Effect of environmental factors on the kinetics of evaporation of droplets containing bacteria or viruses on different surfaces

The efficacy of antimicrobial surfaces at solid-air interfaces can be assessed using standardised testing methods, typically by placing a droplet inoculated with microorganisms onto the surface and monitoring changes in microbial counts over time (hours). However, the mode of action of the putative antimicrobial may rely on the presence of moisture on the surface, thus it is important to know the time taken for the inoculum to dry, since this will affect resultant counts and thereby deduction as to the efficacy of the antimicrobial. Droplet (+/− microorganisms) evaporation time was measured on four different surfaces (copper, PVC, polypropylene and nitrile rubber) where temperature, relative humidity and airflow in the test chamber were controlled. The data were compared with simple models based on external mass transfer for predicting the evaporation time: (i) one assuming constant wetted area (CWA), where the diameter of the drop is unchanged but the volume/height decreases; (ii) constant contact angle (CCA), where the diameter of the droplet decreases but the droplet profile/contact angle remains unchanged; and (iii) a mixed mode model. The mixed mode model gave the best fit to the data, in which evaporation initially followed CWA kinetics, then shifted to CCA when a critical contact angle was reached. The presence of microorganisms consistently and often significantly reduced the evaporation time. Deposited bacteria were visible over the whole wetted area, with a noticeable ring at the original edge of the droplet (the location of the initial solid-liquid-air contact line), consistent with the mixed mode model. Accumulation of microorganisms and the decrease in evaporation time may affect the effectiveness of antimicrobial materials. The speed of droplet evaporation is affected by a wide range of factors: temperature, humidity, airflow, the nature of the surface and the presence (and nature) of microorganisms. If these factors are not adequately recognised or controlled, then results from testing methods carried out under different (unspecified) environmental conditions in different laboratories, are liable to vary and give rise to confusion and misinterpretation.