characterization of two types of unmodified chitosan properties with different molecular weight (MW) and deacetylation degree (DD) was carried out in terms of polymer concentration (a concentration lower than the saturation, the saturation concentration and a supersaturated concentration). The modification of the production conditions allowed to obtain chitosan samples with different properties (MW and DD), which eventually determined its surface activity. Thus, a kind of tailor-made chitosan may be designed to be adapted to different applications. Surface activity has been analysed by mean of measurements of surface tension, dilatational and shear rheology. The results have demonstrated that chitosan possessed a good surface activity at high ionic strength. Also, the increase in the concentration of chitosan in the solution, and the increase in molecular weight (and, therefore, a decrease in the degree of deacetylation) favoured the formation of aggregates, which improved the adsorption kinetics. The latter allowed adequate elasticity values to be reached more quickly. These properties are reached faster for the higher molecular weight chitosan (lower degree of deacetylation), due to the more hydrophobic character that facilitates the interactions between the adsorbed aggregates. Surface rheology revealed a predominantly elastic behaviour of chitosan film. All of these results strengthen the idea that chitosan behaves as a surface-active hydrophilic Pickering agent.

Cruciferin versus napin – Air-water interface and foam stabilizing properties of rapeseed storage proteins

Rapeseed protein extract has high nutritional value and good techno-functionalities, e.g. in foam systems. Yet, its exact interface and foam stabilizing mechanisms are not well understood. Rapeseed proteins comprise mainly of cruciferin and napin. Our aim was to systematically investigate the interface stabilization behaviors of cruciferin, napin and combinations of both at air-water interfaces. We used surface (dilatational and shear) rheology and microstructure imaging (AFM) and linked their behavior to their foaming properties. We observed that napin adsorbed faster at the air-water interface than cruciferin due to its smaller size, leading to 90% higher foam overrun than cruciferin (320%). The interfaces showed distinct differences in structure and mechanical properties, as cruciferin formed stiff solid-like interfaces with Ed’ = 72.5 mN/m and Gi’ = 9.0·10−3 Pa m, leading to high foam stability (half-life time 220 min). Napin formed weaker less stretchable interfaces (Ed’ = 61.8 mN/m; Gi’ = 6.7·10−3 Pa m), leading to substantially lower foam stability (half-life time 23 min). Cruciferin and napin were also mixed at 3:1, 1:1 and 1:3 (w/w) ratios. Napin increased the foamability of all mixtures with foam overrun between 400 and 420%. The mixture at 3:1 cruciferin-to-napin ratio had comparable foam stability with pure cruciferin since cruciferin dominated the mechanical properties of the air-water interface. Higher napin contents largely decreased foam stability with half-life time decreasing to 80 min. These findings provide a comprehensive understanding of the behaviors of rapeseed proteins at air-water interfaces and their link to foaming properties, which can be used to tailor the properties of aerated products stabilized by rapeseed proteins.

A comprehensive methodology to study double emulsion stability

HypothesisThe stability of emulsions requires the fast formation of viscoelastic interfaces between water and oil phases. In double emulsions, two surfactant types (hydrophilic and lipophilic) are present and two interfacial films are involved. Understanding cooperative adsorption of these surfactants and its implication on properties of water/oil/water interfacial films will enable replacing the empirical methodologies used in designing double emulsion systems with a knowledge-based approach. Experiments and modelThe distribution of surfactants between the water/oil interfaces was investigated using single droplet diffusion experiments and simulation of equilibrium surfactant density profiles. The stability of the interfaces against coalescence was characterized by dye transport in a leach cell and coalescence time of single droplets in a model experiment. The conformation of the surfactants at an interface was then examined via surface rheology, sum frequency generation spectroscopy, and dissipative particle dynamics simulation. FindingsTwo selected hydrophilic surfactants combined with a lipophilic surfactant induce very different properties at water/oil interfaces and different dye release behaviour from their corresponding double emulsions. Competitive adsorption of sodium dodecyl sulfate and lipophilic surfactant results in the improvement of encapsulation efficiency, elasticity of the interface, and resistance against coalescence due to the intercalation of surfactant alkyl chains into the oil chains.

Jujube Shell Based-Porous Carbon Composites Double-Doped by MnO2 and Ti3C2Tx: The Effect of Double Pseudocapacitive Doping on Electrochemical Properties.

In this study, manganese-containing porous carbon was synthesized from jujube shells by two-step carbonization and activation and was then covered with Ti3C2Tx to obtain double-doped biomass composites. In order to improve the interfacial properties (surface tension and wettability) between Ti3C2Tx and porous carbon, the effects of two media (deionized water and acetone solution) on the electrochemical properties of the composites were compared. The acetone solution changed the surface rheology of Ti3C2Tx and porous carbon, and the decreased surface tension and the increased wettability contributed to the ordered growth of 2D-Ti3C2Tx on the surface of the porous carbon. Raman analysis shows the relatively higher graphitization degree of JSPC&Ti3C2Tx (acetone). Compared with JSPC&Ti3C2Tx, JSPC&Ti3C2Tx (acetone) can maintain better rectangle-like properties even at a higher scanning rate. Under the effect of the acetone solution, the pseudocapacitive ratio of JSPC&Ti3C2Tx (acetone) increased from 10.1% to 30.7%. At the current density of 0.5 A/g, the specific capacitance of JSPC&Ti3C2Tx (acetone) achieved 96.83 F/g, and the specific capacitance of 58.17 F/g was maintained even at the high current density (10 A/g), which shows excellent magnification. Under the condition of the current density of 10 A/g, JSPC&Ti3C2Tx (acetone) can obtain a power density of 52,000 W/kg while maintaining an energy density of 8.74 Wh/kg. After 2000 cycles, the symmetrical button battery assembled with this material can still have a capacitance retention rate of more than 90%. This method realized the deep utilization of green and low-cost raw materials by using biomass as the precursor of composite materials and promoted the further development of carbon-based supercapacitor electrode materials.

Preparation and ex vivo investigation of an injectable microparticulate formulation for gastrointestinal mucosa polyp resection

Endoscopic submucosal dissection (ESD) and endoscopic submucosal resection (EMR) are non-invasive endoscopic techniques. They allow an early excised gastrointestinal (GI) mucosal precancerous lessions. For their application is necessary to use a submucosal injection that lifts the area to excise. The main objective of this study was the preparation of a microparticulate-based fluid for injection in the GI submucosa. Alginate microparticles (MPs) were developed by the solvent displacement technique and characterized by particle size, surface electrical properties, swelling, degradation, rheology, adhesion, leakage, syringeablity and stability. Furthermore, their potential to form a submucosal cushion was assayed in porcine stomach mucosa and porcine colon mucosa. Results showed MPs sizes below 160 μm, negative surface charge around −50 mV at pH = 6, high rates of swelling and good adhesion. The microparticulate-based fluid exhibited pseudoplastic behavior following the Ostwald-de Waele rheological model. A brief force is sufficient for its injection through a syringe. Finally, formulations were able to provide a submucosa elevation of 1.70 cm for more than 90 min and 120 min in the porcine stomach and colon, respectively.

Mechanisms ruling the lifetimes of films of liquid mixtures

Foams and bubbles formed in liquid mixtures have lifetimes longer by several orders of magnitude than in pure liquids of similar viscosities. We have shown recently that this effect results from slight differences in molecular concentrations between bulk and surfaces, which give rise to a thickness-dependent surface tension of liquid films. We present a quantitative description of the enhanced stability of liquid films in binary mixtures, based on experimental data and theoretical analysis. Experiments were performed with mixtures of different natures and compositions: foams of stationary heights were obtained by continuous injection of gas, on one hand, and single bubbles were swollen under the surface of a liquid bath, on the other hand. Remarkably, the lifetimes measured in both experiments exhibit the same variations with mixture composition, and follow a power law with the microscopic length that characterises the amplitude of the thickness-dependent surface tension. In addition, the lifetimes vary with the squared film thicknesses at the onset of bursting. We suggest that two stages occur between the birth of liquid films and their rupture. We show how the thickness-dependent surface tension allows an equilibrium shape to be reached at the end of a first stretching stage. We give an analytical description of this shape, which is fully consistent with experimental findings. We suggest a possible mechanism for final rupture of the film, and discuss it in light of existing theoretical predictions. Finally, we compare the properties of liquid mixtures and surfactant solutions, and in particular their surface rheology.

Contamination in Sodium Dodecyl Sulfate Solutions: Insights from the Measurements of Surface Tension and Surface Rheology

The presence of contamination in sodium dodecyl sulfate (SDS) solutions in the form of dodecanol (LOH) is known to drastically affect the resulting interfacial properties such as surface tension (SFT) and rheology. Dodecanol molecules, which are the product of SDS hydrolysis and are inherently present in SDS solutions, have higher surface activity compared to SDS because they are less soluble in water. A characteristic dip in the SFT isotherm is an indicator of the dodecanol contamination in the sample. The presence of an electrolyte in the solution impacts the surface activity of SDS and its critical micelle concentration, and could yield SFT isotherms that closely match those obtained for pure SDS samples. The interpretation of the isotherms in such cases could thus lead to misinterpretation of the surface purity. In this work, we have examined the SFT isotherms for SDS solutions in both the absence and presence of electrolyte. We have fitted the isotherms to three different thermodynamic adsorption models to estimate the amount of dodecanol present in the sample. We have applied the estimated values for the LOH content in a two-component rheological model to predict the viscoelasticity of such surfactant-laden surfaces. We have compared these results with the experimentally measured interfacial rheological properties. Our findings demonstrate that the presence of impurities can be captured under dynamic expansion and contractions, even for solutions containing background electrolyte.

Impact of structural flexibility in the adsorption of wheat and sunflower proteins at an air/water interface

Food transition requires the replacement in human diet of animal-based proteins by alternative sources of proteins including plant-based proteins. This calls for a detailed knowledge of the functional properties of plant-based proteins, including their surface activity. In this framework, we provide here a comparative study of the interfacial properties of two plant proteins, extracted respectively from wheat and sunflower. We combine time- and concentration-dependent measurements of the surface tension and the surface rheology, as measured with a pendant-drop set-up, and of the surface excess concentration, as measured by ellipsometry, of plant protein interfacial films. We demonstrate a time-concentration superposition principle for the surface pressure and surface excess concentration, showing that the kinetics for the building of the interfacial films is essentially governed by the diffusion of the proteins from the bulk to the interface. We find that the rheological and structural properties of the interfacial protein films show markedly different behaviors for the two classes of protein, which is encoded in the structural features of the individual proteins: wheat proteins are more surface active than sunflower proteins, are keen to compress and re-arrange at an air-water interface, whereas sunflower proteins do not. This work provides qualitative and quantitative analysis of the comparative interfacial behavior of flexible and rigid plant proteins extracted respectively from wheat and sunflower, and demonstrates that a combination of several experimental techniques is necessary to obtain insightful information on the interfacial properties of any species.

Impact of Polymer Nanoparticles on DPPC Monolayer Properties

The application of surface rheology and Brewster angle microscopy on mixed monolayers of DPPC and polymeric nanoparticles (cationic and anionic) showed that the sign of the particle charge affects the dynamic properties of the monolayers less than the nanoparticles’ ability to aggregate. Under almost physiological conditions, the effect of nanoparticles on the elasticity of DPPC monolayer is insignificant. However, the particles prevent the surface tension from decreasing to extremely low values. This effect could affect the functionality of pulmonary surfactants.

Impact of Amphiphilic Nanostructures on Formation and Rheology of Interfacial Layers and on Foam Film Drainage

The aim of the present studies is to clarify how the surfactant adsorption layer properties are related to the course of the drainage parameters of microscopic foam films in the special case of aqueous solutions containing premicellar amphiphilic nanostructures. The scope of the research covers the adsorption dynamics, construction of equilibrium adsorption isotherms, surface rheology of interfacial layers, and foam film drainage kinetics. It is established that, in the premicellar domain, there are concentration intervals, where the considerable irregularities of adsorption layerproperties are observed: several plateau regions in the surface tension isotherms, unusual changes of the surface rheological characteristics, etc. The systematic investigation of the drainage of foam films obtained from these solutions show that the dependences of basic kinetic parameters of the films on the amphiphile concentration run in synchrony with changes in the adsorption layer properties. Thus, the presence of smaller loose aggregates (premicelles) plays a significant role for the kinetic stability of films. The importance of this research is related toproviding a better insight into the initial stages of self-assembling phenomena and into the factors determining thedrainage and the stability of thin liquid films. The results have implications for the understanding and the correct prediction of properties of foam systems.