Rheology measurements by sessile drop oscillation

• Applicant: Dr.-Ing. Peyman Rostami, Ph.D.
• Subject Area: Fluid Mechanics ; Experimental Condensed Matter Physics
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 579452955

Understanding the rheological properties of complex fluids is crucial for a wide range of applications, including those in the food industry and inkjet printing. Fluids are broadly classified as Newtonian or non-Newtonian. The latter category includes viscoelastic, shear-thickening, yield-stress fluids, and etc. which exhibit complex behaviors. Rheometers are the conventional instruments for probing such properties through measurements such as flow curves, frequency sweeps and Large Amplitude Oscillatory Shear (LAOS). While rheometers are highly informative, they face several limitations, including inertia effects at high frequencies, difficulties with time-dependent fluids (e.g., thixotropic systems), and the requirement for large sample volumes, which is impractical for expensive materials. This proposal aims to complement rheometry by studying how sessile drops of complex fluids oscillate under controlled frequencies and amplitudes, using high-speed imaging. Analysis of drop deformation and stress response will enable the extraction of rheological properties, including the storage modulus (G′) and loss modulus (G″). This technique offers several advantages: it extends measurements to higher frequencies, reduces experimental time, and requires only microliter-scale samples. We will adapt the measurements for the specified fluids and for chirp rheometry, and then compare the results from the drop experiments with both rheometry data and published findings. The main challenge is that natural oscillation modes affect drop dynamics. To overcome this, we will vary drop volume to shift the eigenfrequency. The second possibility is to calibrate method with Newtonian drops and then applying the method to complex fluids. We will also develop machine learning models to interpret responses near eigenfrequencies, trained on data collected at low and high frequencies. This work will be done in collaboration with Professor Constante Amores (University of Illinois Urbana-Champaign). For viscoelastic fluids (e.g., polymer solutions), experiments will be complemented by numerical simulations in Basilisk, conducted in collaboration with Professor Constante Amores (University of Illinois Urbana-Champaign) and Professor Mostert (University of Oxford). Experimental data will be used to validate these simulations, which will in turn expand the accessible parameter space to higher frequencies and amplitudes.

• DFG Programme: Position