Directed Hierarchical Self Assembly – Active Fluid Mechanics at the Micro and Nanoscales

Directed hierarchical self assembly (DHSA) involves the purposeful assembly of a series of different subunits over multiple length scales to create arbitrarily shaped, large structures. We are developing a two stage DHSA procedure comprising of a coarse “far field” fluidic process which brings the sub-elements into the general vicinity of the assembly point and a fine “near field” process which completes the process. In the experiments presented here our sub-elements comprise of lithographically patterned silicon “microtiles” which are 500μm in size and contain a series of functional elements. The far-field assembly of these tiles is controlled fluid dynamically by modulating the translational and rotational shear forces applied to the tiles with a microfluidic structure. The near-field assembly is controlled through on board (i.e. on tile) microchannel jets/sinks, the strengths of which are modulated through the assembly or disassembly of thermally actuated gel based microvalves. This provides two fundamental levels of system control, namely control of the magnitude and direction of fluid flow, as well as addressability and control of each assembled component through on tile valves. In this paper we present our preliminary work on the fundamental fluid mechanics of this assembly at these two stages. In the first section we look at fluid motion and forces using CFD based numerical simulations. In the second we study a valving system based on an aqueous solution of a triblock copolymers that form a gel at temperatures near room temperature. INTRODUCTION Figure 1 is a schematic showing our approach to directed fluidic self assembly. The chamber (a modified version of which will be described in greater detail later) contains a series of microchannel inlets and outlets arranged along an assembly substrate. As mentioned in the abstract, in our envisioned system each tile will also have on board microvalves which allow us to control the near field assembly process. Previously we have demonstrated the manipulation and far-field assembly of 500μm silicon microtiles [1]. As an example of this, Figure 2 illustrates the assembly of two tiles by selectively opening and closing of valves along the assembly substrate. Figure 1: Assembly of tiles using fluidic forces Tiles are manipulated by fluidic forces, and are controlled and addressed by an on-board valving system.