Germs Wijnand Chr, Roeling Erik M, van Ijzendoorn Leo J, Smalbrugge Barry, de Vries Tjibbe, Geluk Erik Jan, Janssen René A J, Kemerink Martijn
Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
Phys Rev E Stat Nonlin Soft Matter Phys. 2012 Oct;86(4 Pt 1):041106. doi: 10.1103/PhysRevE.86.041106. Epub 2012 Oct 2.
Brownian ratchets enable the use of thermal motion in performing useful work. They typically employ spatial asymmetry to rectify nondirected external forces that drive the system out of equilibrium (cf. running marbles on a shaking washboard). The major application foreseen for Brownian ratchets is high-selectivity fractionation of particle or molecule distributions. Here, we investigate the functioning of an important model system, the on/off ratchet for water-suspended particles, in which interdigitated finger electrodes can be switched on and off to create a time-dependent, spatially periodic but asymmetric potential. Surprisingly, we find that mainly dielectrophoretic rather than electrophoretic forces are responsible for the ratchet effect. This has major implications for the (a)symmetry of the ratchet potential and the settings needed for optimal performance. We demonstrate that by applying a potential offset the ratchet can be optimized such that its particle displacement efficiency reaches the theoretical upper limit corresponding to the electrode geometry and particle size. Efficient fractionation based on size selectivity is therefore not only possible for charged species, but also for uncharged ones, which greatly expands the applicability range of this type of Brownian ratchet.
布朗棘轮能够利用热运动来执行有用功。它们通常利用空间不对称性来纠正驱动系统偏离平衡的无定向外力（比如在抖动的搓衣板上滚动弹珠）。布朗棘轮预计的主要应用是对粒子或分子分布进行高选择性分级分离。在此，我们研究一个重要模型系统——用于悬浮在水中的粒子的开/关棘轮——的运行情况，其中叉指式指状电极可以打开和关闭，以产生一个随时间变化、空间周期性但不对称的电势。令人惊讶的是，我们发现主要是介电泳力而非电泳力导致了棘轮效应。这对棘轮电势的（非）对称性以及实现最佳性能所需的设置具有重要意义。我们证明，通过施加一个电势偏移可以优化棘轮，使其粒子位移效率达到与电极几何形状和粒子大小相对应的理论上限。因此，基于尺寸选择性的高效分级分离不仅对带电物种可行，对不带电物种也可行，这极大地扩展了这类布朗棘轮的适用范围。
