Temperature-responsive peristome-structured smart surface for the unidirectional controllable motion of large droplets

Abstract

The manipulation of fast, unidirectional motion for large droplets shows important applications in the fields of fog collection and biochemical reactions. However, driving large droplets (>5 μL) to move directionally and quickly remains challenging due to the nonnegligible volume force. Herein, we fabricated a scalable, bionic peristome substrate with a microcavity width of 180 μm using a 3D printing method, which could unidirectionally drive a large water droplet (~8 μL) at a speed reaching 12.5 mm/s by temperature-responsive wettability. The substrate surface was grafted with PNIPAAm, which could reversibly change its wettability in response to temperature, thereby enabling a temperature-responsive smart surface that could regulate droplet movement in real-time by changing the temperature. A series of temperature-responsive smart patterns were designed to induce water transport along specific paths to further realize controllable droplet motion with the antibacterial treatment of predesignated areas. The ability to achieve temperature-responsive unidirectional motion and dynamic control of droplet movement could allow programmable fluidic biosensors and precision medical devices. A temperature-responsive smart surface was produced to control the unidirectional motion of large droplets between spreading and pinning movement by changing the surface wettability.

Keywords: Chemistry; Microfluidics.

A temperature-responsive smart surface was produced to control the unidirectional motion of large droplets between spreading and pinning movement by changing the surface wettability.

Fig. 1. Design of a temperature-responsive smart surface.

a Schematic diagram of the unidirectional spreading and pinning of droplets on the surface mediated by different temperatures. b Schematic diagram of the preparation process of the temperature-responsive smart surface

Fig. 2. Wettability and surface morphology of the smart surface.

a Wettability change in the PNI surface and PNI/GRO/PER surface in parallel and vertical microgroove directions at high and low temperatures. b FT-IR spectra of the PER substrate (yellow line) and the PNI/GRO/PER surface (blue line). c–e SEM images of the temperature-responsive smart surface at different magnifications. f 2D and (g) 3D optical images and (h) outline map of the microcavity structure on the smart surface from LCM

Fig. 3. Droplet movement with different volumes on various surfaces (PNI surface, PNI/PER surface, PNI/GRO surface and PNI/GRO/PER surface).

a Optical microscopy images of the detailed process showing water spreading on various surfaces with different water volumes. b Comparison of the spreading distance of 5 μL water on various surfaces in the positive direction and negative direction

Fig. 4. Surface morphologies and 3D optical images of the 180 and 360 surfaces.

a, e PNI/GRO/PER surface images with microcavity widths of 180 μm and 360 μm under optical microscopy, respectively. b, f 2D optical images. c, g 3D optical images. d, h Outline maps of the microcavity structures on the 180 and 360 surfaces from LCM, respectively

Fig. 5. Spreading distances and velocities of different water volumes on various surfaces in both directions.

a Comparison of the spreading distances of different water volumes on various surfaces in the positive direction and negative direction. b Comparison of the spreading velocities of different water volumes on various surfaces in the positive direction. c Spreading distances of 5 μL water on various surfaces in the positive direction and negative direction at different tilt angles. d Cross-sectional view of droplet movement of different volumes on various surfaces

Fig. 6. Observation of droplet movement on the PNI/GRO/PER surface at different temperatures.

a As the droplet volume continues to increase, the droplet spreads unidirectionally on the surface. b The PNI/GRO/PER surface allows for reversible unidirectional spreading and pinning of droplets when the temperature is changed. c Spreading distances of the droplets at different temperatures over 10 cycles

Fig. 7. Mechanism of unidirectional motion of droplets on the PNI/GRO/PER surface

a Unidirectional motion of 5 μL water on the 180 surface. b Wetting state of droplets at high and low temperatures. c ${\theta }_{\mathrm{Y}}$ is the intrinsic WCA of the PNI surface at different temperatures. d Force analysis of droplets during unidirectional motion. e Diagram of the microstructure in a single track

Fig. 8. Unidirectional motion of droplets on different linear patterns.

a, b Unidirectional motion of droplets in Y- and W-shaped patterns

Fig. 9. Unidirectional motion of droplets on different curvilinear patterns.

a, b Unidirectional motion of droplets in different curved S-shaped patterns

Fig. 10

Unidirectional motion of droplets on a spiral microgroove pattern

Fig. 11. The antibacterial treatment of predesignated areas on the Y-shaped pattern.

a Photograph of bacterial colonies (original bacterial suspension: OBS, Y-shaped pattern with microgroove structures: Y-PNI/GRO, and Y-shaped pattern with microgroove and peristome structures: Y-PNI/GRO/PER) growing on predesignated areas of different samples. b Statistical amounts of adherent bacteria on the predesignated areas of different samples by the plate counting method. c Schematic diagram of the antibacterial processes on the Y-PNI/GRO and Y-PNI/GRO/PER patterns