We actuate with light & magnets, we fold & unfold DNA, we cell-free express proteins, we play with drops, marbles & interfaces, we organize particles, we harness coffee rings.

Coffee stains...the inside cover of Angewandte!

15 December 2014. Published in the 15 Dec issue of Angewandte Chemie, our paper on coffee rings is also illustrated in the inside cover page!

Abstract: When a colloidal drop dries on a surface, most of the particles accumulate at the drop periphery, yielding a characteristic ring-shaped pattern. This so-called coffee-ring effect (CRE) is observed in any pinned evaporating drop containing non-volatile solutes. Here, we show the first dynamic control of the CRE using light and demonstrate unprecedented reconfigurability of the deposit profile. This is achieved through a new mechanism where particle stickiness is optically tuned on-demand, thus offering reliable modulation of the deposition pattern. Our system consists of anionic nanoparticles and photosensitive cationic surfactants dispersed in water. We show that light-dependent modulation of surfactant-particle interaction dictates particle attraction and trapping at the liquid-gas interface, which allows us to direct particle deposition into a wide range of patterns from rings to homogeneous disks. Patterning from single drops is photoreversible upon changing the wavelength while spatial control in multiple drop arrays is achieved using a photomask (see picture below).


Dynamic photocontrol of the coffee-ring effect with optically-tunable particle stickiness
M. Anyfantakis, D. Baigl*
Angew. Chem. Int. Ed. 2014, 53, 14077–14081
 - doi : 10.1002/anie.201406903