Equilibrium of particle distribution on surfaces due to touch

Particles can be transferred on a surface network by touch. Knowledge of surface particle distribution due to touch is important in two important areas of indoor surface environment - fomite disease transmission and surface microbiome - but such knowledge is lacking. We studied particle contact transfer using an innovative experimental setup. Surface contact was controlled using a newly designed touch machine, which allowed us to precisely control touch force and duration. Four surface materials (polyurethane, rubber, silicone and stainless steel) and fingerpads were tested using different touch parameters. Glo Germ particles were used as the model particles on the tested surfaces and quantified using a fluorescence microscope. We evaluated the equilibrium of particle distribution between each pair of surfaces and tested the so-called transitivity of particle distribution at equilibrium. When the surfaces were touched, the particle transfer between the contacted surfaces reached equilibrium experimentally. At equilibrium, a rubber surface always adhered more particles when in contact with other tested surfaces, while relatively few particles adhered to the steel surface. The contact parameters influenced the particle transfer speed but had no significant effect on the final equilibrium state. Particle distribution between surfaces was attributed to the variation in adhesion force on each surface. An inherent equilibrium of particles existed between any pair of surfaces, independent of contacts. The equilibrium was approximately transitive, implying that the particles reached relative stability as contact occurred in a surface network, even when some surface pairs never made direct contact.