Path-programmable water droplet manipulations on an adhesion controlled superhydrophobic surface

Abstract

Here, we developed a novel and facile method to control the local water adhesion force of a thin and stretchable superhydrophobic polydimethylsiloxane (PDMS) substrate with micro-pillar arrays that allows the individual manipulation of droplet motions including moving, merging and mixing. When a vacuum pressure was applied below the PDMS substrate, a local dimple structure was formed and the water adhesion force of structure was significantly changed owing to the dynamically varied pillar density. With the help of the lowered water adhesion force and the slope angle of the formed dimple structure, the motion of individual water droplets could be precisely controlled, which facilitated the creation of a droplet-based microfluidic platform capable of a programmable manipulation of droplets. We showed that the platform could be used in newer and emerging microfluidic operations such as surface-enhanced Raman spectroscopy with extremely high sensing capability (10(-15) M) and in vitro small interfering RNA transfection with enhanced transfection efficiency of ~80%.

Figure 1. Superhydrophobic PDMS with micro-pillar arrays for manipulations of water droplet motion.

(a) Schematic illustration of the microfluidic platform that used a local dimple structure to manipulate water droplet motions including moving, mixing and analysis on the suspended PDMS substrate with micro-pillar array. (b) SEM image of a regular micro-pillar arrays (2.5 μm radius, 4 μm height). Scale bar, 5 μm. (c,d) Photographs of water droplets on the surface of the PDMS substrate with micro-pillar arrays. Scale bar, 1 cm. (e) Photograph images showing the excellent stretchability of the PDMS substrate.

Figure 2. Vacuum induced local dimple formation on the PDMS substrate with micro-pillar arrays.

(a) Photographic images and schematic illustrations of the local dimple formation on the substrate using a vacuum tip. Scale bar, 5 mm. (b) Cross-sectional photographic image of the negative replica of the dimple structure. PDMS substrate is uniformly stretched by the applied vacuum pressure. Scale bar, 1 mm. (c) Typical SEM image of the duplicated dimple structure. Scale bar, 30 μm. (d) SEM images of the micro-pillar arrays taken from the flat region (i), negative curvature region (ii) and positive curvature region (iii). Scale bar, 5 μm.

Figure 3. Geometric deformation of dimple as a function of vacuum tip diameter.

(a) Cross-sectional profiles of local dimple versus position for five different diameters of vacuum tips. (b) Measured positive/negative bending radii and slope angles of dimple structures as a function of tip diameter. Red, blue and black lines indicate the positive bending radius, negative bending radius and slope angle, respectively. (c) The variation in ε_dist as a function of tip diameter.

Figure 4. Dynamic water adhesion force changes for the manipulation of droplet motions on the PDMS substrate with micro-pillar arrays.

(a) The force-distance curves for the PDMS substrate contacted with a water droplet. (b) Relationship between F_adh and ε_dist on the PDMS substrate. F_adh is decreased as ε_dist is increased. (c) Schematic illustration of the forces on the surface of a dimple structure that affect water droplet motions (left) and time-sequential photographic images of a moving water droplet on the PDMS substrate via the tunable dimple structure (right). Scale bar, 3 mm.

Figure 5. In situ manipulation of water droplet motions on the PDMS micro-pillar arrays for droplet-based microfluidic operations.

(a) A 10 μl moving water droplet follows the trace of a character “N” shape. Scale bar, 5 mm. (b) Droplet operations including transportation, merging and mixing on the superhydrophobic PDMS substrate. Scale bar, 5 mm. (c) Scheme of the SERS measurement system (top) and typical in situ/ex situ SERS analysis spectra with different concentrations of analyte (R6G), obtained from a droplet mixture of R6G/Ag NP and evaporated R6G/Ag NP droplet, respectively (bottom). (d) Scheme of the siRNA-lipidoid complex formation for in vitro transfection. (e) Fluorescent images (top) and flow cytometry analyses of GFP-HeLa cells two days after transfection (bottom). Scale bar, 200 μm (n = 3, **p < 0.01, compared to the conventional group).