Isomeric colloidal clusters with shape-dependent mobility

Fig. 1 (a) Colloidal trimers can be discriminated by the bending angle (q) defined in this panel. (b) Experimental example of an isomer with q 1⁄4 85 . (c) Raw data illustrated: mean-squared angular displacement is Colloidal particles stand at the cross-section of chemistry, biology, and engineering; they underpin technology in fields that span pharmaceuticals, foodstuffs, and chemical sensors. It has been a challenge to understand their mobility under conditions where their shape does not fall into simple geometrical idealizations such as sphere, rod, ellipsoid, and so forth. As a result, computation-based methods have been developed that enable the solution of fluid dynamics equations for objects with complex shapes, however the need to compute each case separately with appropriate software renders these techniques specialized. As an alternative, we present here experiments in which colloidal spheres are assembled into explicitly discrete isomeric shapes and their mobility is tracked individually under a microscope. Considering translation and rotation in the surface plane, parallel to a wall after particles sediment in water to near the bottom of a sample cell, we focus on trimers as the simplest chain structure that can provide insight into how isomerizationmatters. As is well known, in this situation lubrication forces slow translational diffusion relative to that in the bulk, and in a prior related study we considered this quantitatively. However, in-plane symmetry of the surface causes lubrication forces to affect equally those translational motions parallel and perpendicular to the surface; the absolutemagnitudes are reduced but not their ratios, which are the main point of the present study. Using CCD detection we have accumulated large statistical samples; this enables us to contrast, without concern even in principle about sample heterogeneity, the influence of isomerization on translational and rotational diffusion for rigid assemblies whose masses are identical but whose shapes differ. To produce colloidal isomers, we build upon methods developed by many previously to form close-packed colloidal monolayers, except that to produce isomers, instead we form submonolayers onto which a thin (30 nm) layer of silicon oxide is coated by electron beam deposition. This produces coherent, bridged objects that are easy to remove by mild ultrasonication, and the resulting objects present a large family of different cluster sizes and shapes. The parent particles, from which the colloidal isomers were formed, are colloidal silica spheres, 1.57 mm in diameter (Duke Scientific). During experiments, salt (3 mMNaCl) is added to keep the electrostatic screening length negligible relative to the particle diameter. Single-particle