Puff rheometer for fast and contactless measurements of viscosity and surface tension

To enable in-situ and in-line characterization of the viscoelastic and interfacial properties of soft materials, we are developing a fast and non-manipulative apparatus. An air puff with a known pressure profile is applied to the surface of the material and the resulting surface deformation depends on the rheological, interfacial and physical properties of the material. Our puff rheometer combines measurements of the air-deformed surface and a numerical model within an optimization procedure. The numerical model, based on the finite element method, is used to model the deformation of the fluid caused by the air puff. The experimental setup uses pressurized air and a solenoid valve to control the air puff, and measures the deformation of the fluid surface with a laser sensor. With an optimization procedure, the numerical model can be matched to the live measurements, giving access to the surface tension and rheological properties. We analyze the sensitivity of the measurement results to various input parameters and material characteristics to quantify the potential of this method. Moreover, a simple analytical model capturing the main physical mechanisms is shown to accurately describe the experimental and numerical data for Newtonian fluids, which facilitates data extraction.