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Rocking Brownian motors achieve directional motion in highly diffusive environments by combining an inherently asymmetric static potential energy landscape and an oscillating “rocking” force [1, 2]. Our nanofluidic imple- mentation [3] features a confining wall with a ratchet-type nano-topography and AC electric fields as driving force. The potential energy landscape is due to electrostatic interactions between the charged particles and wall surfaces in close proximity.
Based on this scheme, we have developed a nanoparticle size-separation device [4]. Along the particle transport direction, one of the confining walls features a sawtooth-like topography with a superimposed slope. In turn, we obtain a ratchet-shaped potential with increasing potential energy barrier heights caused by the decreasing nanofluidic gap. A sharp drop in particle current occurs as the potential barriers increase. The drop happens the later, the smaller the particle size. This allows for a sequential separation of the particle suspension into multiple sub-populations.
The device was modeled by solving the Fokker-Planck equation. We show that the physics of the separation mechanism is governed by the energy landscape under forward tilt of the ratchet. The separation resolution is thus only dependent on the applied force. Experimentally, we demonstrate the separation of spherical gold particles of nominal 80 and 100 nm diameters with an applied voltage of 3.5 V and a sorting time of 20 s. We achieve a resolution of 2 nm, in accordance with our simulation results.
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