Oriented assembly of anisotropic particles by capillary interactions† Eric P. Lewandowski, a Jorge A. Bernate, a Alice Tseng, a Peter C. Searson ab and Kathleeen J. Stebe * c Received 17th July 2008, Accepted 15th October 2008 First published as an Advance Article on the web 3rd December 2008 DOI: 10.1039/b812257a Partially wet anisotropic particles at an otherwise planar fluid interface create distortions as the interface bends to satisfy boundary conditions at the three phase contact line. Overlapping distortions create capillary interactions that depend strongly on the particle shape. Since the excess surface area associated with the distortions can be locally elevated at certain regions near the particle, the resulting capillary interactions drive assembly in preferred orientations. In this work, arguments relating particle aspect ratio to the preferred orientation for assembly are developed for particles of constant cross section. End-to-end registry of particle faces is achieved by exploiting short-range capillary interactions between particles with complex end faces. Finally, insoluble surfactant monolayers are used to arrest the assembly process. Introduction Particle assembly by capillary interactions is an area of active interest. 1–10 While the assembly of spherical objects has been well studied, the assembly of anisotropically shaped objects at fluid interfaces has received less attention, despite the fact that the ability to create such objects is rapidly developing. 12 Interest in the assembly of complex-shaped objects was spurred by the work of Bowden et al., 5 who used complex shapes and surface func- tionalization to drive assembly of non-spherical particles on the mesoscale. Studies which address shape anisotropy on the microscale include the work of Loudet and collaborators, 6,7 who studied assembly of ellipsoidal particles, and Brown et al., 10 who studied the ordered structures formed by micron-sized bimetallic particles. Nanowire-like objects at fluid interfaces have also been reported to form 2-D assemblies with high orientational order. 11 Particle assembly by capillary attraction is driven by the decrease in liquid–vapor surface area as the particles approach each other. Small particles on otherwise planar interfaces deform the interface. Assuming small slopes, the resulting shape of the interface can be described in terms of a multipole expansion. Gravitational forces are negligible for small particles.‡ In the absence of other applied forces and torques, the first admissible term in the expansion is a quadrupole, which defines the slowest decaying term in the interface distortion. 1 Interface distortion creates excess surface area. When the disturbances created by two neighboring particles overlap, the excess area can decrease, creating capillary attractions that drive assembly. The interac- tion of quadrupolar distortions dictates orientation, with quad- rupolar rise axes attracting, or quadrupolar depression axes attracting equally. Depending on the shape of the contact line, other modes can contribute to the deformation and attraction close to the particle. These arguments were developed to explain interactions between spherical objects with undulating contact lines created by heterogeneous surface energies or roughness. In the case of random roughness or heterogeneity, there is little control over the dominant modes which drive assembly as the particles approach. In this work, particle shape is used to create the contact line undulation. In order to satisfy boundary conditions at the three phase contact line, the interfacial area increases at certain locations on the particle, creating preferred orientations for capillary assembly. The relationship between particle shape and oriented assembly is explored. Surfactants can retard the mobility of fluid interfaces through a number of mechanisms ranging from Marangoni stresses 14 to surface viscosities. 15 Insoluble surfactant monolayers can form tangentially immobile structures. 16 We explore the use of highly condensed insoluble monolayers in arresting particle assembly. Collapsed monolayers of pentadecanoic acid (PDA) on acidic subphases are to create immobile interfaces which arrest assembly. Solubilization of the PDA by adding base to the subphase can be used to restore interface mobility and allow oriented assembly to proceed. Materials and methods A variety of particles with constant cross section were fabricated from a photoresist (SU-8 2000 series) by contact lithography. 17,18 The particle cross sections were defined by the mask and the thicknesses were defined by the thickness of the resist. The four characteristic cross sections studied are shown in Fig. 1. The particles, which were partially wet, assume an equilibrium configuration which makes the largest hole in the interface. Top a Department of Chemical and Biomolecular Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA; Fax: +410-516-5510 b Department of Materials Science and Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA. E-mail: searson@jhu.edu; Fax: +410-516-5293; Tel: +410-516-8145 c Department of Chemical and Biomolecular Engineering, University of Pennsylvania, 311A Towne Blg, 220 S. 33rd Street, Philadelphia, PA 19104-6393, USA. E-mail: kstebe@seas.upenn.edu; Fax: +215-573-2093; Tel: +215-898-4515 † Electronic supplementary information (ESI) available: Optical microscopy movies for Fig. 4, Fig. 6, Fig. 7a, Fig. 7b, and Fig. 8. See DOI: 10.1039/b812257a ‡ The ratio of gravitational to surface tension-related forces is expressed as a Bond number Bo ¼ ðr particle  r liquid Þ gL particle 2 g where L particle is the particle dimension, r particle and r liquid are the densities of the particle and liquid, respectively, g is the surface tension, and g is the gravitational acceleration constant. The magnitude of capillary interactions caused by gravity was reported by Paunov et al. 13 The excess liquid–vapor energy associated with a particle of length scale L has pre-factor g L 2 Bo 2 . Hence, for Bo  1, gravitational effects are negligible. 886 | Soft Matter , 2009, 5, 886–890 This journal is ª The Royal Society of Chemistry 2009 PAPER www.rsc.org/softmatter | Soft Matter