Steven D. Hudson

National Institute of Standards and Technology

Physical Scientist

Fellow, Elected 2023

Dr. Steven Hudson has made significant contributions to rheology across of a broad range of soft matter systems through the development of creative microfluidic methodologies and by extraction of insightful parameters that highlight important phenomena. Notably he was an early pioneer in the incorporation of microfluidics into the field of rheology and built experimental platforms for microfluidic metrology – such as velocimetry, interfacial tensiometry and droplet dynamics – that helped develop this field. Here, we indicate three areas where Dr. Hudson has made profound impact.

High Shear Rate Micro-Rheometers: Rheometric methods are always challenged in the high shear rate regime by instabilities and sample volume restrictions. Dr. Hudson’s work to develop high shear rate micro-rheometers has smashed these limitations. Their recent capillary and slit rheometers access shear rates of approximately one million/s. The first of these rheometers was especially for small volume (10 uL) of protein solutions, in support of the biopharmaceutical industry’s significant interest in small volume and convenient techniques. Concentrated self-associating protein solutions exhibited Newtonian behavior at modest rate and shear thinning at higher rates, which were correlated to structural relaxation rates measured by light scattering.

Viscoelastic Flow: Dr. Hudson’s work in viscoelastic flow measurements via microfluidic metrology has pushed the field further by demonstration that simultaneous velocimetry and stress are achievable. Dr. Hudson and his team utilize high-speed imaging that combines two modalities by switching the illumination every other frame (at several thousand fps). The system thus can measure the flow in three dimensions and the polymer stress in two dimensions. This instrument is therefore able to examine normal stress differences explicitly, locally and in real time. This allowed them to show for the first time that for viscous flow, principal axes of flow and stress are aligned, but they are misaligned when the normal stress difference is finite. This local measure of normal stress can be used to understand flow instabilities in cross-slot flow of polymer solutions. During unstable flow, the normal stress accumulates before each switching event.

Shear Thickening in Colloids: Dr. Hudson’s work in colloids showed the importance of normal force measurements to understand shear thickening. Together with collaborators, Dr. Hudson’s early measurements of normal force during continuous shear thickening (CST) exhibited a smooth transition from negative to positive N1. At the time, the focus was on discontinuous shear thickening. By testing CST, however, the transition was more accessible to experimental analysis. Based on the viscosity data, Wyart’s concept of a fraction of frictional contacts was adapted effectively to define when N1 becomes positive and to demonstrate limits to the formation of such contacts at lower colloidal volume fraction.