When a coffee drop dries on a solid, a ring pattern is usually observed as most of the suspended particles gather at the drop periphery [1]. The so-called coffee-ring effect (CRE) is a ubiquitous phenomenon observed in any pinned evaporating drop containing non-volatile solutes. Despite the vivid interest in controlling the CRE, methods reported so far were based on adapting the drop composition [2] and solute characteristics [3], hindering any reconfigurability for a given system. Additionally, on-demand modulation of the deposit profile has not been yet demonstrated.
An alternative strategy is the use of external stimuli which can affect the drying process. Photocontrol of particle deposition emerges as an advantageous candidate, since light offers a contact-less and highly reconfigurable stimulus. Here, a novel optical approach to dynamically control particle deposition from evaporating drops is presented. We designed aqueous suspensions consisting of nanoparticles and photosensitive surfactants where the attractive particle interactions (‘stickiness’) can be modulated by LED light. An increase in particle stickiness favours particle-particle attraction and trapping at the liquid/gas interface, which results in deposit homogenization. A wide range of patterns from rings to homogeneous disks is achieved by varying the irradiation time that directly controls particle stickiness. Patterning from single drops is photoreversible upon changing the wavelength while spatial control in multi-drop arrays is achieved using a photomask. The reported effect is robust as evidenced from the excellent pattern reproducibility [4].
References
[1] R. D. Deegan et al., Nature, 389, 827-829 (1997)
[2] H. Hu and R. G. J. Larson, Phys. Chem. B, 110, 7090-7094 (2006)
[3] P. J. Yunker et al., Nature, 476, 308-311 (2011)
[4] M. Anyfantakis and D. Baigl, submitted.
emmanouil.anyfantakis@ens.fr
