Sprays and droplet impact
A major motivation for our work on complex fluids comes from agricultural spraying in the Netherlands, where spray drift and droplet bounce-off on hydrophobic crops cause substantial pesticide loss to soil, groundwater, and surface water. Conventional surfactant-based additives can improve deposition but often worsen drift and raise environmental concerns. Polymeric additives offer an alternative: they increase average droplet sizes and broaden the size distribution, thereby reducing drift while maintaining or improving deposition.
We link these changes to modifications of the liquid sheet emerging from the nozzle—specifically its breakup length and the wavelength of the flapping instability that drives sheet fragmentation—and we are extending this to emulsions and to aerial spraying under wind. Drop-impact experiments with high-speed imaging show that dynamic surface tension is a key control parameter for whether droplets stick to or bounce from plant surfaces, and we are exploring how polymer additives and formulation chemistry tune this behavior.
Rheology and interfacial dynamics of complex fluids
Beyond sprays, we study the fundamental dynamics of polymeric fluids and emulsions. In viscoelastic pinch-off, we have clarified when the elastic regime sets in during filament thinning and derived universal scalings for the transition radius and time, showing how this depends on protocol and solution properties. Complementary work demonstrates that filament-thinning rheometry (CaBER and related methods) does not always yield a unique material relaxation time: the inferred “elastic” relaxation time can depend on plate or nozzle size and flow history, especially for semidilute solutions in low-viscosity solvents.
In parallel, we compare surfactant-stabilized and particle-stabilized (Pickering) emulsions, finding that both can strongly lower interfacial tension and that clear-cut distinctions based solely on equilibrium tension are not always possible, while coalescence stability can differ between the two types. Together, these projects connect spray formation and drop impact to the rheology and interfacial physics of polymeric and particulate complex fluids.
Contact: Daniel Bonn (d.bonn@uva.nl)