January 25, 2011 § Leave a comment
Over the last 20 years or so we’ve been realizing that even bacteria don’t always live alone. Instead, individual cells often stick to each other to make a biofilm, secreting a substance colloquially known as slime (also known to biologists as extracellular matrix). Slime sounds wet, or sticky; but new work from Joanna Aizenberg’s lab (Epstein et al. 2011 Bacterial biofilm shows persistent resistance to liquid wetting and gas penetration. PNAS doi: 10.1073/pnas.1011033108) now shows that biofilms have the least wettable surfaces known on earth: they’re considerably more resistant to wetting than Teflon.
How do you quantitate wettability? Wettability is a function both of the liquid and of the surface. What you do is take a drop of the liquid, put it on the surface, and measure the angle of contact between the drop and the surface. If the surface is highly wettable by that liquid, the drop spreads and the angle is low — less than 90°. If not, i.e. if it’s energetically unfavorable for that particular liquid to spread on that particular surface, the drop stays round and the angle is large. The theoretical limit of non-wettability is a contact angle of 180°.