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We built a device to apply tuned two-dimensional acoustic fields to assemble millions of microparticles into hundreds of crystallites in a single step within a resonating chamber.
Owens, CE, CW Shields, DF Cruz, P Charbonneau, GP Lopez (2016) Highly parallel acoustic assembly of microparticles into well-ordered colloidal crystallites, Soft Matter, 2016, 12, 717. (http://pubs.rsc.org/en/content/articlepdf/2016/SM/C5SM02348C)
Acoustic Assembly of Microparticles_Graduation with Distinction (1).pdfFinal project presentation
The above report gives the overall project summary. Below are videos of the system and simulation in action!
As an introduction, standing waves are the superposition of reflected or superimosed traveling waves. (gif source: http://resource.isvr.soton.ac.uk/spcg/tutorial/tutorial/Tutorial_files/Web-standing-nature.htm)
We developed a device to apply standing waves with one and two orthogonal dimensions of control.
Here are sequential applications of 1D and 2D periodic standing waves, showing the motion of microparticles to the pressure nodes.
In addition to positive nodes, there are standing antinodes at equal half-wave spacings. Materials selected to have negative acoustic contrast factor, i.e, being very compressible or "squishy," move to these antinodes while rigid particles with positive acoustic contrast factors move to the nodes. Using fluorescently colored particles, we can distinguish these two types and achieve facile binary separation after starting with a homogeneous mixture.
In addition to a physical system, a 3D Brownian dynamics simulation (BDS) was created to imitate the device performance. The previous videos show simulations of particle assembly at multiple and individual nodes.
MATLAB code of our BDS is available upon request.
Below, the power of the simulation is shown in more detail to mark particles by final position, and to test parameters (i.e., high pressure leading to many-layer stacking) outside the reasonable scope of experimentation.
This is compared to experimental stacking imaged using confocal imaging during active acoustics-directed stacking. Further analysis showed good predictive power of the simulation for marking these monolayer-to-stacked transitions, despite real-world secondary acoustic effects.
We can simulate assemblies of shaped particles as well, which opens to door to more elaborate crystallite generation than what we can form from simple packed spheres.
Different particle shapes direct local packing structure, while the acoustic field still dictates the total particle count, stacking, and size per node.
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