School of Nano Science and Technology, National Institute of Technology , Calicut 673 601, India.
Centre for Applied Nanosciences, Department of Atomic and Molecular Physics, Manipal University , Karnataka 576 104, India.
ACS Appl Mater Interfaces. 2017 Aug 23;9(33):28046-28054. doi: 10.1021/acsami.7b07451. Epub 2017 Aug 9.
In spite of the reported temperature dependent tunability in wettability of poly(N-isopropylacrylamide) (PNIPAAm) surfaces for below and above lower critical solution temperature (32 °C), the transport of water droplets is inhibited by the large contact angle hysteresis. Herein, for the first time, we report on-demand, fast, and reconfigurable droplet manipulation over a PNIPAAm grafted structured polymer surface using temperature-induced wettability gradient. Our study reveals that the PNIPAAm grafted on intrinsically superhydrophobic surfaces exhibit hydrophilic nature with high contact angle hysteresis below 30 °C and superhydrophobic nature with ultralow contact angle hysteresis above 36 °C. The transition region between 30 and 36 °C is characterized by a large change in water contact angle (∼100°) with a concomitant change in contact angle hysteresis. By utilizing this "transport zone" wherein driving forces overcome the frictional forces, we demonstrate macroscopic transport of water drops with a maximum transport velocity of approximately 40 cm/s. The theoretical calculations on the force measurements concur with dominating behavior of driving forces across the transport zone. The tunability in transport velocity by varying the temperature gradient along the surface or the inclination angle of the surface (maximum angle of 15° with a reduced velocity 0.4 mm/s) is also elucidated. In addition, as a practical application, coalescence of water droplets is demonstrated by using the temperature controlled wettability gradient. The presented results are expected to provide new insights on the design and fabrication of smart multifunctional surfaces for applications such as biochemical analysis, self-cleaning, and microfluidics.
尽管有报道称聚 N-异丙基丙烯酰胺（PNIPAAm）表面的润湿性在低于和高于下临界溶液温度（32°C）时具有温度依赖性可调性，但由于大接触角滞后，水滴滴的传输仍受到抑制。在此，我们首次报道了在 PNIPAAm 接枝结构化聚合物表面上使用温度诱导润湿性梯度实现按需、快速和可重构的液滴操纵。我们的研究表明，接枝在本征超疏水表面上的 PNIPAAm 在低于 30°C 时表现出高接触角滞后的亲水性质，而在高于 36°C 时表现出超低接触角滞后的超疏水性质。30 和 36°C 之间的过渡区域的特征是水接触角（∼100°）发生很大变化，同时接触角滞后也发生变化。通过利用这个“传输区”，其中驱动力克服摩擦力，我们展示了水滴滴的宏观传输，最大传输速度约为 40 cm/s。对力测量的理论计算与传输区中驱动力主导行为一致。通过沿表面改变温度梯度或表面倾斜角（最大角度为 15°，速度降低至 0.4 mm/s）来调节传输速度的可调性也得到了阐明。此外，作为实际应用，通过使用温度控制润湿性梯度演示了水滴的聚结。预计所提出的结果将为设计和制造用于生化分析、自清洁和微流控等应用的智能多功能表面提供新的见解。
