ac/dc Magnetic Fields for Enhanced Translation of Colloidal Microwheels
ac/dc Magnetic Fields for Enhanced Translation of Colloidal Microwheels
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DOI：
10.1021/acs.langmuir.8b04084
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发表时间：
2019-03-05
期刊：
影响因子：
3.9
通讯作者：
Marr, David W. M.
中科院分区：
文献类型：
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作者：
Disharoon, Dante;Neeves, Keith B.;Marr, David W. M.
微型装置必须克服流体的可逆性，以在粘性力占主导地位的环境中推进自身。胶体微轮（mu轮）使用的一种方法是利用附近的表面来提供摩擦力，其中胶体微轮由超顺磁性颗粒组装而成，并由旋转的交流磁场提供动力。在这里，我们使用全内反射显微镜显示，个别8.3 μ m的颗粒滚动效率低下，由于20-80 nm的颗粒表面流体间隙的显着滑移。我们确定，间隙宽度和滑移随着颗粒旋转速率的增加而增加，当负载力仅由重力提供时，从而提供平移速度的上限。通过在交流磁场上叠加直流磁场梯度施加额外的负载力，我们能够减小差距宽度，从而提高平移速度。例如，由直流场梯度提供的0.2 F-g的附加载荷力将40 Hz旋转速率的平移速度从40 μ m/s增加到80 μ m/s。平移速度随着间隙宽度的减小而增加，无论是通过直流场梯度诱导的负载力来改变差距，还是通过用盐减小德拜长度来改变间隙。这些结果提出了一种策略，以加速微尺度颗粒的表面使能滚动，并打开独立于重力场的高速μ轮滚动的可能性。
Microscale devices must overcome fluid reversibility to propel themselves in environments where viscous forces dominate. One approach, used by colloidal microwheels (mu wheels) consisting of superparamagnetic particles assembled and powered by rotating ac magnetic fields, is to employ a nearby surface to provide friction. Here, we used total internal reflection microscopy to show that individual 8.3 mu m particles roll inefficiently with significant slip because of a particle-surface fluid gap of 20-80 nm. We determined that both gap width and slip increase with the increasing particle rotation rate when the load force is provided by gravity alone, thus providing an upper bound on translational velocity. By imposing an additional load force with a dc magnetic field gradient superimposed on the ac field, we were able to decrease the gap width and thereby enhance translation velocities. For example, an additional load force of 0.2 F-g provided by a dc field gradient increased the translational velocity from 40 to 80 mu m/s for a 40 Hz rotation rate. The translation velocity increases with the decreasing gap width whether the gap is varied by dc field gradient-induced load forces or by reducing the Debye length with salt. These results present a strategy to accelerate surface-enabled rolling of microscale particles and open the possibility of high-speed mu wheel rolling independent of the gravitational field.