Interfacial rheology goes contactless
Fluorescence microscopy enables measuring the mechanical properties of liquid interfaces without disturbing them.
The mechanical properties of complex liquid interfaces, i.e. interfacial rheology, are crucial for understanding the stability of emulsions and foams, including everyday examples such as skin cream and whipped cream.
Conventional techniques to measure interfacial rheology rely on macroscopic probes, like a needle or a ring, directly attached to the water-oil interface. However, this couples the interfacial flow to the bulk flows in the water and oil phases, thereby limiting measurement sensitivity. Moreover, the presence of a macroscopic probe in the water-oil interface could significantly affect the structure of the interface itself, which would mean that the mechanical properties of the measured interface are not quite the same as those of the interface without a measurement probe in it.
In their Journal of Rheology paper (and corresponding Scilight showcase), a team of researchers from The University of Edinburgh, Delft University of Technology, and the University of Gothenburg present a contactless method to measure the interfacial rheology of liquid-liquid interfaces using fluorescence microscopy.
One of the authors, Dr Job Thijssen from the School of Physics and Astronomy, explains the method: "Our typical sample is a layer of oil on top of a layer of water, with micron-sized particles attached to the interface between the water and the oil layer. We then rotate the upper (oil) layer in a controlled manner and we measure the response of the particle-laden water-oil interface using a microscope. As we control the rotation of the upper (oil) layer, we can calculate the stress (force per unit area) applied at the interface. From the microscopy videos, we can extract the strain (deformation) of the interface in response to that stress. Hence, we can quantify the mechanical properties of the interface in a flow curve (plot of stress vs shear)."
Importantly, the method should only require relatively common equipment, for example a fixed-rate motor and an optical (fluorescence) microscope, so the team hopes that their paper will contribute to interfacial-rheology measurements becoming more accessible. Through the Edinburgh Complex Fluids Partnership, they are also hoping to apply the novel contactless interfacial rheology technique to everyday and industrial samples.