Rheology Of Thin Films Research Articles

Thin-Film Rheology and Tribology of Imidazolium Ionic Liquids.

Ionic liquids (ILs) are organic molten salts with low-temperature melting points that hold promise as next-generation environmentally friendly boundary lubricants. This work examines the relationship between tribological and rheological behavior of thin films of five imidazolium ILs using a surface force apparatus to elucidate lubrication mechanisms. When confined to films of a few nanometers, the rheological properties change drastically as a function of the number of confined ion layers; not only the viscosity increases by several orders of magnitude but ILs can also undergo a transition from Newtonian to viscoelastic fluid and to an elastic solid. This behavior can be justified by the confinement-induced formation of supramolecular clusters with long relaxation times. The quantized friction coefficient is explained from the perspective of the strain relaxation via diffusion of these supramolecular clusters, where higher friction correlates with longer relaxation times. A deviation from this behavior is observed only for 1-ethyl-3-methylimidazolium ethylsulfate ([C2C1Im][EtSO4]), characterized by strong hydrogen bonding; this is hypothesized to restrict the reorganization of the confined IL into clusters and hinder (visco)elastic behavior, which is consistent with the smallest friction coefficient measured for this IL. We also investigate the contrasting influence of traces of water on the thin-film rheology and tribology of a hydrophobic IL, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, [C2C1Im][FAP], and a hydrophilic IL, [C2C1Im][EtSO4]. [C2C1Im][EtSO4] remains Newtonian under both dry and humid conditions and provides the best lubrication, while [C2C1Im][FAP], characterized by a prominent solid-like behavior under both conditions, is a poor lubricant. The results of this study may inspire molecular designs to enable efficient IL lubrication.

Thin film rheology via focused laser spike thermocapillary dewetting

Focused laser spike (FLaSk) dewetting employs a localized heat source to create thermocapillary-induced trench-ridge morphologies. Using a universal heating substrate coupled with optical microscopy, we have studied the dewetted ridge feature for several distinct thin films of glass-forming materials. The evolution of the ridge’s radius over time can be modeled to derive a maximum dewetted radius and a characteristic decay time. The decay time shows an Arrhenius behavior when compared to the mean melt pool temperature, leading to a characteristic activation energy. An effective viscosity can also be defined. These descriptors demonstrate FLaSk’s potential for rapid metrology.

Contactless thin-film rheology unveiled by laser-induced nanoscale interface dynamics.

One of the classical limitations for the investigation of the local rheology of small scale soft objects and/or confined fluids is related to the difficulty to control mechanical contact and its consequences. In order to overcome these issues, we implement a new local, active, fast and contactless optical strategy, called optorheology, which is based on both the optical radiation pressure of a laser wave to dynamically deform a fluid interface and interferometry to probe this deformation with nanometric resolution. This optical approach is first validated by measuring the surface tension and the viscosity of transparent Newtonian liquids. We also show how non-equilibrium situations, such as continuous evaporation, can be used to deduce the thickness dependence of the rheology of thin films and the concentration dependence of the viscosity of binary liquid mixtures and suspensions. We further extend the investigation to elasticity and viscoelasticity measurements of polymer solutions. Finally, since liquids may absorb light, we discuss the influence of a weak laser heating and the triggering of interface deformations by thermocapillary tangential stresses that could represent a complementary approach to probe the rheology at small scale.

PEKMEZ-TAHİN KARIŞIMLARININ TRİBOLOJİSİ

A new field of texture analysis called “tribology” or “thin film rheology” that might explain the food texture throughout oral processing has become important. In this study, tribological experiments for samples containing pekmez and tahin at 30:70, 40:60 and 50:50 (w/w) were conducted by using a modified pin-on-disk tribometer. In the Stribeck curves hydrodynamic and boundary regions were determined. For the mixture containing pekmez:tahin at 50:50, hydrodynamic region started at a lower speed compared to other samples, which indicated a developing a thin lubrication film between contact surfaces. The friction decreased with the oil content of the sample. This was more profound in the lower speed region for all samples. Tribological data provided more distinct evaluation compared to the viscosity data which were close to each other due to similar composition of samples. Further research are needed on tribological properties of foods, their relation to rheology and sensory analysis.

Kappa carrageenan fluid gel material properties. Part 2: Tribology

Semi-solid and liquid food thickeners typically take the form of either polymeric or particulate structures. These structures are known to control flow properties and mixing efficiency which can influence performance, texture and the perception of tastants and aromas. However, their structural influence on thin-film rheology (tribology), which is also relevant for texture perception, is not so well understood. In this investigation, the tribology in a boundary regime of lubrication is tested using kappa carrageenan lubricants formulated both in solution and as gelled particles in suspension (fluid gels) to provide new insights into the structural influence of thickener type on tribology. Polymeric lubricated systems were shown to be dominated by elastic deformation of the tribo surfaces and particulate suspensions were dominated by particles acting as contacting asperities of the mating surfaces. The tribology of gelled particles was shown to depend strongly on particle elasticity where less deformable (stiffer) particles reduce surface–surface contact and therefore reduce friction coefficients. The effect of particle volume fraction on friction coefficient is related to the number of particles entraining the contact and not particle–particle interactions or bulk rheological behaviour.

Rheology of thin films from flow observations

A major challenge regarding thin films is the characterization of their rheology and the measurement of the fluid physical parameters. For complex fluids, performing direct rheology measurements is extremely difficult considering the geometric characteristics of thin films. In this paper, we present a method for characterizing the film rheology based on measurements at regular time intervals of the film surface topography. These measures allow us, by solving an inverse problem, to validate a model of rheology for the thin fluid film and to determine the physical parameters specific to this model.

Thin liquid film lubrication under external electrical fields: Roles of liquid intermolecular interactions

One of the important features of the nanoscale liquid film lubrication is the formation of ordered layers at the solid/liquid interface. In this paper, the effect of the intermolecular interaction in liquid lubricant films on the formation of ordered layers after applying external electric fields (EEFs) has been investigated by measuring the central-film-thicknesses of liquids in concentrated point contacts and then inferring the thin film rheology. It has been found that the film formation properties of both pure liquid n-alkanes and liquid n-alcohols with relatively long chains have weak responses to EEFs, while those of their mixed solutions could be enhanced more notably by EEFs. In addition, the effect of the dispersive interactions between solvent molecules on the formation of ordered layers in thin lubrication films under EEFs was also discussed.

Thin Film Rheology of the Paint Film Formation Process and the Multilayer Paint Film

The solvent-based paint has been changed into a water-based paint and higher non-volatile paint because of environmental problems. In addition, the paint film formation process was shortened such as the wet on wet process. In this paper, we measured film formation process using the electric field tweezers in open systems. In multilayer paint film formed by wet on wet process, we measured the dynamic glasstransition temperature of single layer using the differential scanning calorimetry, and we measured crosslinking density of single layer using the micro vickers hardness tester.

3D Force and Displacement Sensor for SFA and AFM Measurements

A new device has been designed, and a prototype built and tested, that can simultaneously measure the displacements and/or the components of a force in three orthogonal directions. The "3D sensor" consists of four or eight strain gauges attached to the four arms of a single cross-shaped force-measuring cantilever spring. Finite element modeling (FEM) was performed to optimize the design configuration to give desired sensitivity of force, displacement, stiffness, and resonant frequency in each direction (x, y, and z) which were tested on a "mesoscale" device and found to agree with the predicted values to within 4-10%. The device can be fitted into a surface forces apparatus (SFA), and a future smaller "microscale" microfabricated version can be fitted into an atomic force microscope (AFM) for simultaneous measurements of the normal and lateral (friction) forces between a tip (or colloidal bead probe) and a surface, and the topography of the surface. Results of the FEM analysis are presented, and approximate equations derived using linear elasticity theory are given for the sensitivity in each direction. Initial calibrations and measurements of thin film rheology (lubrication forces) using the "mesoscale" prototype show the device to function as expected.

Basics of mixed lubrication

AbstractThis paper reviews recent research in mixed lubrication, focusing on the current understanding of lubricant entrainment, and thus film thickness, and of friction in thin‐film, rough‐surface lubricated contacts. By combining research using optical interferometry on the experimental side and numerical modelling on the theoretical side, we now have a reasonable understanding of micro‐elastohydrodynamic lubrication, although design rules are still lacking. The regime of true mixed lubrication, where there are both elastohydrodynamic and boundary lubricated regions within a single contact, remains quite poorly understood. New experimental techniques as well as new information about very thin‐film rheology under high‐strain and high‐pressure conditions are probably needed before much further progress can be made in this area.

Oral behaviour of food hydrocolloids and emulsions. Part 1. Lubrication and deposition considerations

Biopolymers (polysaccharides and proteins) and their mixtures are used in the Food Industry to impart stability, texture and appearance to fabricated foods. This paper reviews the physical measures conducted to elucidate textural properties such as creaminess, smoothness, sliminess and thickness of food products and discusses oral processing mechanisms in relation to the behaviour of hydrocolloids and emulsions in the oral cavity during eating. In particular, this article covers the use of Tribology and Evanescent Wave Spectroscopy techniques that enable the study of the lubrication and deposition behaviour of food components. Comparison of the physical measurements with sensory properties indicate that thin film rheology and surface deposition phenomena make an important contribution to sensory properties such as fattiness, smoothness and astringency.

Thin film rheology and lubricity of hyaluronic acid solutions at a normal physiological concentration.

Using a surface forces apparatus to measure forces, and optical (multiple beam) interferometry to measure surface shapes and separations (to +/-1 A), the normal, viscous, and shear (lubrication) forces between smooth mica surfaces in aqueous hyaluronic acid (HA) solutions were measured. The experimental conditions of loading pressures, pH, and HA concentration were set to closely correspond to physiological human knee-joint conditions. From the force and optical (refractive index) measurements, it was concluded that, like other negatively charged polyelectrolytes, HA does not naturally adsorb on the mica surface which is hydrophilic and weakly negatively charged at physiological conditions: the polymer solution exhibits the bulk viscosity (22.5 +/- 1.5 cP) for films thicker than about 0.4 miccrom of the polymer, whereas for thinner films, the viscosity decreases monotonically toward the value of the pure electrolyte solution (1 cP) as HA is extruded from between the surfaces. This is indicative of a repulsive "depletion" interaction of HA with each mica surface and to a weakly attractive polymer-mediated force between the two surfaces. Thus, free HA in synovial fluid is not expected to act as a good "boundary lubricant." Relaxation measurements on approaching and receding surfaces in HA solutions were also performed, and it is shown that the presence of HA in the bulk solution can improve "hydrodynamic" modes of lubrication, for example, by assuaging the compression stroke. The study includes information that is beneficial to researchers working with biomaterials viscosupplementation devices.

LITERATURE ABSTRACTS: GENERAL PRINCIPLES

Texture Degradation Kinetics of Gels Made from Pacific Whiting Surimi. The Geometry of Drop Distortion. Biophysical Basis of Pale, Soft, Exudative (PSE) Pork and Poultry Muscle: A Review. The Geometry of Drop Distortion. Biophysical Basis of Pale, Soft, Exudative (PSE) Pork and Poultry Muscle: A Review. Conformational and Rheological Dynamics of Semiflexible Macromolecules Undergoing Shear Flow: A Nonequilibium Brownian Dynamics Study. Rheology of Full‐fat and Lo‐fat Cheddar Cheeses as Related to Type of Fat Mimetic. Factors Affecting Viscosity of Slurries of Oat Groat Flours. Effects of Lactic Acid, Acetic Acid, and Table Salt on Fundamental Rheological Properties of Wheat Dough. K. Effects of Granular Structure on the Pasting Behaviors of Starches. of Gliadin‐Rich Subfractions of Glenlea Wheat on the Mixing Characteristics of Wheat Flour. Water Distribution in Frozen Lean Wheat Doughs. J. Puncture and Stress Relaxation Behavior of Blackgram (Phaseoh rnunggo) Flour‐Based Papad Dough. Rheological Characterization of Kappa‐carrageenan Milk Gels. Effects of Concentration and Temperature on the Ftheology of Concentrated Milk. Dynamic Rheological Properties of Mozzarella Cheese During Refrigerated Storage. Viscoelastic Characterization of Surimi Gel: Effects of Setting and Starch. New Method to Determine Wall Shear Stress Distribution. Measurement of Extrusion Effects by Viscosity Profile Using the Rapid Visco Analyser. Measure of Fracture Toughness of Bread Crumb Factors Affecting the Visco‐Amylograph and Rapid Visco‐Analyzer Evaluation of the Impact of Annealing on Starch Pasting Properties. Comparison of the Brabender Viskograph and the Rapid Visco Analyser. 1. Statistical Evaluation of the Pasting Profile. Prediction of the Compressive Deformabilities of Multilayered Gels and Texturized Fruit, Glued Together by Three Different Adhesion Techniques. Spectrum Analysis of Mixing Power Curves for Neural Network Prediction of Dough Rheological Properties. Peach Firmness Determination Using Two Different Nondestructive Vibrational Sensing Instruments. Mass and Drop‐Height Influence on Kiwifruit Firmness by Impact Force. Bruising on Blush and Non‐Blush Sides in Apple‐to‐Apple Impact. Thin Film Rheology and Tribology of Chocolate. Measuring Texture of Crackers with a Multiple‐point Sheet Sensor. K. Comparison of Work Input Requirement on Laboratory‐Scale and Industrial‐Scale Mechanical Dough Development Mixers. and Color Retention in Blanched Green Beans and Green Bell Pepper (Res. Note). Flavor Quality and Texture of Modified Fatty Acid High Monoene, Low Saturate Butter. Changes in Viscosity Properties of Potato Starch During Growth. The Influence of High Intensity Ultrasound Baths on Aspects of Beef Tenderness. Oxidative Stability, Cooking Yield and Texture of Pork Treated with a Low‐Sodium Salt. Changes in the Perceived Textural Properties of Cheddar Cheese During Maturation. Textural and Viscoelastic Changes of Canned Biscuit Dough During Microwave and Conventional Baking. Processing Parameter Effects on Textural Characteristics of Reduced Fat Pork Sausage. Irreversible Shear Thinning and Thickening of Tomato Juice. Study of Prestorage Heat Treatment Effect on Apple Texture: Destructive and Nondestructive Measurements. Effect of Oil Droplets on Brittleness of O/W Emulsion Gel. Studies on Development of Reduced Sugar Apple Jelly. Approaches to Translation Problems of Sensory Decriptors. Sensory Profile Description of Mozzarella Cheese and Its Relationship with Consumer Preference.

Thin Film Rheology and Tribology of Chocolate

ABSTRACTWe investigated relations between food texture and thin film properties rather than bulk properties. Such thin film rheological or‘tribological’effects may characterize physical behavior and physical perceptions of food products. Experiments were done on three chocolate samples. The Surface Forces Apparatus (SFA) proved to be effective for studying the very complex chocolate system. Very complex thin film behavior was found to be quite different for the three samples. Fat constitution and average particle size were important in determining the type of friction forces generated between shearing/sliding surfaces separated by thin films of chocolate. Such studies can help characterize specific physical qualities of chocolate.

Thin Film Rheology and Tribology of Confined Polymer Melts: Contrasts with Bulk Properties

A modified Surface Forces Apparatus was used for making rheological and tribological measurements of thin fluid films, and polybutadiene (PBD) of Mw ≈ 7000 was studied in detail. At low shear rates, PBD exhibits bulklike properties in films thicker than about 200 nm. In thinner films (200−20 nm), the shear viscosity ηeff and moduli G‘ and G‘‘ become quantitatively modified from those of the bulk. However, a sinusoidal input still produces a sinusoidal output, and a modified WLF representation remains applicable to such films. On entering the tribological regime (film thickness <30 nm) polybutadiene exhibits highly nonlinear behavior and yield points, indicative of phase transitions to “glassy” or “solidlike” states. Other features are (i) a shift of the slip plane, (ii) an additional normal force component, and (iii) the emergence of new scaling relationships for ηeff, G‘, G‘‘, and the friction force F as a function of frequency, sliding speed, and other system parameters.

The Rheology of Thin Films

Friction and its reduction by lubrication are areas of physics that are directly encountered by most people. The property is, however, one that is not easily measured and cannot be predicted a priori; fortunately, frictional effects may be uncoupled and this permits a measure of understanding.