Asymmetric pores in a silicon membrane acting as massively parallel brownian ratchets

Abstract

The brownian motion of mesoscopic particles is ubiquitous and usually random. But in systems with periodic asymmetric barriers to movement, directed or 'rectified' motion can arise and may even modulate some biological processes. In man-made devices, brownian ratchets and variants based on optical or quantum effects have been exploited to induce directed motion, and the dependence of the amplitude of motion on particle size has led to the size-dependent separation of biomolecules. Here we demonstrate that the one-dimensional pores of a macroporous silicon membrane, etched to exhibit a periodic asymmetric variation in pore diameter, can act as massively parallel and multiply stacked brownian ratchets that are potentially suitable for large-scale particle separations. We show that applying a periodic pressure profile with a mean value of zero to a basin separated by such a membrane induces a periodic flow of water and suspended particles through the pores, resulting in a net motion of the particles from one side of the membrane to the other without moving the liquid itself. We find that the experimentally observed pressure dependence of the particle transport, including an inversion of the transport direction, agrees with calculations of the transport properties in the type of ratchet devices used here.

Entropic transport: a test bed for the Fick–Jacobs approximation

“…Diffusion of Brownian particles through narrow, tortuous confining structures such as micropores and nanopores, zeolites, biological cells and microfluidic devices plays a prominent role in the dynamical characterization of these systems (Barrer 1978;Volkmuth & Austin 1992;Liu et al 1999;Kettner et al 2000;Müller et al 2000;Hille 2001;Nixon & Slater 2002;Matthias & Müller 2003;Berezhkovskii & Bezrukov 2005;Siwy et al 2005). Effective control schemes for transport in these systems require a detailed understanding of the diffusive mechanisms involving small objects and, in this regard, an operative measure to gauge the role of fluctuations.…”

Rectification Through Entropic Barriers

“…The geometric restrictions to the system's dynamics results in entropic barriers and regulate the transport of particles yielding important effects exhibiting peculiar properties. The results have prominent implications in processes such as catalysis, osmosis and particle separation [1][2][3][4][5][6][7][8][9][10][11][12] and, as well, for the noise-induced transport in periodic potential landscapes that lack reflection symmetry (Brownian ratchet systems) [13][14][15] or Brownian motor transport occurring in arrays of periodically arranged asymmetric obstacles, termed "entropic" ratchet devices [16][17][18][19][20]. Motion in these systems can be induced by imposing different concentrations at the ends of the channel, or by the presence of external driving forces supplying the particles with the energy necessary to proceed.…”

Bio-manufacturing technology based on diatom micro- and nanostructure

“…The mantle and girdle band of Coscinodiscus also have myriads of pores (diameter 100 nm; Figure 3(d)). Particularly, the pores arranged on the girdle have an asymmetric pore structure, affecting the Brownian motion of mesoscale particles [43]. The center process, satellite process and septa of some diatom frustules may also have micro-or nano-scale pores [10].…”