Directional Movement of Droplets in Grooves: Suspended or Immersed?

Abstract

The behavior of droplets trapped in geometric structures is essential to droplet manipulation applications such as for droplet transport. Here we show that directional droplet movement can be realized by a V-shaped groove with the movement direction controlled by adjusting the surface wettability of the groove inner wall and the cross sectional angle of the groove. Experiments and analyses show that a droplet in a superhydrophobic groove translates from the immersed state to the suspended state as the cross sectional angle of the groove decreases and the suspended droplet departs from the groove bottom as the droplet volume increases. We also demonstrate that this simple grooved structure can be used to separate a water-oil mixture and generate droplets with the desired sizes. The structural effect actuated droplet movements provide a controllable droplet transport method which can be used in a wide range of droplet manipulation applications.

Introduction

The dynamic behavior of droplets has drawn much attention in recent years due to its fundamental effect in many practical applications. Generally, droplets can be manipulated by a surface free energy gradient^{1,2,3,4,5,6,7,8}, temperature gradient^9,10, or forces in confined geometric structures^{11,Supplementary Figure 2). The grooved structure was folded from a copper sheet and connected to step** drives on each side. In this way, the cross section angle β of the groove could be adjusted in real time to the desired values. The microsyringe system was used to generate droplets with the desired volumes for the wetting mode measurements and to adjust the droplet volume in real time for the droplet growth observations. The surface wettabilities, including the apparent contact angle θ, advancing contact angle θ_a and receding contact angle θ_r, were measured by an OCAH200 contact angle system (Dataphysics, Germany, ±0.1°) with the results listed in Table 1. The experiments are conducted in an ambient environment with room temperatures of 18~24 °C and relative humidities of 35~45% RH.}

Experimental procedure

The droplet growth was simulated with a 2 μL water droplet placed in the groove with a microsyringe needle then inserted into the droplet to supply water at a constant flow rate of 0.06 μL/s. The effect of the inserted needle on the droplet movement can be neglected since its contact angle is ~90° and it has a relatively small diameter. In the evaporation experiments, a 4 μL water droplet was placed in the groove and then the droplet was allowed to evaporate by exposure to the ambient environment.

Imaging

The experimental images were captured by the CCD camera of the OCAH200 system with a resolution of 752 × 484 pixels and frame rate of 60 fps. The image overlap** in Fig. 5 was conducted in Photoshop with the “Difference” blend mode.

Materials

The V-shaped grooved structures were folded from a copper sheet. The groove surfaces (inner walls) were treated with W0.5 abrasive paste polishing, 180# sandpaper grinding and chemical etching⁴⁰ followed by the addition of a self-assembled monolayer (SAM) of n-Octadecyl mercaptan⁴¹. The fabricated groove structures are referred to as Smooth, Smooth SAM, Rough SAM and Etched SAM grooves depending on the fabrication method. The PVD surface was fabricated using physical vapor deposition⁴² of 1H, 1H, 2H, 2H-Perfluorooctyltrichlorosilane on the chemically etched copper surface⁴⁰. Deionized water, glycerol, kerosene and ethylene glycol were used to generate the droplets and for observations of the resting state.