本論文主要探討DNA分子於直流電場驅動後馬上切換交流電場所呈現的運動行為(第三章)。我們發現即使在沒有任何電場梯度作用下，這些DNA分子會往其載入的方向移動。這樣的不尋常的泳動現象並非是來自於介電泳，然而，他們可能來自於電雙層中相反電荷離子因直流電場作用重新分佈所產生之淨偶極電荷所致。此電偶極再與交流電場作用可產生不對稱的庫倫力，進而驅動DNA分子運動。我們從以下四個觀察結果證實了此現象為不對稱的極化所致。第一，在同一實驗反覆切換不同電場則會有遲滯現象發生；第二，將交流電場延遲一段時間後開啟，DNA仍有淨移動行為的發生；第三，其淨移動的遷移率與DNA大小有關；第四，此淨移動行為會與物質本身的帶電量有所相關，且會因所帶電性不同而有相反的淨移動方向，而淨移動方向與電泳的運動方向相反。我們還發現這種現象並不局限於DNA，也可能發生於其他的帶電膠體粒子。因此我們推測此奇特現象普遍存在於帶電之膠體粒子懸浮液中。
此外，我們探討DNA分子在電場或流場作用下於親疏水表面的運動行為。於第四章我們發現靠近玻璃表面的DNA分子在流場作用下會有拉伸、回縮以及翻滾等現象；但於電場作用下卻無這些現象。在第五章我們製作三維結構的流道來拉伸DNA分子，並發現於三維突縮流道的DNA分子其延展程度比經常所使用的二維突縮流道更好。以及討論奈米流道在製程上所碰到的問題。

This thesis is mainly focused on the motion of DNA molecules when one switches to an AC field right after an application with a DC field (in Chapter 3). We find, surprisingly, that these DNAs can drift toward the injection end without acquiring any field gradients. Such anomalous drift is not due to dielectrophoresis. Rather, it could arise from the net dipole charge due to field-induced polarization of counterions within the electrical double layer to cause asymmetric Coulomb force created by this charge. This asymmetric polarization is borne out by the observations of (i) unusual hysteresis phenomenon when applying different fields in one experiment, (ii) non-vanished DNA drift when delaying the switching to an AC field, (iii) size-dependent drift mobility, and (iv) the drifting phenomenon is symmetric with respect to the charge attribute of an object—the drifting direction is always opposite to that of electrophoresis that is determined by the sign of the native charge. We also find that the phenomenon is not limited to DNA but can also occur to other charged colloids. We thus speculate that this peculiar phenomenon commonly exists in charged suspensions.
In addition, we investigate the motion of DNA molecules in electric fields or hydrodynamic flows on hydrophilic or hydrophobic surfaces. We find in Chapter 4 that near-surface DNAs can exhibit stretching, recoiled, and tumbling in hydrodynamic flows but not in electric fields. In Chapter 5, we fabricate structured three-dimensional channels for stretching DNA molecules. We observe that DNAs in such channels exhibit greater extensions than those in regular two-dimensional channels. Problems in fabrication of nanochannels are also discussed.

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