Drop impact of non-Newtonian dairy-based solutions

Abstract

Vertical drop impacts of dairy-based solutions on to smooth stainless steel surfaces have been experimentally studied using high-speed photography. Seven test fluids were prepared using infant formula, whole milk, and whey protein powders, reconstituted at 10–45% total solids content in water. Their shear-thinning rheology yielded Herschel-Bulkley consistency indices up to 385 (mPa s) n with a flow index of n = 0.67. Impacts covered ranges of drop diameters (1.75–3.15 mm) and impact velocities (0.90–3.50 m s −1 ) producing Weber numbers between 35 and 605. Observed outcomes for different fluids were dominated by differences in rheology, and shear-thinning was evident when comparing maximum spread of the two most viscous solutions. To describe the normalised maximum spreading diameter ( β max ), a model which interpolates between capillary and viscous regimes, and which was developed for Newtonian fluids, was successfully adapted for shear-thinning drops. The time taken to reach maximum spread ( t s ) was best described using power-law models in which the dimensional time is proportional to either the Reynolds or Ohnesorge number raised by an exponent. This research should assist with understanding spray-drying of milk, and will be more broadly applicable to processes involving spraying and impact of shear-thinning droplets. • A detailed study of dairy-based liquid drop impacts using high-speed photography. • The fluids with high solids concentrations are non-Newtonian (shear-thinning). • Outcomes for different fluids are dominated by differences in rheology. • Appropriate models are identified for the extent and timing of maximum spread. • The study has broad relevance for food processing using stainless-steel surfaces.