On sticky wicket
doi:10.1038/nindia.2008.312 Published online 6 November 2008
Indian researchers have studied the simple-sounding but not so well understood process of sticking of particles on solid surfaces. They have done this by using a laser-tweezer to hold a small micron-sized particle and a piezoelectrically-controlled stage to precisely position a plate close to the particle1.
"When you spill coffee on your clothes, the colloidal particles in the beverage stick to the clothe fibres and stain them. One would expect that such a common phenomenon is well-understood, but that is not the case," says one of the researchers Shankar Ghosh.
The results of the team's experiments show that such an apparently simple problem is actually quite complex when the particle is soft and deformable. The dynamics of the particles suspended in liquid are mainly dominated by its viscous coupling with the liquid surrounding it.
However, as the particles stick to a surface, the coupling changes from viscous to elastic. "Generically we find that the colloidal particles get better stuck with time. This is consistent with our everyday observation that the longer the coffee-stains are left unattended, the more difficult it becomes to clean them," Ghosh says.
- Sharma, P. et al. Microrheology of a sticking transition. Nature Phys. doi: 10.1038/nphys1105 (2008)