液滴潤濕行為之研究:幾何結構效應

在日常生活中常見的液滴潤濕現象與薄膜除潤行為的相關研究，提供在工程科學、奈米技術等應用上的相關資訊。在本論文將探討幾何結構效應對液滴潤濕的影響與高分子薄膜在光滑表面上的除潤機制，分為下列四個部分: (1) 利用多體耗散粒子動力學和表面進化器的兩種模擬方法，來探討液滴位於凹槽(groove)正上方的潤濕行為。凹槽內與基材表面上有著相同的潤濕性(接觸角)，依據凹槽內一開始的潤濕情況可決定兩種臨界角度θ_Y^c 和θ_Y^(c*)。只要模擬設定的基材潤濕性(θ_Y)同時大於θ_Y^c 和θ_Y^(c*)的角度時，就無法讓凹槽內被潤濕。其存在三種區域：(i)當θ_Y≤θ_Y^c時，凹槽內總是會被液滴潤濕；(ii)當θ_Y^c<θ_Y<θ_Y^(c*)時，可觀察到浸透(impregnated)和沒有潤濕(unwetted)的兩種狀態；(iii)當θ_Y≥ θ_Y^(c*)時，凹槽內部就無法被液滴浸透。當液滴的體積逐漸增加時，θ_Y^c和θ_Y^(c*)兩個臨界角度都會跟著減小，但是當液滴體積夠大後臨界角度就不再受影響。表面粗糙度會阻礙液體潤濕凹槽內部。與平滑表面相比可發現在規則淺坑(shallow pits)的粗糙表面上，粗糙度會使液滴兩個臨界角度θ_Y^c 和θ_Y^(c*)都會減小。因此即使基材表面呈現稍微親水的潤濕性，大體積的液滴也無法潤濕粗糙表面上的凹槽。當凹槽內的表面結構從規則淺坑改為直線溝槽(straight trenches)時，臨界接觸角則會更進一步的降低。根據模擬結果顯示對於液體吸收(imbibition)的行為，凹槽內部的幾何結構是關鍵因素。在不經由化學改質的情況下，可以製造出防止液體濕潤的親水性凹洞(cavities)。 (2) 藉由多體耗散粒子動力學和表面進化器兩種模擬來進行研究，探討液滴位於基材表面上裂縫(crevice)所構成的閥(valve)時，是會通過(passage)裂縫或是阻塞(blockage)裂縫。雖然一般情況下認為液滴容易去潤濕親液性的裂縫，但研究結果顯示可有效阻止液滴滲透(penetration)進入具特定結構的裂縫當中。根據模擬結果製作出接觸角(θ_Y)和楔角(α)變化組成的相圖，並決定了三種液滴潤濕狀態：無滲透、部分滲透、以及完全滲透。發現當楔角(α)夠小時，即使在親液性基材表面上液滴都會從裂縫上方滑走產生無法滲透的狀態。對於適當的楔角(α)與較小接觸角(θ_Y)，液滴會傾向於破裂(break up)並且只有部分液體去潤濕裂縫產生部分滲透的狀態。模擬結果顯示出只要簡單地去調整基材上裂縫的楔角(α)，就可以製造出用來控制液滴通過的親液性毛細管所構成的奈米閥。 (3) 利用多體耗散粒子動力學來探討液滴在完全潤濕表面上的擴展動態過程。模擬在不同的基材擴展係數(S)下，液滴在平滑與粗糙表面的潤濕情況。擴展(spreading)膜的主要特徵在於液滴所呈現的球帽潤濕半徑(Rp)、表觀基底半徑(Rb)和接觸角(θ)。其中Rp和Rb之間的差異代表了前驅(precursor)膜的存在。可以使用冪定律：Rp~t^m、Rb~t^n、與 θ~t^(-α) 來描述液滴潤濕的動態行為。液滴在不同表面粗糙度上，指數 n=0.1 和 α=0.3 都遵守Tanner定律，並與擴展係數(S)無關。然而前驅膜的擴展取決於表面粗糙度和擴展係數。隨著對應粗糙度的凹洞(cavity)尺寸減小或是擴展係數(S)的增加，指數 m 可以大約從0.1上升至0.2。也就是自發擴展是由擴展係數所驅動，但也會受到表面粗糙度的阻礙。另外液滴在光滑表面上的強制擴展會導致非等向性(anisotropic)潤濕擴展。沿著施力方向 L(t) 長度會遵循冪定律 L∝t^p，其指數 p≈0.274 且不受擴展係數(S)的影響；然而此與施力垂直方向的長度(W)則是由自發擴展主導。僅在中等施力的情況之下，可觀察到液滴的後端會有接觸線被釘住的現象。 (4) 藉由多體耗散粒子動力學來探討高分子薄膜於平滑且部分潤濕的表面上其自我修復和除潤現象的動態過程。根據模擬決定三種除潤現象：(i)旋節(離相)分解；(ii)成核成長；(iii)介穩態的自我修復。其結果取決於基材表面的潤濕性質(θ_Y)、高分子薄膜的厚度(h_0)和乾孔半徑(R_0)大小。對於特定的乾孔大小可利用 θ_Y 和 h_0 關係來製作除潤機制相圖。當表面潤濕性降低也就是增加 θ_Y 時，成核與自我修復區間交界處的臨界膜厚度(h_c)也隨之增長。因此透過自我修復過程可以保持薄膜的介穩定性。除了潤濕性和乾孔大小之外，臨界膜厚(h_c)也取決於高分子的長度(N)。發現當高分子較長時，則需要較厚的薄膜來抑制透過成核現象產生的薄膜除濕。提出了兩種抑制除濕的方法。透過加入短的高分子或採用緊密的聚合物如星形高分子，可以促進具有大分子量高分子薄膜的介穩定性。在加入緊密聚合物方法中，星形狀的高分子隨著星形臂數(p)的增加，可有效提升奈米薄膜的穩定性。

關鍵字

液滴潤濕；接觸角；凹槽；裂縫；擴展系數；薄膜除潤；多體耗散粒子動力學；表面進化器

並列摘要

The droplet wetting and spreading are commonly observed phenomena in our daily life. The study about the wetting, spreading, and film dewetting can offer a concept to the application in engineering, science, and nanotechnology. In this dissertation, there are four major parts to investigate the effects of geometry structure on a droplet wetting and polymer thin film dewetting on a smooth surface. (1) The wetting behavior of a nanodrop atop a nanogroove on a smooth or rough surface is explored by many-body dissipative particle dynamics and Surface Evolver. The nanogroove possesses the same contact angle (θ_Y) as that of the surface. Depending on whether the groove is initially wetted or not, two critical contact angles (θ_Y^c,θ_Y^(c*)) beyond which the groove cannot be wetted are determined. Three regimes are identified: (i) as θ_Y≤θ_Y^c, the groove is always wetted; (ii) as θ_Y^c<θ_Y<θ_Y^(c*), both impregnated and unwetted states can be observed; (iii) as θ_Y≥θ_Y^(c*), the groove cannot be impregnated. As the drop volume is increased, both θ_Y^c and θ_Y^(c*) decrease but become insensitive to the volume eventually. Surface roughness tends to hamper the impregnation of grooves by liquid. Compared to a smooth surface, both critical contact angles of a rough surface with regular shallow pits are smaller. As a result, a large drop is unable to wet the groove with a rough surface even when the surface becomes slightly hydrophilic. When the surface structure within the groove is modified from shallow pits to straight trenches, the critical contact angle is further reduced. Our simulation outcomes show that the surface structure within the groove is crucial for liquid imbibition and it is possible to fabricate hydrophilic cavities that can prevent impregnation, without resorting to chemical modification process. (2) The passage or blockage of nanodrops through a nanovalve made of nanocrevice is explored by proof-of-concept simulations, including many-body dissipative particle dynamics and Surface Evolver. Although it is generally believed that the drops wet lyophilic crevices readily, we show that the penetration of the drops into such crevices with specific structures can be prevented. The morphological phase diagram in terms of the contact angle (θ_Y) and wedge angle (α) are constructed and three regimes are identified: non-penetration and partial penetration, in addition to complete penetration. It is interesting to find that as long as α is small enough, the drop always runs away from the crevice even on lyophilic surfaces, leading to the non-penetration state. For intermediate α and small θ_Y, the drop tends to break up and only a portion of liquid wets the crevice, corresponding to the partial penetration state. Our simulation results demonstrate that a lyophilic capillary nanovalve for controlling the droplet passage can be fabricated by simply adjusting the wedge angle of the crevice. (3) The spreading dynamics of a nanodrop on a total wetting surface is explored by many-body dissipative particle dynamics. Both smooth and rough surfaces with various spreading coefficients (S) are considered. The evolution of the spreading film is mainly characterized by the radius of the wetting area (Rp) and the apparent base radius (Rb) and contact angle (θ) of the spherical cap. The difference between Rp and Rb reveals the presence of the precursor film. The dynamic behavior can be described by the power law: Rp~t^m, Rb~t^n, and θ~t^(-α). Regardless of the surface roughness, the exponents n=0.1 and α=0.3 agree with the Tanner’s law and are independent of the spreading coefficient. However, the expansion of the precursor film depends on the surface roughness and the spreading coefficient. As the cavity size corresponding to roughness decreases or S increases, the exponent m can rise approximately from 0.1 to 0.2. That is, the spontaneous expansion is driven by the spreading coefficient but impeded by the surface roughness. Forced spreading of a nanodrop on a smooth surface leads to anisotropic expansion. The length along the force direction L(t) follows the power law L∝t^p and the exponent p≈0.274 is insensitive to S. Nonetheless, the length along the direction perpendicular to the force direction is dominated by the spontaneous spreading. Contact line pinning of the rear end is only observed for intermediate forces. (4) The self-healing and dewetting dynamics of a polymer nanofilm on a smooth, partial wetting surface are explored by many-body dissipative particle dynamics. Three types of dewetting phenomena are identified, (i) spinodal decomposition, (ii) nucleation and growth, and (iii) metastable self-healing. The outcome depends on surface wettability (θ_Y), polymer film thickness (h_0), and radius of the dry hole (R_0). The phase diagram of the dewetting mechanism as a function of θ_Y and h_0 is obtained for a specified R_0. As surface wettability decreases (increasing θ_Y), the critical film thickness associated with the nucleation/self-healing crossover (h_c) grows so that the metastability of the film can be retained by the self-healing process. In addition to θ_Y and R_0, h_c depends on the polymer length (N) as well. It is found that longer polymer requires thicker nanofilm to avoid dewetting by nucleation. Two strategies for dewetting suppression are proposed. The metastability of the film of polymers with large molecular weight can be promoted either by the addition of short polymers or by employing compact polymers such as star polymers. In the latter approach, the increment of arm number enhances the nanofilm stability.