Nanofluidic rocking Brownian motors

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Science  30 Mar 2018:
Vol. 359, Issue 6383, pp. 1505-1508
DOI: 10.1126/science.aal3271

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Shaking the small from the even smaller

Gently oscillate a can of mixed nuts, and eventually the larger Brazil nuts will rise to the top. Skaug et al. created three-dimensional patterned surfaces to which they applied electric fields. Combined with gentle shaking to induce a rocking Brownian motion, they were able to guide particles smaller than 100 nm along complicated paths. They could also quickly separate particles with different sizes and shapes.

Science, this issue p. 1505


Control and transport of nanoscale objects in fluids is challenging because of the unfavorable scaling of most interaction mechanisms to small length scales. We designed energy landscapes for nanoparticles by accurately shaping the geometry of a nanofluidic slit and exploiting the electrostatic interaction between like-charged particles and walls. Directed transport was performed by combining asymmetric potentials with an oscillating electric field to achieve a rocking Brownian motor. Using gold spheres 60 nanometers in diameter, we investigated the physics of the motor with high spatiotemporal resolution, enabling a parameter-free comparison with theory. We fabricated a sorting device that separates 60- and 100-nanometer particles in opposing directions within seconds. Modeling suggests that the device separates particles with a radial difference of 1 nanometer.

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