Mekala Krishnan

Dynamically programmable fluidic assembly

A major challenge in fluidic assembly is the dynamically programmable fabrication of arbitrary geometries from basic components. Current approaches require predetermination of either the assembly machinery or the component interfaces for the specific target geometries. We present an alternative concept that exploits self-assembly forces locally but directs these forces globally, allowing fabrication and manipulation of target structures without tailoring the substrate or interfaces. By controlling the flow in a microfluidic chamber, components are directed to their target locations where local interactions align and bond them. Following this approach, we demonstrate and quantify the experimental assembly of structures composed of two to ten components.

Optothermorheological flow manipulation

Optical methods for microfluidic flow manipulation offer a flexible, noncontact technique for both fluid actuation and valving. At present, however, such techniques are limited by their high laser power requirements, low achieved flow rates, or poor valve switching times. Here we demonstrate a microfluidic valving technique based on optothermorheological manipulation using a low-power 40 mW laser with switching times on the order of 1 s at high flow rates of 1 mm/s. In our approach a laser beam incident on an absorbing substrate is used to locally heat a thermorheological fluid flowing in a microfluidic channel. The resulting gelation in the heated region creates a reversible fluid valve.

Hydrodynamically Tunable Affinities for Fluidic Assembly

Most current micro- and nanoscale self-assembly methods rely on static, preprogrammed assembly affinities between the assembling components such as capillarity, DNA base pair matching, and geometric interactions. While these techniques have proven successful at creating relatively simple and regular structures, it is difficult to adapt these methods to enable dynamic reconfiguration of the structure or on-the-fly error correction. Here we demonstrate a technique to hydrodynamically tune affinities between assembling components by direct thermal modulation of the local viscosity field surrounding them. This approach is shown here for two-dimensional silicon elements of 500 microm length using a thermorheological fluid that undergoes reversible sol-gel transition on heating. Using this system, we demonstrate the ability to dynamically change the assembly point in a fluidic self-assembly process and selectively attract and reject elements from a larger structure. Although this technique is demonstrated here for a small number of passive mobile components around a fixed structure, it has the potential to overcome some of the limitations of current static affinity based self-assembly.

Interfacing methods for fluidically-assembled microcomponents

Here we present the design and implementation of electrical and mechanical interfaces for fluidically-assembled planar MEMS. We discuss the design and fabrication of systems of passive mechanical latches to bond microcomponents together and of electrical layers capable of establishing electrical connections with each other. We evaluate the ability of components with these interfaces to bond together within a microfluidic channel and to establish electrical circuits when assembled. This work supports the development of a novel microassembly strategy that bridges the gap between bottom-up self-assembly and top-down direct-manipulation technique. The ultimate goal of this research is the development of MEMS devices capable of the on-demand self-assembly, repair, and reconfiguration.

Increased robustness for fluidic self-assembly

Self-assembly methods have been developed at the micro- and nanoscale to create functional structures from subelements stochastically dispersed in a fluid. Self-assembly paradigms have limitations in terms of achievable complexity of the final structure, ability to perform error correction, and scalability. Fluidic self-assembly attempts to overcome these limitations by incorporating a controlled flow structure and/or complex geometric interactions to improve the assembly rate and the specificity of the final positioning. Since the initial position and orientation of a subelement in a stochastic system are indeterminate, the most robust of these schemes are those for which the dependence on the initial condition will be the weakest. In this paper we develop an analytical/numerical model for the fluid forces and torques on a two-dimensional subelement involved in a fluidic self-assembly process and describe the translational and rotational motions of the element due to these forces. We use this model to de...

Creating optically reconfigurable channel based microfluidic systems

Channel based microfluidic systems are devices where flow channels and valves are designed and fabricated a priori, and large scale reconfiguration of flow networks cannot be done on-the-fly. This limits the versatility and robustness of these devices since, for instance, analyses to be performed on samples flowing within the device cannot be revised based on results upstream and a single chip cannot be used for many diverse applications. Optofluidic based approaches to overcome this limitation have been developed, many involving the use of polymer gels [1, 2]. However, these approaches usually suffer from poor reconfiguration times or require the use of high power lasers. Here we present a rapid optofluidic approach to creating reconfigurable channel based microfluidic systems.

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.Copyright

Optofluidically reconfigurable channel based microfluidics

Here we demonstrate the use of optofluidics to create rapidly reconfigurable channel based microfluidic systems, implemented through reversible rheological changes in a polymer solution flowing within the microfluidic device and dynamic photomasking.

Fluid Dynamically Driven Assembly in Three Dimensions for Programmable Matter

In this work we describe our development of fluidic assembly of 3D blocks as an approach to achieving programmable matter. Devices made of this matter are assembled from small building blocks, and their shape and function can be reconfigured on demand. In our implementation of such a system small building blocks are assembled in parallel to create a target structure using fluid forces in a chamber. The building blocks are centimeter-scale cubes and are attracted to sinks within an assembly chamber. In effort to investigate the behavior of these blocks with respect to the sink numerical simulations and experiments were carried out. The simulation results indicated that blocks which align with the gravity field (due to an unbalanced mass distribution) are better suited for proper alignment during assembly. Experiments have shown that blocks can be attracted to an assembly site in a robust and timely manner. Blocks with the ability to interact geometrically have been experimentally tested and the assembly and rejection of a block at a given location has been demonstrated. Finally, several methods of making block assembly more repeatable have been proposed.© 2009 ASME