Short Course #1
Interfacial Rheology and Fluid Mechanics: The Thin Film Mechanics in Foams, Emulsions, and the Human Body
Date: August 02, 2020 8:30 - 17:30 hs
Prof. Gerald Fuller
Chemical Engineering, Stanford University
Numerous natural phenomena, technological applications, and advanced materials involve thin liquid film dynamics and, when free surfaces are present, interfacial rheology and surface tension gradients must be taken into account. The course will begin with a general overview of capillarity and the construction of boundary conditions at fluid-fluid interfaces. Next, the course will introduce the participants to the interfacial material functions that must be considered when constructing the boundary conditions that must be used when considering free surface flows. Both shearing and dilatational interfacial deformations will be discussed along with the associated material functions of interfacial shear and dilatational viscosity and viscoelastic moduli. Measurement techniques for each of these material functions will be described. In the case of shearing deformations, the use of magnetic probes (the Interfacial Shear Rheometer with a magnetic needle and the “rotating button”), the double wall ring accessory, and the bi-cone will be explained. Dilatational rheometry using inflating/deflating pendant bubbles and drops will be described.
To properly understand thin film fluid dynamics and applications in emulsion/foam stability and problems in human health, experimental platforms with the ability to control and measure interfacial flows must be used. For this purpose, the students will be introduced to a number of instruments whereby interfacial flows can be monitored with great precision. The i-DDrOP (interfacial drainage and dewetting optical platform) will be described. This instrument allows one to track the drainage of liquids over curved surfaces while controlling the surface pressure of insoluble surfactant layers. The DFI (dynamic fluid film interferometer) is an instrument that serves two purposes: the measurement of dilatational viscoelasticity and monitoring fluid film drainage during the approach and ultimate coalescence of bubbles and drops against interfaces. The modification of the DFI to measure biological layers, such as the tear film directly on the human eye, is described. In addition, the classical Sheludko capillary cell is presented, as well. Each of these interfacial flow instruments utilizes thin film, optical interferometry as a means determining the thickness of draining films in space and time. The design of interferometers and the inversion of color interference patterns to film thickness determination is also described.
The key concepts and experimental methods described above are best illustrated through examples and the following applications will be described:
1. Oil/water emulsion stability in the presence of asphaltenes. Asphaltenes, a complex, aromatic class of constituents of heavy oils, are classified through their solubility: they are soluble in toluene but insoluble in n-heptane. Their strong propensity to adsorb at interfaces, makes them problematic in the transportation of heavy oils and in breaking water-in-oil emulsions. This latter challenge to the production of heavy oils implicates the strong interfacial viscoelasticity that accompanies the adsorption of asphaltenes onto oil/water interfaces. The use of both shear and dilatational interfacial rheology is reviewed as well as the recent discovery of spontaneous emulsification by the presence of asphaltenes.
2. Stability of the tear film. The tear film is a structured, thin film that protects the cornea against dehydration and infection. However, the phenomena of dry-eye affects a large population of people and occurs when the tear film dewets from the cornea surface. There are numerous reasons for this affliction, such as an inability to produce the correct Meibomian lipid mixture. This insoluble layer is spread upon blinking and has been determined to be interfacially viscoelastic. This non-Newtonian behavior is thought to be critical to the success of this layer in postponing dewetting and protecting against dry eye.
3. Stabilization of nonaqueous foams. The stability of foams is linked to the drainage and ultimate coalescence of bubbles a foam. Resistance to coalescence and coarsening of foams can arise through Marangoni stresses that result from spatial variations in surface tension or through increased viscoelasticity of the bubble interfaces. The former mechanism can result through temperature gradients, redistribution of surfactant, or, surprisingly, by evaporation. Stabilization of foams through enhanced interfacial viscoelasticity is a result of an increased resistance to film rupture through a strengthening of the bubble interfaces. The DFI is demonstrated to provide clear evidence of the mechanism of stabilization.
4. Investigations of biofilm mechanics. Bacteria colonies produce biofilms as a means of protecting the colony against desiccation and mechanical stress. These films also promote genetic diversity. One large class of biofilms are termed pellicles, which exist at the interface between the aqueous bacterial colonies and air, making them suitable for exploring their mechanical properties using interfacial rheology. The use of surface shear viscoelasticity provides a convenient and quantitive method to follow the kinetics of biofilm growth and to determine the particular proteins that contribute to the strength of these structures.
Short Course #2
Structure and rheology of soft colloids: fundamentals and applications
Date: August 01, 2020 8:30 - 18:30 hs
Prof. Dimitris Vlassopoulos
Materials Science & Technology, University of Crete and Institute of Electronic Structure & Laser, FORTH, Greece
Prof. Michel Cloitre
Molecular, Macromolecular Chemistry, and Materials
ESPCI Paris, CNRS, PSL University, France
A great number of advanced responsive materials and applications involve formulations based on soft colloids. Softness imparts a plethora of rheological phenomena and transitions in colloidal suspensions and offers ample opportunities for tailoring their structure and flow behavior. Soft colloids are often seen as hybrids encompassing polymeric and colloidal features which affect their properties. The course will introduce this class of soft materials by providing fundamental concepts for understanding their internal microstructure, interactions and macroscopic properties. In the second part, specific examples from flow applications will be discussed.
Basic elements of colloid and polymer physics and fluid mechanics
Classification of soft colloids and the importance of model systems: particle microstructure and elasticity, shape, solvent or temperature effects, solvent-free colloids.
2. Interactions and structure
Volume fraction, state diagrams, flow-induced transitions.
Linear viscoelasticity, diffusion, signatures of glass/jamming transitions at different conditions, analysis (modeling, simulations).
4. Nonlinear shear rheology
Transient stress, yielding and flow, instabilities (slip, shear banding, fracture), analysis (modeling, simulations).
5. Mixtures with varying interactions
Osmotic effects, role of different additives, gel and glass states, analysis (effective interactions, simulations).
6. Special topics and applications
Aging and slow dynamics, rheo-physical instrumentation, thixotropy, attractive soft colloids, industrial formulations (cosmetics, food, energy).