Overcoming the “Coffee-Stain” Effect by Compositional Marangoni- Flow-Assisted Drop-Drying Mainak Majumder, †,‡ Clint S. Rendall, † J. Alexander Eukel, † James Y. L. Wang, † Natnael Behabtu, † Cary L. Pint, § Tzu-Yu Liu, † Alvin W. Orbaek, § Francesca Mirri, † Jaewook Nam, † Andrew R. Barron, §,⊥ Robert H. Hauge, §,⊥ Howard K. Schmidt, §,⊥ and Matteo Pasquali* ,†,§,⊥ † Department of Chemical & Biomolecular Engineering, § Department of Chemistry, and ⊥ The Smalley Institute for Nanoscale Science & Technology, Rice University, Houston, Texas 77005, United States ‡ Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria 3800, Australia * S Supporting Information ABSTRACT: Attempts at depositing uniform films of nanoparticles by drop-drying have been frustrated by the “coffee-stain” effect due to convective macroscopic flow into the contact line. Here, we show that uniform deposition of nanoparticles in aqueous suspensions can be attained easily by drying the droplet in an ethanol vapor atmosphere. This technique allows the particle-laden water droplets to spread on a variety of surfaces such as glass, silicon, mica, PDMS, and even Teflon. Visualization of droplet shape and internal flow shows initial droplet spreading and strong recirculating flow during spreading and shrinkage. The initial spreading is due to a diminishing contact angle from the absorption of ethanol from the vapor at the contact line. During the drying phase, the vapor is saturated in ethanol, leading to preferential evaporation of water at the contact line. This generates a surface tension gradient that drives a strong recirculating flow and homogenizes the nanoparticle concentration. We show that this method can be used for depositing catalyst nanoparticles for the growth of single-walled carbon nanotubes as well as to manufacture plasmonic films of well-spaced, unaggregated gold nanoparticles. 1. INTRODUCTION Nanoparticles (1-100 nm) synthesized or dispersed in solutions provide a convenient starting point for the manufacturing of nanostructured materials for various applications of nanotechnology. Presently, a variety of nano- particles, for example, gold, silver, quantum dots, metal oxides, and carbon nanotubes (CNTs), is available commercially or can be synthesized easily in the laboratory. Such nanoparticles have diverse uses in photonic devices, in catalysis, and in printable electronics. Yet, better methods are needed for spreading nanoparticles uniformly over surfaces. Spin-coating is used routinely for making nanoparticle films; however, spin-coating inevitably leads to considerable loss of nanoparticles. Moreover, the ability to form films requires tailoring the viscosity and/or volatility of the solvents. A significantly more convenient method is drop-drying of nanoparticle-laden suspensions on the substrate of interest; however, drop-drying is prone to particle aggregation at the edge of pinned drops due to faster solvent evaporation at the contact line, commonly known as “coffee- stain”. 1 While the coffee-stain is sometimes desirable, for example, in printing repetitive fine lines, 1,2 it is detrimental to the formation of uniform films. Uniform deposition has been attained by convecting the particles to the air-liquid interface, by leveraging particle anisotropy, 3 and by controlling the composition of the system so as to achieve gelation during drying. 4,5 Yet, a general method for uniform deposition is still desired, and generality dictates that such a method be based on controlling the internal droplet hydrodynamics. 6 Flow can be driven by surface tension gradients arising from spatial variations of composition or temperature, the Marangoni effect, known since the 1800s. 7 This effect leads to the well- known example of the formation of tears in strong wines. 8 When properly controlled and engineered, Marangoni flow can be useful in enhancing heat transfer, 9,10 rapid droplet movement on surfaces, 10 cleaning of surfaces, 11 inkjet printing, 12 and patterning of nanoparticles. 13 Nonuniform evaporation at the free surface of droplets can generate temperature gradients and hence Marangoni flow; such flow can induce strong convection and is prominent in highly volatile solvents such as octane, ethanol, and pentane but difficult to generate in aqueous suspensions. 14 Hu and Larson demonstrated that this thermal Marangoni flow can reduce or eliminate the coffee-stain effect for particles dispersed in evaporating octane droplets; 15 they explained that thermal Marangoni flow is not observed in water-based suspensions because of the combination of lower volatility and higher heat Received: January 30, 2012 Revised: May 10, 2012 Published: May 15, 2012 Article pubs.acs.org/JPCB © 2012 American Chemical Society 6536 dx.doi.org/10.1021/jp3009628 | J. Phys. Chem. B 2012, 116, 6536-6542