Li Yiliang, Huang Jinmei, Cheng Jiang, Xu Shouping, Pi Pihui, Wen Xiufang
School of Chemistry and Chemical Engineering, Guangdong Provincial Key Lab of Green Chemical Product Technology, South China University of Technology, Guangzhou 510640, PR China.
ACS Appl Mater Interfaces. 2021 Apr 7;13(13):15857-15865. doi: 10.1021/acsami.1c00517. Epub 2021 Mar 25.
The wedge-shaped Ag/Cu surface with a contact angle (CA) [droplet of 30 vol % propylene glycol (PG)] of 18.6° in the wedge track and 64.6° at its periphery was fabricated through a facile gradient displacement reaction on the Cu substrate. The aqueous droplet of 30% PG could realize directed motion on the wedge track without back-end pinning, moving in a two-stage process of front-end spreading and subsequent back-end shrinking. A wettability gradient from 64.6 to 18.6° on the wedge surface could enhance the droplet motion, especially during the second stage. A favorable length of the wettability gradient (15 mm) was obtained, capable of moving the droplet the farthest displacement of 21.6 mm at a velocity of 0.53 mm/s on a wedge track with the wedge angle of α = 10° and length of 25 mm. The driving force arising from the wettability gradient during the second stage was evaluated theoretically to elucidate the effect of the length of the wettability gradient on the movement. Finally, three T-shaped self-driven surface micromixers composed of a mixing zone with uniform wettability and a transportation zone with different gradients were designed to test the drainage ability of droplets away from the surface. The wedge track combined with the wettability gradient was found to be capable of removing the mixed droplet completely out of the mixing region and flowing away, while the droplet would attach or stay in the mixing zone if actuated by the shape gradient or the wettability gradient alone.
通过在铜基板上进行简便的梯度置换反应，制备出了楔形的银/铜表面，其在楔形轨迹中的接触角（CA）[30体积%丙二醇（PG）液滴]为18.6°，在其周边为64.6°。30%PG的水滴能够在楔形轨迹上实现定向运动，而不会出现后端钉扎现象，以前端铺展和随后后端收缩的两阶段过程移动。楔形表面上从64.6°到18.6°的润湿性梯度可以增强液滴运动，特别是在第二阶段。获得了一个合适的润湿性梯度长度（15毫米），在α = 10°、长度为25毫米的楔形轨迹上，该梯度能够使液滴以0.53毫米/秒的速度移动到最远21.6毫米的位移。从理论上评估了第二阶段润湿性梯度产生的驱动力，以阐明润湿性梯度长度对运动的影响。最后，设计了三个由具有均匀润湿性的混合区和具有不同梯度的传输区组成的T形自驱动表面微混合器，以测试液滴从表面排出的能力。结果发现，楔形轨迹与润湿性梯度相结合能够将混合液滴完全从混合区域中排出并流走，而如果仅由形状梯度或润湿性梯度驱动，液滴会附着或停留在混合区。
