Direct-write Self-assembly of 3d Colloidal Microstructures

We present a direct-write technique for assembly of microscale 3D colloidal crystals on substrates. We use a custombuilt high-resolution liquid manipulation system to dispense colloidal suspensions through a capillary tip. Using this system, we establish a liquid bridge between the capillary tip and a temperature-controlled substrate, initiating crystal growth upward from the substrate. We demonstrate construction of cone-shaped and tower structures by controlling the dynamic shape of the liquid meniscus during crystal precipitation. Interplay between lateral capillary forces and granular cohesion governs assembly. Finally, we show that confinement of the meniscus on microfabricated template features enables assembly of discrete particle clusters. INTRODUCTION Spanning from the nanometer to millimeter scales, discrete components can serve as building blocks for optical components, filters, biosensors, micromechanisms, electronics, and many other novel materials and devices. New techniques are needed to assemble a growing library of these building blocks with high precision and throughput, ranging from semiconductor nanoparticles to polymer microspheres and miniature circuit elements. In particular, the proportional relationship between liquid meniscus curvature and capillary pressure is a versatile means for ordering ensembles of particles at liquid-air interfaces, and enables fabrication of closely packed layers and discrete clusters of micro and nanoparticles on substrates. Collectively, these techniques are known as evaporative self-assembly, and are, for example, implemented by dispensing droplets onto the target substrate by inkjet printing [1], or by drawing a meniscus across the substrate [2]. However, it is highly challenging to arrange particles into well-organized 3D shapes by current methods. Here, we demonstrate that 3D colloidal assemblies can be directly fabricated on flat and microstructured surfaces by dynamic mechanical control of a liquid meniscus during evaporation. Figure 1: (a) Liquid dispensing system (adapted from [3]); (b) schematic of 4-DOF motion control, and (c) schematic of directwrite assembly process. To facilitate this study, we designed and built a precision instrument (Fig. 1a, [3]), which can simultaneously deposit and manipulate nanoand microliter liquid droplets on a temperature controlled substrate, in four degrees of freedom (Fig. 1b). In this paper, we introduce a technique for fabricating 3D colloidal microstructures using vertical stage motion. We contact the heated substrate with a liquid that is displaced slightly from the end of the capillary tip, thereby creating a liquid bridge between the capillary tip and substrate (Fig. 1c). Particles in the liquid migrate towards the substrate due to sedimentation and/or evaporation-driven liquid flow. When the evaporation rate locally exceeds the rate of liquid replenishment, the meniscus recedes upward from the substrate, building a colloidal crystal whose geometry is determined by the dynamic shape of the receding meniscus. Via concerted control of vertical substrate motion and liquid dispensing rate, we can control the liquid bridge profile between the tip and the heated substrate. As shown in Figure 2, this control allows us to fabricate colloid crystals, consisting of 10μm polystyrene particles, in the shape of cones and towers on flat substrates. Figure 2: Extrusion of crystals of 10μm polystyrene spheres into conical and tower structures by drawing from a capillary tip (all scale bars = 500μm unless noted) PROCESS FUNDAMENTALS In order to understand how these structures precipitate from the liquid bridge, we present a conceptual description of the particle assembly mechanisms observed during crystal growth, which arise due to capillary forces. Specifically, the local curvature of a liquid meniscus is proportional to the pressure difference across the interface, as described by the Laplace-Young equation. This capillary pressure, in concert with the effects of surface tension, produce two distinct phenomena which we call surface assembly and bulk cohesion. Formation of these crystal structures may be understood by considering the interaction between colloid organization at the liquid-air interface and coherence in the bulk material. 9780964002494/HH2012/$25©2012TRF 129 Solid-State Sensors, Actuators, and Microsystems Workshop Hilton Head Island, South Carolina, June 3-7, 2012 Assembly at the Three-Phase Contact Line Suspended colloid particles that are at the liquid-air interface locally deform the liquid meniscus, which gives rise to lateral capillary attraction between particles due to interface tension and Laplace capillary pressure. This drives the colloids to assemble into closed-packed domains along the liquid-air interface. This has been explored thoroughly in the context of self-assembly of closed-packed monolayers and multilayers of particles on surfaces, using a variety of techniques, including [4] drop-casting, bladecasting, and Langmuir-Blodgett drawing. After evaporation, particles remain arranged in the crystal lattice due to Van der Waals attraction to neighboring particles and to the surface. Our process may be regarded as a 3-dimensional (3D) analog to these well established planar self-assembly techniques. However, here are important differences with regard to our process due to the 3D contour of the liquid bridge. As shown in Fig. 3, evaporation-induced flows pull particles towards the liquidsubstrate contact line, where the particles pack due to lateral capillary forces at the interface, thus causing a colloid crystal nucleate and to grow. In this respect, the colloid microstructure may be thought of as precipitating from the meniscus-crystal contact line in the direction of meniscus recession. Figure 3: (a) Schematic illustrating the surface assembly process within the liquid bridge; (b) isometric view with wedge cut illustrates crystal growth tangent to the meniscus, and inwards radial crystal growth, which adds thickness to the crystal. Meniscus recession may occur tangent to the meniscus surface, thereby precipitating a closed-packed outer surface; and radially inwards, thereby thickening the closed packed surface to form a multi-layered ‘crust’ (Fig. 3). Note that evaporationinduced convection can transport particles to the meniscus surface for tangential crystal growth, and to the underside of the hexagonal-packed surface for radial crystal growth. Several factors influence the integrity of the closed packed crystal. One obvious culprit is monodispersity in colloid diameter. Larger-than-average particles in the crystal lattice create defects that can propagate through the crystal. This may be minimized by using highly monodisperse particles. Additionally, crystal growth may nucleate at multiple locations, which leads to grain boundaries at the intersection of crystals with different lattice orientations. These defect mechanisms are well documented in the literature for 2D capillary self-assembly [4]. Geometry provides a unique constraint for our process. The crystallization process is confined by the curvature of the meniscus, and defects may be created to accommodate this geometric confinement. One can imagine that the defect density will increase radially inwards from the outer surface, ultimately resulting in jammed, amorphously packed colloids at the center. Bulk Cohesion In contrast to the surface assembly just described, capillary forces can effectuate bulk cohesion between particles. Here, the colloid particles behave as an amorphously packed wet granular material, which may exist in four distinct phases according to the volume fraction of liquid in the void spaces between the particles [5]. At very low saturation, small liquid bridges form between particles near contact points (Fig 4a-i). Surface tension and capillary pressure within the liquid bridges constitute a normal capillary force attraction, which causes high cohesion amongst the particles. Increase in liquid content fills more of the void space between the particles, which leads to coalescence of capillary bridges (Fig. 4a-ii) and a decrease in particle cohesion. At the limit where all void space is filled with liquid (Fig. 4a-iii), the particles experience weak cohesion due to lateral capillary forces at the peripheral liquid air interface. Further increase in liquid content dilutes particles into a slurry (Fig. 4a-iv), and cohesion is lost. Figure 4: (a) Schematic illustrating the phases of granular bulk cohesion [5]; (b) illustration of particles sedimenting within the liquid bridge; (c) isometric view with wedge cut illustrates the contact angle, θc, created at the contact diameter at the interface between the cohesive bulk phase and liquid bridge slurry. During extrusion of a crystal structure by our method, particles may sediment onto the substrate inside the liquid bridge (Fig. 4b). Substrate heating accelerates liquid evaporation, thereby introducing air voids in the interstitial spaces and effectuating cohesion amongst the sedimented particles (i.e., phases i and ii, Fig. 4a). Collectively, the particles behave as a distinct bulk wet granular material that, we believe, develops a discernable interface with respect to the liquid bridge slurry above (Fig. 4c). The meniscus of the liquid bridge slurry pins to the cohesive bulk at the peripheral contact diameter with contact angle θc, as illustrated. An amorphous crystal structure may be grown by simply pulling on the meniscus so that this contact diameter recedes by slipping over the cohesive bulk material. RESULTS Based on this understanding, we demonstrate control of the process parameters to direct the crystal structure contour, and realize 3D structures, ranging from cones to towers. Utilizing the capabilities of our liquid dispenser system, we are able to control the following parameters for fabricating colloid microstructures: profile of the liquid bridge between the capillary tip and