Insights into surrogate respiratory droplet behaviour on inclined surfaces: Implications for disease transmission

Disease transmission via fluid ejections from infected individuals presents a significant public health challenge. The ejected droplet frequently lands on substrates with varying orientations, including horizontal, vertical, and inclined. However, the behaviour of disease-causing droplets on inclined substrates remains unexplored. Recent research has demonstrated that droplet evaporation, flow, and colloidal deposition are significantly altered when surfaces are inclined. Changes in contact angle impact evaporative flux and induce flow between the top and bottom edges of the droplet. This study examines the behaviour of simulated respiratory droplets containing NaCl, mucin, and micrometer-sized particles as pathogen surrogates, focusing on substrate inclinations of 0°, 45°, and 90° on glass surfaces. As respiratory droplets evaporate, their solute concentration rises, altering both surface tension and viscosity. The study explores the coupled effects of solute concentration variations and asymmetric evaporative flux at varying relative humidity conditions and droplet volumes. The presence of salt and mucin induces Marangoni flow, potentially altering evaporation, flow patterns, and deposition dynamics. Additionally, sedimentation flow influences crystal nucleation sites and precipitation patterns. Results reveal unprecedented crystallization dynamics on inclined substrates, with notable differences in nucleation and growth based on the angle of inclination. Optical profilometry and confocal microscopy further show that surrogate bacterial particles accumulate excessively at the bottom edge of inclined droplets, suggesting an elevated risk of pathogen survival on inclined fomites. These findings highlight the critical importance of considering substrate orientation in understanding the transmission of disease through surface-bound droplets, offering insights that could inform public health strategies.