Fresh raspberries repel honey (?)

Science Talk
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Hi, not a scientist here, but I noticed an unusual phenom while having lunch today.

I had a pint of fresh raspberries at room temp. They came in a PET clamshell. I drizzled a tablespoon of honey evenly over them. After about a minute, nary a single drop of honey was sticking to any of the berries. Rather the honey completely rolled off and is at the bottom of the clamshell. There is no sign that honey was ever added save for the puddle at the bottom of the clamshell. The berries look pristine.

Should I be clearing my bookshelf for a Nobel or is this old hat?

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Interestingly, although honey is much more viscous than water, it has similar wettability, which is what makes it sticky (Honey - Wikipedia), so what you saw would not be a contact angle phenomenon unique to honey.

And indeed raspberries can be hydrophobic to water! (https://www.reddit.com/r/mildlyinteresting/comments/g8abrm/raspberries_are_hydrophobic/) What you are seeing is the effect of the fine hairs on raspberries, which clearly trap air well enough to leave water in the Cassie-Baxter state (Cassie's law - Wikipedia), where water effectively sees a half-air surface.

This state is the basis for much biological hydrophobicity, such as lotus leaves and feathers. I suppose that in this context the hydrophobicity inhibits bacterial adhesion (Influence of the hydrophobicity and surface roughness of mangoes and tomatoes on the adhesion of Salmonella enterica serovar Typhimurium and evaluation of cleaning procedures using surfactin - ScienceDirect), which would be helpful for protecting the yummy sweet fillings of a raspberry!

Interestingly, the raspberry has been cited as inspiration for the morphology of nanoparticles for superhydrophobic coatings since 2005 (https://pubs.acs.org/doi/10.1021/nl0517363). So no Nobel Prize for you or me, sadly.

1 Like
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I didn’t expect such a thorough answer. Thank you.