Long-range spontaneous droplet self-propulsion on wettability gradient surfaces

Chaoran Liu,Lufeng Che,Jing Li,Chenghao Xiang,Zuankai Wang,Xiaofeng Zhou,Jing Sun

doi:10.1038/s41598-017-07867-5

Abstract

The directional and long-range droplet transportation is of great importance in microfluidic systems. However, it usually requires external energy input. Here we designed a wettability gradient surface that can drive droplet motion by structural topography. The surface has a wettability gradient range of over 150° from superhydrophobic to hydrophilic, which was achieved by etching silicon nanopillars and adjusting the area of hydrophilic silicon dioxide plane. We conducted force analysis to further reveal the mechanism for droplet self-propulsion, and found that the nanostructures are critical to providing a large driving force and small resistance force. Theoretical calculation has been used to analyze the maximal self-propulsion displacement on different gradient surfaces with different volumes of droplets. On this basis, we designed several surfaces with arbitrary paths, which achieved directional and long-range transportation of droplet. These results clarify a driving mechanism for droplet self-propulsion on wettability gradient surfaces, and open up new opportunities for long-range and directional droplet transportation in microfluidic system.

Highlights

The spontaneous and directional liquid droplet transportation on a solid surface without any externally applied force has attracted increasing interests in microfluidic systems, especially for analytical chemistry and bioassay applications in recent years[1,2,3,4,5,6,7,8,9,10,11]
The valleys of the stripes are covered silicon nanopillars fabricated by deep reactive ion etching (RIE), which exhibits a static contact angle (CA) of 166.0°
Here, ASiO2 and ASNP are the area of SiO2 region and silicon nanopillars region, respectively

Summary

Introduction

The spontaneous and directional liquid droplet transportation on a solid surface without any externally applied force has attracted increasing interests in microfluidic systems, especially for analytical chemistry and bioassay applications in recent years[1,2,3,4,5,6,7,8,9,10,11]. Elegant approaches to break the wettability symmetry of a droplet on a surface have been developed by leveraging on the gradients of chemical[2, 7, 14, 15], structural topography[16,17,18,19,20,21], temperature[22], electric force[23,24,25,26], mechanical vibration[12, 27], PH-induced[28, 29] or their combinations[30,31,32,33] Among these strategies, the creation of wettability gradient by structural topography or chemical heterogeneity has gained increasing attention owing to its advantages such as the alleviation of external energy supply and easy operation[13, 28, 34,35,36,37,38,39,40,41]. More droplet motion paths besides the reported path are achieved by this surface wettability gradient method

Methods

Results

Conclusion