Water drops on hydrophobic mold surface

[rjlittlefield]

There's a story, of course.

I opened a container of yogurt that had been sitting too long, and was surprised to see that
(a) it was completely covered by a layer of mold filaments,
(b) there were some water drops on the surface of the mold, and
(c) most of the water drops would just roll around like they were on an oiled surface.

OK, so maybe (a) and (b) were not all that surprising, but I surely did not recall having noticed (c) before. The water drops simply were not wetting the surface.





Reading a little about this phenomenon, I was also surprised to learn that the water repellency is apparently due to specialized proteins, not lipids. See the discussion of "hydrophobins" at https://en.wikipedia.org/wiki/Hydrophobin .

Shot with two undiffused Jansjo lamps, one in front and one positioned above and behind the subject, so as to focus light through the foreground drop. Canon T1i camera (APS-C), Canon EF 100mm F2.8 L IS USM macro lens plus 1.4X teleconverter, at about 1.4:1 overall.

This is an aperture stack -- just one focus point, but varying aperture from f/4 to f/32 nominal (roughly f/8 to f/64 effective), then treated as a focus stack using Zerene Stacker PMax plus a little retouching from individual source frames to clean up specular reflections. ISO 100, shutter speed from 1/30 to 1 second.

Curves adjustment in Photoshop, an S-shaped curve to bring out the texture of the mold.

--Rik

[TheLostVertex]

Reading a little about this phenomenon, I was also surprised to learn that the water repellency is apparently due to specialized proteins, not lipids. See the discussion of "hydrophobins" at https://en.wikipedia.org/wiki/Hydrophobin .

I learned something new. When I first saw the image I immediately thought it would be due to the lotus effect that I see on many plants. But if I am reading correctly, the proteins themself are strictly hydrophobic, and it is not due to the hairy fibrous structure that the fungus makes.

[rjlittlefield]

When I first saw the image I immediately thought it would be due to the lotus effect that I see on many plants. But if I am reading correctly, the proteins themself are strictly hydrophobic, and it is not due to the hairy fibrous structure that the fungus makes.

I think it's directly analogous to the lotus effect, the big difference being that the lotus's waxes are replaced by the hydrophobic proteins.

I let the mold film dry out and played with it under various conditions.

Small water droplets and low velocity streams would stay on top of the fiber mat, touching only the tips of the fibers and apparently kept from going deeper by surface tension.

But a higher velocity stream would penetrate the mat, and once that happened the surface appeared wetted and lost its droplet-shedding capabilities.

I did not attempt to explore the transition in any more detail. I don't know if the water was actually wetting some surfaces below the tips of the fibers, or if it was just trapped within the fiber mat and available for grabbing onto new drops on contact, or maybe the stream had somehow disrupted the surface structure so as to expose wettable parts.

--Rik

[BugEZ]

This is quite interesting and I reviewed the Wikipedia reference.

I found myself considering the hydrophobic corneal lenses of flies. But worthy of a separate post...