Directed Growth of Virus Nanofilaments on a Superhydrophobic Surface Giovanni Marinaro, †,‡ Manfred Burghammer, †,# Luca Costa, † Thomas Dane, † Francesco De Angelis, ‡ Enzo Di Fabrizio, §,⊥ and Christian Riekel* ,† † ESRF-European Synchrotron Radiation Facility, CS 40220, F-38043 Grenoble Cedex 9, France ‡ Nanostructures Department, Istituto Italiano di Tecnologia, Via Morego 30, Genova 16163, Italy § Physical Science and Engineering Divisions, KAUST (King Abdullah University of Science and Technology) , Jeddah, Saudi Arabia ⊥ BIONEM Lab, University of Magna Graecia, Campus Salvatore Venuta, Viale Europa, 88100 Germaneto-Catanzaro, Italy # Department of Analytical Chemistry, Ghent University, Krijgslaan 281, S12B-9000 Ghent, Belgium *S Supporting Information ABSTRACT: The evaporation of single droplets of colloidal tobacco mosaic virus (TMV) nanoparticles on a superhydrophobic surface with a hexagonal pillar-pattern results in the formation of coffee-ring type residues. We imaged surface features by optical, scanning electron, and atomic force microscopies. Bulk features were probed by raster-scan X-ray nano- diffraction. At ∼100 pg/μL nanoparticle concentration, the rim of the residue connects to neighboring pillars via fibrous extensions containing flow-aligned crystalline domains. At ∼1 pg/μL nanoparticle concentration, nanofilaments of ≥80 nm diameter and ∼20 μm length are formed, extending normal to the residue-rim across a range of pillars. X-ray scattering is dominated by the nanofilament form-factor but some evidence for crystallinity has been obtained. The observation of sheets composed of stacks of self-assembled nanoparticles deposited on pillars suggests that the nanofilaments are drawn from a structured droplet interface. KEYWORDS: droplet microfluidics, superhydrophobic surface, virus nanoparticles, nanofilaments, X-ray nanodiffraction ■ INTRODUCTION Nucleation and crystallization processes during polymer extrusion can be probed for laboratory-scale extruders by synchrotron radiation (SR) small-angle (SAXS) and wide-angle (WAXS) X-ray scattering (summarized here as X-ray diffraction; XRD). 1 Local deformation and fracture zones in fibers and other extruded parts can be spatially resolved by raster-scan microbeam XRD (μXRD). 2 Biopolymers are, however, often only available in small quantities, requiring scaled-down devices for microstructural studies of assembly processes. Indeed, fibroin protein self-assembly can be probed using continuous-flow microfluidics combined with XRD. 3,4 The aim of this article is exploring a further reduction of sample volumes by making use of digital microfluidics 5 and probing bulk assemblies by nanobeam XRD (nanoXRD) as well as surface features by optical microscopy (OM), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Indeed, ultrasmall quantities of colloidal biological particles can be concentrated during evaporation of discrete droplets on a superhydrophobic surface (SHS) under quasi contact-free conditions. 6 The evaporation results in convective- flow mediated mass transport and the formation of a viscous interface-layer which is at the origin of self-assembly processes. 6 Similar processes occur on wetting surfaces resulting in coffee- ring type residues. 7 Shear- and capillary-flow alignment at the droplet interface on a SHS allows generating filamentary morphologies. Indeed, λ-DNA nanofilaments, composed of an assembly of few molecular chains, can be obtained from ultradilute droplets on pillared Si-SHSs. 8 This process does not, however, involve large-scale self-assembly as the length of the molecular chains scales with the pillar distance. 9 We will explore in this text self-assembly of tobacco mosaic virus (TMV) nanoparticles 10 on a pillared SHS extending experiments on TMV self-assembly on a wetting surface. 11 The rodlike particles have a length of ∼300 nm, a diameter of ∼18 nm with a ∼4 nm central bore. 12,13 TMV shows a rich phase diagram forming notably several liquid crystalline phases depending on the salt concentration and ionic strength (For a review see: 14 ). The nanoparticles show head-to-tail self- assembly properties 15 and provide access to supramolecular materials such as 2D- and 3D-aggregates. 11,16-19 The potential for forming filamentous morphologies by self-assembly is suggested by the observation of highly oriented domains down Special Issue: Forum on Polymeric Nanostructures: Recent Advances toward Applications Received: October 29, 2014 Accepted: January 6, 2015 Forum Article www.acsami.org © XXXX American Chemical Society A DOI: 10.1021/am507509z ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX