Acoustically-Driven Directed Assembly in Three Dimensional and Microfluidic Environment

Directed or selective assembly can create ordered microsystems for a broad range of applications from sensing to the life sciences. One example is templated assembly by selective removal (TASR), which has been shown to be able to selectively assemble microscale objects and biological cells by size into predetermined locations on a 2D surface based on the surface's geometry and acoustic excitation. Although directed assembly excels at ordering structures on exposed 2D surfaces, it is less well developed for forming 3D and hierarchical architectures such as are typically required for metamaterials, tissue engineering or medical diagnostic devices. One potential solution to the challenge of creating 3D and hierarchical systems using directed assembly is to integrate the assembly techniques with other microscale technologies such as microfluidics or the folding of 2D surfaces into the third dimension. However, the physics of directed assembly processes are not independent of their context, and applying directed assembly to folding and microfluidic systems requires that their physics be understood and taken into account. This research examines the engineering principles that underlie the integration of the TASR process into folding and microfluidic systems and demonstrates TASR's successful integration into these systems.;The first part of the research examines the capabilities and limits of the TASR process in folding systems. Because folding rotates planar surfaces through non-horizontal angles, the key requirement for integrating TASR into folding systems is that TASR must function effectively on non-horizontal surfaces. An analytical model was developed for predicting the performance of the assembly process on non-horizontal substrates. The model predicts that TASR-based assembly will be both effective and selective for low angles of tilt, below about 45 degrees, and that it will lose its effectiveness and selectivity as the surface rotates to higher angles. Experiments were carried out to demonstrate the effects of angle of tilt on the assembly of different sizes of microspheres into correct (well-matched) and incorrect (poorly-matched) assembly sites. The experimental results show good agreement with the model predictions, with microspheres successfully assembling into their correct assembly sites and successfully avoiding incorrect assembly sites at low angles. At higher angles, both the assembly and its selectivity are lost as predicted by the model.;The second part of the research examines the ability of TASR to guide selective assembly inside of microfluidic channels as a function of channel height, width, geometry, and operating conditions. The results demonstrate that microchannel dimensions are primarily important insofar as they affect the number of assembly components that are available relative to the number of assembly sites to be filled. However, the width of the microchannels is found to be a second and independent factor in determining the assembly results, with assembly being unsuccessful below a characteristic length scale that is determined by the acoustic excitation. The assembly is found to be largely independent of the microchannels' layout and the placement of the assembly sites within the channels.