We experimentally study the time evolution of the distance between two identical spheres settling down in polyacrylamide shear-thinning solutions in mixtures of water and glycerin. In order to analyze the behavior of the spheres under different rheological conditions, we varied the concentration of the polymer and glycerin. To examine the influence of wall effects, the experiments were performed in two containers of different size. We found that depending on the fluids' rheological properties, two different regimes are revealed for the same diluted polymer. In one of them (I), the separation between the spheres grows in time if the initial separation $S_i$ is beyond a critical distance $S_c$, while the separation decreases for an initial distance shorter than $S_c$. In the second observed regime (II), the spheres aggregate if $S_i<S_c$ while they remain unchanged in their separation if $S_i>S_c$, i.e., repulsion is not detected. The transition from one to another regime can be produced at constant polymer concentration by changing the solvent viscosity ${\eta }_s$. In our experiments this shift has been done modifying the glycerin-water proportion. Although these two behaviors were reported in previous works for different viscoelastic fluids, a result that we show is that both regimes can be observed for the same polymer. The elusive regime I, in which far-field repulsion is exhibited, is of particular interest since it has been previously reported only by Riddle et al. [ J. Non-Newtonian Fluid Mech. 2, 23 (1997)]. Thus, our results confirm that this behavior is also feasible.

Coughing and sneezing are critical mechanisms for the transmission of airborne respiratory diseases, dispersing pathogen-laden aerosols into the environment. Previous human volunteer studies provided valuable insight into aerosol dynamics but lacked reproducibility due to individual variations. This paper presents a novel, replicable experimental setup using three dimensional models of the upper respiratory tract and nasal cavity to simulate isothermal human-like coughs and sneezes. Results indicate that nasal cavity involvement decreases horizontal aerosol cloud spread while enhancing vertical dispersion. Incorporating this experimental data with theoretical models improves predictive accuracy of aerosol cloud evolution, particularly for indoor environments. Finally, a single novel analytical expression for the evolution of the particle cloud tip is derived that accurately predicts the cases studied.