Mechanically Switchable Wetting Petal Effect in Self-Patterned Nanocolumnar Films on Poly(dimethylsiloxane)

Abstract

Switchable mechanically induced changes in the wetting behavior of surfaces are of paramount importance for advanced microfluidic, self-cleaning and biomedical applications. In this work we show that the well-known polydimethylsiloxane (PDMS) elastomer develops self-patterning when it is coated with nanostructured TiO₂ films prepared by physical vapor deposition at glancing angles and subsequently subjected to a mechanical deformation. Thus, unlike the disordered wrinkled surfaces typically created by deformation of the bare elastomer, well-ordered and aligned micro-scaled grooves form on TiO₂/PDMS after the first post-deposition bending or stretching event. These regularly patterned surfaces can be reversibly modified by mechanical deformation, thereby inducing a switchable and reversible wetting petal effect and the sliding of liquid droplets. When performed in a dynamic way, this mechanical actuation produces a unique capacity of liquid droplets (water and diiodomethane) transport and tweezing, this latter through their selective capture and release depending on their volume and chemical characteristics. Scanning electron and atomic force microscopy studies of the strained samples showed that a dual-scale roughness, a parallel alignment of patterned grooves and their reversible widening upon deformation, are critical factors controlling this singular sliding behavior and the possibility to tailor their response by the appropriate manufacturing of surface structures.

Keywords: GLAD coatings; PDMS; anisotropic wetting; droplet sliding; self-surface patterning.

Figure 1

SEM analysis of the surface state of TiO₂/PDMS samples subjected to different deformations after a first activation event by bending or stretching. (a) Schematic representation of the different configurations used to take the FESEM images; (b,c) FESEM micrographs of 60°-TiO₂/PDMS (middle) and 85°-TiO₂/PDMS surfaces (right). From top to bottom: (i) surface images of activated samples in a flat configuration after a first activation by bending; (ii,iii) concave and convex configurations during activation by bending and (iv) stretched samples in a flat configuration. The insets show magnified views of the groove regions. GD: average groove distance, as measured in the images.

Figure 2

AFM images of PDMS and 85°-TiO₂/PDMS surfaces in flat (after a single bending event, (a1,a2), stretched (d1,d2) and bent configurations, this latter in the form of either a concave (b1,b2) or a convex (c1,c2) surface, respectively.

Figure 3

(a,b) Sliding angles (α^II parallel and α^N perpendicular to the direction of ordered microgrooves, respectively) vs. type of strain configuration for PDMS and 60°- and 85°-TiO₂/PDMS samples. The volumes of the water and diiodomethane droplets were 30 µL and 5 μL, respectively. (c) Images showing the sliding behavior of 2, 5, 10 and 30 μL water drops left on a 60°-TiO₂/PDMS sample at increasing tilting angles (left), and images of a 10 μL water and 5 μL diiodomethane drops left on the 85°-TiO₂/PDMS sample at increasing tilting angles (right).

Figure 4

Rolling-off control of liquid droplets on TiO₂/PDMS samples. (a) Photographs of 30 µL water droplet on a 85°-TiO₂/PDMS sample in a highly stretched state (top), which is progressively released (bottom); (b) idem for 5 µL diiodomethane droplets on 60°-TiO₂/PDMS sample. Green arrows indicate the direction and magnitude of stretching. Red arrows indicate the sequence of images. The schemes at the left and right sides represent the self-patterned surface for the samples in the stretched (top) and non-stretched (bottom) states, highlighting the different distribution of PDMS (dark regions in the grooves) and TiO₂ (light regions) zones in the examined samples. Positive sliding is represented by green circles and pinned situations by red circles.