Polymer Patterns in Evaporating Droplets on Dissolving Substrates Manoj Gonuguntla and Ashutosh Sharma* Department of Chemical Engineering, Indian Institute of Technology at Kanpur, Kanpur 208016, India Received November 26, 2003. In Final Form: February 1, 2004 Self-organized polymer patterns resulting from the evaporation of an organic solvent drop on a soluble layer of polymer are investigated. The patterns can be modulated by changing the rate of evaporation and also the rate of substrate dissolution controlled by its solubility. Both of these affect the contact zone motion and its instabilities, leading to spatially variable rates of substrate etching and redeposition that result from a complex interplay of several factors such as Rayleigh-Benard cells, thermocapillary flow, solutal Marangoni flow, flow due to differential evaporation, osmotic-pressure-induced flow, and contact-line pinning-depinning events. The most complex novel pattern, observed at relatively low rates of evaporation, medium solubility, and without macroscopic contact-line stick-slip, consists of a regularly undulating ring made up of a bundle of parallel spaghetti-like threads or striations and radially oriented fingerlike ridges. Increased rate of evaporation obliterates the polymer threads, producing more densely packed fingers and widely separated multiple rings due to a frequent macroscopic pinning-depinning of the contact line. Near-equilibrium conditions such as slow evaporation or increased solubility of the substrate engender a wider and less undulating single ring. Introduction When a solution or a particle containing suspension droplet dries, the solute is often distributed on the substrate in interesting ring patterns, usually in the vicinity of a (pinned) contact line. The mesoscopic struc- tures thus produced have been extensively studied due to their potential importance in mesopatterning by organi- zation of particles and other solutes, surface cleaning and coating technologies, preparation of polymer films, optical elements, surface-adhered proteins assays, data storage, microelectronics, and other applications requiring small- scale soft patterns. Since patterns are formed only in the immediate vicinity of the contact line, it is possible to greatly downsize the printed features relative to the size of the solution droplet, leading to novel soft lithography schemes. Some interesting examples of evaporation- assisted patterning are the formation of micron-sized copper lines from a ribbon of copper hexanoate solution, 1 long-range ordering of diblock copolymers by strong droplet pinning, 2 and nanostructuring of conjugate molecules by a stamp-assisted pinning. 3 Much of the effort has been directed toward patterns formed by evaporation of colloidal particle dispersions. The “coffee-stain problem” studied by Deegan et al. 4-6 reported the formation of ring deposit of particles by drying of a suspension drop. Adachi et al. 7 reported the formation of concentric multiple rings of particles attributed to repeated pinning/depinning of the contact line. In all of the previous studies, the self-organizing features near a contact line have been studied in evaporating drops of solutions (for example, suspensions of metal or latex microspheres, polymer or protein solutions) on nondis- solving substrates. The objective of this study is to investigate the self-organized patterns that result from the evaporation of an initially pure solvent drop on a dissolving substrate. To clearly compare and contrast this study with the earlier works on the drying of solution droplets, we first present some of the known salient features and mechanisms of the patterns in drying solution drops. The rate of evaporation in a droplet is maximum near the contact line, which causes a flow of liquid and solute toward the contact line, 2,4 often pinning it for some time. The solute concentration thus rises near the contact line. The phenomenon of ringlike stain formation in a dried drop of coffee was explained by this mechanism. Deegan et al. 5 proposed a model to predict the flow velocity, growth rate of the deposited ring, and the distribution of the solute. Formation of distinct multiple concentric rings was also observed in drops containing small particles (0.1 μm), but multiple rings were absent when larger (1 μm) particles were used that prevented depinning. 5 An increase in the localized rate of evaporation near the apex of a droplet yielded a more uniform deposit of solute (rather than a ring pattern) as the redistribution of solute toward the contact line became weaker. 6 The formation of multiple rings was also reported by Adachi et al. 7 when a suspension droplet was evaporated on a glass slide. In this study, the three-phase contact- line motion seemed to be an oscillatory stick-slip, for which a model was also proposed. The oscillation was suggested to result from the competing friction and surface tension at the contact line. Maeda 8 investigated the concentric ring patterns from evaporating droplets of collagen solutions. The formation of multiple rings and the dynamics of the motion of the contact line were also * To whom correspondence should be addressed. E-mail: ashutos@iitk.ac.in. (1) Cuk, T.; Troian, S. M.; Hong, C. M.; Wagner, S. Appl. Phys. Lett. 2000, 77, 2063. (2) Kimura, M.; Misner, M. J.; Xu, T.; Kim, S. H.; Russell, T. P. Langmuir 2003, 19, 9910. (3) Cavallini, M.; Biscarini, F. Nano Lett. 2003, 3, 1269. (4) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature (London) 1997, 389, 827. (5) Deegan, R. D. Phys. Rev. E 2000, 61, 475. (6) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. Rev. E 2000, 62, 756. (7) Adachi, E.; Dimitrov, A. S.; Nagayama, K. Langmuir 1995, 11, 1057. (8) Maeda, H. Langmuir 1999, 15, 8505. 3456 Langmuir 2004, 20, 3456-3463 10.1021/la0362268 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/17/2004