IITB-Monash Research Academy, Indian Institute of Technology Bombay , Mumbai 400076, India.
Department of Mechanical Engineering, Indian Institute of Technology Bombay , Mumbai 400076, India.
Langmuir. 2017 Sep 26;33(38):9620-9631. doi: 10.1021/acs.langmuir.7b02183. Epub 2017 Sep 12.
We experimentally investigate the impact dynamics of a microliter water droplet on a hydrophobic microgrooved surface. The surface is fabricated using photolithography, and high-speed visualization is employed to record the time-varying droplet shapes in the transverse and longitudinal directions. The effect of the pitch of the grooved surface and Weber number on the droplet dynamics and impact outcome are studied. At low pitch and Weber number, the maximum droplet spreading is found to be greater in the longitudinal direction than the transverse direction to the grooves. The preferential spreading inversely scales with the pitch at a given Weber number. In this case, the outcome is no bouncing (NB); however, this changes at larger pitch or Weber number. Under these conditions, the following outcomes are obtained as a function of the pitch and Weber number: droplet completely bounces off the surface (CB), bouncing occurs with droplet breakup (BDB), or no bouncing because of a Cassie to Wenzel wetting transition (NBW). In BDB and NBW, the liquid partially or completely penetrates the grooves beneath the droplet as a result of the wetting transition. The former results in droplet breakup alongside bouncing, while the latter suppresses the bouncing. These outcomes are demarcated on the Weber number-dimensionless pitch plane, and the proposed regime map suggests the existence of a critical Weber number or pitch for the transition from one regime to the other. CB and BDB are quantified by plotting the coefficient of restitution of the bouncing droplet and the volume of the daughter droplet left on the surface, respectively. The critical Weber number needed for the transition from CB to BDB is estimated using an existing mathematical model and is compared with the measurements. The comparison is good and provides insights into the mechanism of liquid penetration into the grooves. The present results on microgrooved surfaces are compared with published results on micropillared surfaces in order to assess the water-repelling properties of the two surfaces.
我们实验研究了微升水滴在疏水微槽表面上的冲击动力学。该表面是通过光刻制造的,采用高速可视化技术记录横向和纵向随时间变化的液滴形状。研究了槽面节距和韦伯数对液滴动力学和冲击结果的影响。在低节距和韦伯数下,发现最大液滴扩展在纵向方向上大于横向方向。在给定韦伯数的情况下,优先扩展与节距成反比。在这种情况下,结果是不反弹(NB);然而,在更大的节距或韦伯数下会发生变化。在这些条件下,作为节距和韦伯数的函数,得到以下结果:液滴完全从表面反弹(CB)、反弹时液滴发生破裂(BDB)或由于 Cassie 到 Wenzel 润湿转变而不反弹(NBW)。在 BDB 和 NBW 中,由于润湿转变,液体部分或完全渗透到液滴下方的凹槽中。前者导致液滴破裂和反弹,而后者抑制了反弹。这些结果在韦伯数-无量纲节距平面上进行了划分,并提出了一个状态图,表明从一种状态到另一种状态的转变存在一个临界韦伯数或节距。通过绘制反弹液滴的恢复系数和留在表面上的子液滴体积来定量 CB 和 BDB。使用现有的数学模型估计从 CB 到 BDB 转变所需的临界韦伯数,并将其与测量值进行比较。比较结果良好,并提供了对液体渗透到凹槽中的机制的深入了解。将微槽表面上的结果与已发表的微柱表面上的结果进行比较,以评估两种表面的疏水性。