Key Applications of Interfacial Rheology
Interfacial rheology is the study of the deformation and flow properties of liquid interfaces. When mixing two immiscible fluids, such as oil and water, a fluid-fluid or fluid-air interfacial layer is formed. This interfacial layer can be of any surface-active material such as a surfactant, polymers, or even nanoparticles. By studying the fundamental forces that drive interfacial dynamics, one can achieve optimisation of product performance and stability of suspensions, emulsions, froth and foams. This optimisation can be vital for applications such as those found in oil recovery, personal care products, paints, food and pharmaceuticals, that all require optimum composition for performance and stability.
How does interfacial rheology work?
Surfactants (surface-active agents) absorb to the surface and influence the interfacial properties, and so, are usually added to assist with stabilising an emulsion. Surfactants lower the interfacial tension between the two liquids as they align so that the hydrophobic part is in the air (or oil) and the hydrophilic part is in water. This process forms small oil drops, which creates a mechanical, steric barrier, electrical barrier, or both a steric and electrical barrier between the droplets. The result is a continuous phase which decreases droplet coalescence and maintains stability. Interfacial tension measurements are commonly used to study this occurrence, but these are poor predictors of drop stability. Interfacial dilatational rheology offers a unique tool suitable to investigate the elasticity of air-liquid or liquid-liquid interfaces under dynamic conditions.
How is interfacial rheology measured?
Interfacial Rheology measurements help us to understand the surfactant kinetics and the viscoelastic properties of the adsorbed interfacial layer. By changing the layer size or shape, it is possible to study the elasticity and viscosity of dilation and shear. By analysing the dilation/compression of a hanging droplet, known as the Pulsating Drop method, one can measure Interfacial rheology.
Using the Attension Theta Optical Tensiometer combined with pulsating droplet module, PD-200, drop shape analysis makes dilatational interfacial measurements. In the pulsating drop measurement, the volume of the drop receives pulses with the known frequency. The area variation, as well as the surface tension change as a function of frequency, is detected. One uses this information to calculate the surface elasticity.
The role of an automated optical tensiometer
An automated optical tensiometer is a contact angle meter used in research, development, and quality control for the study of surfaces and interfaces. This meter records drop images and analyses the drop shape automatically. The drop shape is a function of the surface tension of the liquid, gravity, and density.
On a solid, the drop shape and the contact angle also depends on the properties of the solid (e.g., surface free energy, topography). A drop profile method analyses the captured image to determine the contact angle and surface tension. Performing contact angle measurements with several known liquids calculates surface free energy. As a method, the measurement precision of optical tensiometers depends on the quality of the pictures and the analysis software.
Attension Optical Tensiometers utilise a high-quality monochromatic cold LED light source to minimise undesirable sample evaporation. A high-resolution digital camera with quality optics guarantees image quality and the accuracy of the drop fitting method.
Common applications for interfacial rheology
Interfacial rheology is applicable in many interesting areas, including the following common scenarios:
Medical implants and their biocompatibility
Various materials, such as metals, ceramics, and polymers, used as implants, are usually modified through mechanical roughening, chemical treatment, or both to enhance biocompatibility with the surrounding host tissue. Implant development and quality control benefit from a deeper understanding of the impact between chemical and mechanical treatment.
Paper and board coatings
Paper and board coatings is another application where understanding the effect of roughness on wetting may help with optimising coating formulations for enhanced appearance and durability.
Emulsifying agents and proteins
Some proteins are naturally surface-active, which means they are good foaming and emulsifying agents and widely used in food industry products such as mayonnaise, butter, milk, and coconut milk. Interfacial rheology discloses the stability of the interfacial layer, the layer between the dispersed and continuous layers.
Crude oil components
Commonly produced, stable oil-water emulsions appear in various oil production processes. Asphaltenes, resins, waxes, in crude oil adsorb to the oil-water interface forming rigid interfacial films that are extremely difficult to break. Interfacial rheology studies display the properties of the formed interfacial layer and facilitate the development of better demulsifiers.
Phospholipids are the primary building blocks of the cell membrane, which needs to be rigid enough to support the cell structure but at the same time allow lateral transportation of different substances. Interfacial rheology can give valuable information on phospholipid function. One active research area is in nanotoxicity studying the effect of nanoparticles incorporated into cell membranes.
Interfacial rheology offers a tool to study the fundamental behaviour of surfactants, found in cleaning products and various industrial areas from coatings and inks to enhanced oil recovery agents. Additionally, interfacial rheology optimises stability and meets environmental standards.
Obtaining accurate measurements with the Attension Theta
The Attension Theta automated contact angle meter is modular, meaning it can be suited to meet the application. This includes:
- A compact, optional module, the pulsating drop module, measures surface dilatational viscosity and elasticity based on the modified pendant drop technique. This automated measurement determines interfacial rheology in both liquid-air and liquid-liquid interfaces. The Theta Flex Optical Tensiometer, equipped with a piezoelectric transducer (pulsating droplet module, PD-200), is a powerful combination for studying flow behaviour and determining dilatational interfacial rheological parameters.
- The pulsating drop module oscillates drops and provides complete control of both frequency and amplitude as well as wave shape (sine, triangle, square, etc.). A unique, automatic volume adjustment controls the experiment. It compensates g for evaporation, which enables quick and easy interfacial rheology measurements to provide viscoelasticity information for surfactant applications and to predict emulsion stability.
- The 3D topography module (unique to Biolin Scientific) measures 3D surface roughness and contact angle on precisely the same sample location from where the contact angle is defined. The 3D topography method can simultaneously perform 2D and 3D characterisation at pixel-level resolution while the software – OneAttension – automatically calculates surface roughness and roughness corrected contact angle and surface free energy.
What separates the Attension Theta from other solutions?
Until now, contact angle and surface roughness have only been measured individually by using an optical tensiometer and a separate roughness measurement instrument. This has meant that researchers have relied on the roughness corrected contact angle to assess the influence of surface roughness and chemical modification separately on wetting and adhesion properties. This feature is essential when working with different types of surface modifications with altered surface chemistry and surface topography.
The Attension Theta Flex is a fully automated optical tensiometer used to characterise surface properties such as contact angle and surface tension between gas, liquid, and solid phases. To learn more about the Attension Theta Flex or any material science solutionsspeak to ATA Scientific today.