Effect of gradient wettability on capillary imbibition in open semicircular copper channel

Open semicircular copper channels with a gradient wettability of 45 degrees -3 degrees or 3 degrees -45 degrees (contact angle of 1,2-propanediol), homogeneous wettability of 3 degrees, and segmented wettability on their inner surfaces were constructed. The capillary imbibition of 1,2-propanediol in these channels demonstrates that, compared with the surface with homogeneous wettability, there are an additional driving force and resistance on the surfaces with a gradient wettability of 45 degrees -3 degrees and 3 degrees -45 degrees, respectively. Meanwhile, the channel with gradient wettability on the first half segment and homogeneous wettability on the second half segment could accelerate capillary imbibition better than that with gradient wettability on the whole channel. Furthermore, a smaller length ratio, i.e., length of gradient segment to that of homogeneous segment of the channel, demonstrates a better liquid acceleration effect. Based on the Lucas-Washburn equation, a theoretical model to describe capillary imbibition was established by introducing an additional force arisen from the positive gradient wettability and a retardation coefficient caused by the surface roughness. The flow front position vs flow time could be predicted by the established model in this work, and the relative error between the theoretical prediction and experimental result was less than 20.7%, indicating the rationality of the proposed model.