Pial Turash Haque, Sachar Harnoor Singh, Desai Parth Rakesh, Das Siddhartha
Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742, United States.
ACS Nano. 2021 Apr 27;15(4):6507-6516. doi: 10.1021/acsnano.0c09248. Epub 2021 Apr 2.
Controlling the direction and strength of nanofluidic electrohydrodyanmic transport in the presence of an externally applied electric field is extremely important in a number of nanotechnological applications. Here, we employ all-atom molecular dynamics simulations to discover the possibility of changing the direction of electroosmotic (EOS) liquid flows by merely changing the electric field strength in a nanochannel functionalized with polyelectrolyte (PE) brushes. In exploring this, we have uncovered three facets of nanoconfined PE brush behavior and resulting EOS transport. First, we identify the onset of an overscreening effect: such overscreening refers to the presence of more counterions (Na) within the brush layer than needed to neutralize the negative brush charges. Accordingly, as a consequence of the overscreening, in the bulk liquid outside the brush layer, there is a greater number of co-ions (Cl) than counterions in the presence of an added salt (NaCl). Second, this specific ion distribution ensures that the overall EOS flow is along the direction of motion of the co-ions. Such co-ion-dictated EOS transport directly contradicts the notion that EOS flow is always dictated by the motion of the counterions. Finally, for large-enough electric fields, the brush height reduces significantly, causing some of the excess overscreening-inducing counterions to squeeze out of the PE brush layer into the brush-free bulk. As a result, the overscreening effect disappears and the number of co-ions and counterions outside the PE brush layer become similar. Despite that there is an EOS transport, this EOS transport, unlike the standard EOS transport that occurs due to the imbalance of the co-ions and counterions, occurs since a larger residence time of the water molecules in the first solvation shell of the counterions (Na) ensures a water transport in the direction of motion of the counterions. The net effect is the reversal of the direction of the EOS transport by merely changing the strength of the electric field.
在许多纳米技术应用中，控制纳米流体在外部施加电场情况下的电渗驱动输运方向和强度极为重要。在此，我们采用全原子分子动力学模拟来探究通过仅仅改变用聚电解质（PE）刷功能化的纳米通道中的电场强度，改变电渗（EOS）液体流动方向的可能性。在探索这一问题时，我们揭示了纳米受限PE刷行为及由此产生的EOS输运的三个方面。首先，我们确定了过屏蔽效应的起始：这种过屏蔽是指刷层内存在比中和负性刷电荷所需更多的抗衡离子（Na）。因此，由于过屏蔽，在添加盐（NaCl）的情况下，刷层外部的本体液体中，同离子（Cl）的数量多于抗衡离子。其次，这种特定的离子分布确保了整体EOS流沿着同离子的运动方向。这种由同离子主导的EOS输运直接与EOS流总是由抗衡离子运动主导的观念相矛盾。最后，对于足够大的电场，刷高度显著降低，导致一些导致过屏蔽的过量抗衡离子挤出PE刷层进入无刷本体。结果，过屏蔽效应消失，PE刷层外部的同离子和抗衡离子数量变得相似。尽管存在EOS输运，但这种EOS输运与由于同离子和抗衡离子失衡而发生的标准EOS输运不同，它的发生是因为水分子在抗衡离子（Na）的第一溶剂化壳层中的停留时间更长，确保了水沿抗衡离子运动方向的输运。净效应是仅仅通过改变电场强度就能使EOS输运方向反转。
