Directional motion of resonant drops on a hydrophobic ratchet with gradient inclination

Abstract

A hydrophobic ratchet which consisted of teeth with gradient inclination was prepared in this work. The dynamic behaviors of resonant drops on the ratchet were experimentally investigated using a high-speed camera. According to our results, there are four types of behaviors of a resonant drop on the ratchet, including deformation in situ, wriggling, jumping and breaking up. For a wriggling or jumping drop, the drop can move directionally from the big to small inclination area. A new method to manipulate drop motion is proposed through combining the ratchet with vertical mechanical vibration. Moreover, the motion mechanism is presented on the basis of force analysis. Besides, the effects of amplitude, initial position and drop size on the drop acceleration and velocity are also discussed.

Introduction

The manipulation of drop motion on solid surface is of great significance in the fields of biology analysis [1], microfluidics [2,3], resistance reducing [4,5] and droplet collection [6], and it has attracted increasing attention. A large number of methods, including thermal stimuli [[7], [8], [9]], electrical [10,11], photochemical [12] and acoustic methods [13], have been applied in the manipulation of drops.

The control of drop motion usually involves a variety of microstructured gradient surfaces, and numerous surface microstructures, like radial tension gradient surfaces [14], micro-ridged hydrophobic surfaces [15], wedge-shaped gradient surfaces [16], and gradually increased structural roughness surfaces [17], have been designed and tested. However, drops move very slowly on these surfaces due to the lack of external stimuli.

In recent years, ratchets have been introduced to control drop motion as a novel manipulation approach. For instance, Linke et al. [35] designed a ratchet with even teeth and found that a Leidenfrost drop was able to self-propel on the ratchet. From then on, a number of studies have been carried out focusing on this kind of ratchet. To improve the drop mobility on the gradient surfaces, vibration is applied to manipulate the liquid drops. Moreover, the vibration modes of drops are investigated at various frequencies and movement modes of the three-phase contact lines, which has laid solid foundation for subsequent studies [[18], [19], [20]]. Thereafter, the Wenzel–Cassie and Cassie–Wenzel wetting transitions are analyzed in plenty of works [[21], [22], [23]]. Furthermore, the dynamical behaviors of vibrated drops are analyzed [[24], [25], [26], [27], [28], [29], [30], [31]], the influencing factors are discussed [31], the motion mechanism of vibrated drops is illustrated [[32], [33], [34], [35], [36], [37], [38], [39], [40]], and the friction properties between the drop and the solid surface are explored [4,5,42].

This paper aimed to manipulate drops by mechanical vertical vibration combined with a hydrophobic ratchet with gradient inclination. To this end, a gradient ratchet was prepared, and its wettability was changed by chemical modification. Then, the drop motion was realized by adjusting the characteristic parameters of mechanical vibration. In addition, the mechanism of motion was modeled and the effects of drop size, initial position and vibration energy on the motion characteristics were also discussed.