A novel shear and dilatational interfacial rheometer for the study of complex interfaces applied to polymers at the air-water interface.

The radial trough has been introduced to facilitate the study of dilatational interfacial rheology of complex fluid interfaces by overcoming the mixed interfacial flow fields encountered in the standard rectangular Langmuir trough or the pedant drop. For the measurement of interfacial shear rheology, it has been determined that control of the surface pressure during measurements as carried out in the Double Wall Ring device mounted on a Langmuir trough (DWR-T¹) or the ISR, is required for obtaining reliable reproducible data.  In this contribution, we utilize a new trough instrument, the Quadrotrough   (Y.S. Tein et al. Review of Scientific Instruments, 93, 093903 (2022)), with the capability of performing on the same device (and sample) independent shear and dilatational deformations at the air/liquid interface under strain and surface pressure control. This enables unique control of the shear history of the interface as well as providing a method for shear annealing of the interface, enabling determination of the true equilibrium interfacial surface pressure and rheology.  Furthermore, dynamic oscillatory deformations enable determining the frequency dependent bulk and shear moduli, enabling calculation of the Poisson ratio for the interface.  Brewster angle microscopy imaging, particle tracking, and neutron reflectometry can be carried out in situ simultaneously with the rheological measurements to provide microstructural characterization from the molecular to mesoscale for determining rheological structure-property relationships at interfaces. Instrument, performance is validated by measuring the interfacial rheology of poly (tert-butyl methacrylate, PtBMA) at the air/water interface. Shear and dilatational deformation measurements were carried out over a wide range of surface concentrations. BAM images were obtained during shear and compression. The results obtained by the Quadrotrough are shown to be in agreement with measurements performed previously with the DWR-T and the radial trough. Measurements of the shear and dilatational moduli both in nonequilibrium equilibrium states as well as at finite frequencies and amplitudes are presented for several polyacrylates with increasing glass transition temperature and compared with prevailing theories for polymers at the interface.