奈米自泳動粒子在窄道及兩不相溶液體中的行為

Abstract

主動粒子又稱為自泳動粒子，是一個現今十分熱門的研究領域。其包含了自然界中常見的細菌或是人工合成之Pt-SiO2的Janus膠體粒子。他們的運動行為常常藉由run-and-tumble運動模型來描述，其中特徵參數包含了由驅動力的主動速度(va)及重新改變方向的運行時間(τ)。由於粒子自行泳動的特性會造成在微小尺度下的能量耗散，含有自泳動粒子的主動流體被視為是一個非熱力學的系統，而其中的主動粒子也因而能表現出一些熱力學平衡之外的行為。常見的行為如表面累積、整流、動力學聚集效應等等。根據其能自行移動的特性以及這些特殊現象，自泳動粒子在藥物輸送系統展現了極大的潛力。本論文採用耗散粒子動力學模擬了奈米自泳動粒子在極窄的狹道和兩不相溶液體中所表現的特殊行為。 在第一個題目中(章節3.1)，本研究主要探討自泳動粒子在極窄通道中的行為。一般粒子在極窄的通道中會受到體積排擠效應及亂度的限制，使得一般粒子不容易進入窄道。然而在本研究中方向，自泳動粒子會更喜歡往狹窄通道中鑽入，並且隨著va及τ的增加待在窄道中的粒子數也更多。更令人意外的是當自泳動力夠大時，隨著通道高度的下降，待在通道中的粒子數反而是上升的，因此我們提供三種粒子濃度分布圖來說明這個現象。 第二個題目中(章節3.2)，本研究主要探討自泳動粒子在兩不相溶的液體中所表現的運動行為。發現若粒子在兩液體都能維持自泳動機制，那粒子就可以輕鬆穿過液體界面並有可能使親水粒子在更喜歡待在油中。同樣的，隨著va及τ的增加在油相中的親水粒子也會增加，本文也探討了親疏水性對粒子分布的影響和界面累積效應所扮演的角色。本研究提出了一個熱力學模型以及一個動力學模型分別描述其近熱力學平衡及非熱力學平衡時的行為。

Active particles or swimmers exist in nature and can be fabricated artificially as well. Their motion is frequently described by the run-and-tumble model, in which the linear trajectory is driven by the constant velocity (va) and reorientation after a specific run time (τ). The suspension of active particles is considered as the non-equilibrium system due to energy dissipation at the microscopic level, and therefore it can exhibit the behaviors absent in equilibrium systems. Its out-of-equilibrium phenomena include surface accumulation, rectified motion, swarming, and collective behavior, just to name a few. Consequently, active particles have a great potential in drug delivery systems. In this work, we investigated the behaviors of nanoswimmers towards a nanoslit and an immiscible liquid by dissipative particle dynamic (DPD) simulations. In the first project (Sec. 3.1), partition of nanoswimmers between narrow channel and reservoir is evaluated. In opposite to passive colloids, nanoswimmers prefer to stay in the slit rather than in the reservoir for sufficiently large active force (Fa) or run time (τ). The partition ratio (φ) increases with Fa and τ. Interestingly, as the slit height decreases, φ grows accordingly until the confinement effect dominates. Three types of the concentration profile in the slit are identified as a consequence of the competition between surface accumulation and entropic barrier. In the second project (Sec. 3.2), the behavior of nanoswimmers which can self-propel in two immiscible liquids such as water-oil systems and are able to cross the interface. At steady-state, the partition ratio (φ) of nanoswimmers between two immiscible liquids was obtained, and it depends on the active force (Fa), run time (τ), and swimmer-solvent interactions. Partition ratio φ is found to grow generally with increasing Fa2 τ. At sufficiently large Fa, it is surprising to find that hydrophilic nanoswimmers prefer to stay in the oil phase rather than in the water phase, i.e., φ>1. The partition ratio is also influenced by the hydrophobicity of swimmers in the oil phase. Two simple models are proposed to describe the partition ratio, including a near-equilibrium model and a kinetic model. Surface accumulation appeared at impenetrable interface is also observed at the fluid-fluid interface for small Fa but it vanishes for sufficiently large Fa.