Viscosity is a parameter describing a fluid’s resistance to flow. A fluid with low viscosity moves easily with little internal friction, while fluids with high viscosity resist motion because of a high internal friction. The SI unit of viscosity is pascal second (Pa S), but it is also often expressed as centipoise (cP), which is equivalent to mPa S.

Controlling viscosity is an important part of commercial protein production in fields ranging from biotherapeutics to industrial enzymes with concerns including manufacturability and deliverability. Many factors influence the viscosity of a biologic sample, such as surface charge, pH, and ionic strength. However, the biggest factor influencing viscosity is temperature. For most liquids, increasing temperature decreases viscosity, as the increased thermal movement of molecules helps to overcome attractive forces between them. As an example, water has a viscosity of 1.00 cP at 20 °C, which decreases to 0.65 cP at 40 °C.

Viscosity in DLS experiments

Knowing the viscosity of a sample is a prerequisite to properly determine the hydrodynamic radius of particles from a dynamic light scattering measurement on Prometheus Panta. Only when particles move freely due to Brownian motion is the diffusion coefficient inversely proportional to particle size. While this condition is typically met for dilute samples in water-based buffers, high concentration samples (e.g. late-stage mAb formulations of >100 mg/mL) are more likely to experience particle-solvent and inter-particle interactions which can increase the solution viscosity. For these samples, the solvent viscosity does not accurately describe the system. Instead, the bulk viscosity (including the solvent and solvated particles) must be determined and used.