Rheological design of thickened alcohol-based hand rubs

The handleability and sensory perception of hand sanitisers by consumers affect the hygiene outcome. Spillage may result in under-dosing and poor sensory properties can lead to under-utilisation. We first propose four principles (low run off, spreadability, smoothness and non-stickiness) for designing the rheology of thickened alcohol-based hand rubs with acceptable handleability and hand feel. We then evaluate a commercial hand gel and a variety of simplified formulations thickened with microgels (Carbopol 974P, Carbopol Ultrez 20 and Sepimax Zen), or linear polymers (Jaguar HP 120 COS), and evaluate them against these design criteria. All four additives provide acceptable spreadability by shear thinning to $\eta \approx 10^{-1} Pa \, s$ at $\dot\gamma \sim 10^3 s^{-1}$. Either the finite yield stress conferred by the microgels ($\sigma_y \gtrsim 10 Pa$) or the increase in low-shear viscosity provided by the linear polymer ($\eta \gtrsim 1 Pa \, s$ at $\dot\gamma \lesssim 0.1 s^{-1}$) give rise to acceptably low run-off. However, the formulation using the linear polymer shows a filament breakage time of $\tau_{\rm b} \approx 1 s$ in capillary rheology, which may result in stickiness and therefore a less than optimal hand feel.