The surface topography of the modification PDMS

A stiff film (oxidized PDMS polymers) was capped on the soft foundation (pristine PDMS polymers) that was confirmed in the study. Such bilayer systems such as Al/PS films [91], Au/PDMS films [72] and SiOx/PDMS films [73] have been demonstrated the formation mechanism of wrinkles by several research groups. The young’s modulus of a stiff layer mismatched greatly with that of an underlayer elastical materials. The mismatched young’s modulus would lead to the generation of compress stress. In order to release the compress stress, the surface would form wrinkles and remain eventually the incompressible status. Because one of formation methods for wrinkles is a thin metallic film covered with an elastomer, we used only optical microscopy (OM) to observe the modified surface of PDMS in order to avoid the influence of a metal/elastomer system.

Fig.5.19 (a) shows that OM image of disordered wrinkles formed by dipping into H2SO4/ HNO3 solutions during five seconds. Fig. 5.19 (b) shows that the dipped time as a function of the periodicity of wavy structures. The periodicity ranged from 11 to 165μm

during one second to three minutes was increased very rapidly with increasing the dipped time. Wrinkles generated in a wet process were discontinuous and bug-like in initial stages, but were continuous in later stages, which is because growing wrinkles would merge smaller ones. However, wrinkles generated in metal/elastomer systems were continuous in initial stages and these wrinkles grow hardly into several ten micrometers.

For wrinkles on bilayer systems, the periodicity (λ) of the buckles can be expressed as Eq., Poisson’s of the stiff layer and the soft foundation layer and the thickness[72]. Because we had only a very limited knowledge about the oxidized PDMS layer, we could not judge whether this Eq. described wrinkles on the surface exactly. However, this equation could explain the reason for the increase of wavelength with the increase of dipping time.

In this Eq., the wavelength of wrinkles depended mainly on the thickness of the capped layer. In other words, the thickness of oxidized PDMS layer grew rapidly with the increase of dipping time.

Es v_s E_p v_p

5.4.4 The control for the arrangement of wavy structures

Some reports have been verified that the arrangement of waves formed on the bilayers system could be effectively controlled. There are mainly three approaches to align waves:

(1) the arrangement of waves was always perpendicular to the surface of bas-relief patterns; (2) waves were ordered on a cylindrical surface; (3) waves are ordered after releasing a prestretched substrate. In this study, we tried these approaches to arrange wrinkles on the modified PDMS surface.

To study the effect of the pattern on the orientation of the wrinkles, a single circular step-like pattern was first examined. The circular pattern shown in Fig. 5.20(a) was with a diameter of 750 µm and height of 10 µm and disordered wrinkles were respectively found on the top and bottom of the step-like pattern on PDMS plates. Then, wrinkles on the PDMS plate with patterns in array were also considered. An array with two-by-two

square patterns, each pattern having a side length of 100 µm, height of 10 µm, and pitch of 100 µm, was studied. Disordered wrinkles were still found in this array. These wrinkles would not be regularly arranged along the sides of step-like patterns regardless of the change of the size and shape of step-like patterns. The resultant image is shown in Fig.

5.20(b). Therefore, as the scale of wrinkles grew with dipped time was bigger than that of step patterns, the patterns would be devoured by wrinkles.

Then, to study the effect of the curved surface on the orientation of the wrinkles, curved surface with different curvatures were respectively examined. Wrinkles which were still found on different curved surfaces arranged randomly with the change of curvatures. The resultant image is shown in Fig. 5.20(c). Although disordered wrinkles was formed on curved faces, the ring-like wrinkles, which were different form the bug-like wrinkles on non-curved surfaces, were first found on it.

Finally, the effect of uniaxial forces applied to stretch a PDMS substrate before oxidization was studied. No wrinkles were found before releasing the oxidized substrate.

But, ordered wrinkles, shown in Fig. 5.20(d), were found after releasing the oxidized substrate. Each of wrinkles faced almost a single direction which was perpendicular to the stretched direction of a PDMS plate. The magnitude of the compressive stress on the oxidized PDMS/PDMS due to the substrate contraction was smaller than the stretched tension stress introduced by the preload, leading to be able to arrange regularly wrinkles on the surface.

Experiment results revealed that wrinkles formed by a wet process were merely ordered through the guidance of external forces. These results were corresponding incompletely with previous literature regarding the arrangement of wrinkles formed through metal deposition or plasma treatments. We considered that metallic particles or plasma radicals merely deposited or implanted the surface of PDMS vertically by physical processes, which are unable to modify the sidewalls of steps on PDMS substrate.

The generated compress stress, due to the surface contraction, in the sidewalls of steps was zero but was rapidly increased beyond the edge of steps. The gradient of stress would force to arrange regularly wrinkles. But, the orientation of wrinkles formed by a wet process were easily random because the foundation shrank homogeneously, included the surface of PDMS and the sidewalls of steps. Each part of PDMS surfaces in wet

processes was uniformly oxidized regardless of the sidewall of pattered steps or curved surfaces, leading PDMS surfaces were simultaneously contracted without the generation of the gradient of stress. Hence, a wet process to modify PDMS surfaces can be regarded as an isotropy modification.

Fig.5.1 Optical micrographs of buckled surfaces prepared by plasma oxidation of heated poly(dimethylsiloxane) (PDMS) sheet comprising (a)homogeneous PDMS layer and (b) PDMS substrate with posts (height: 5 mm high, diameter: 30 mm) separated by 70 mm.

The buckles were formed upon cooling the sample to room temperature. (c) Scanning force microscopy image of disordered buckling waves. [72]

Fig.5.2 Scanning force microscopy images (20 6 20 mm2) of a poly(dimethylsiloxane) substrate with relief patterns prior to and after oxygen plasma modification. [73]

Fig.5.3 Optical micrographs of patterns formed when a thin layer of gold was deposited onto warm PDMS and the sample was cooled to room temperature. (a) Disordered patterns, (b) circles (radius: 150 μm), and (c) flat squares (side: 300 μm) elevated by 10–20 μm relative to the surface showed ordered patterns of waves on the recessed regions and no buckling on the plateaus. [74]

Fig.5.4 Alignment of buckles in thin films on PDMS patterned onto regions differing in Young’s modulus and coefficient of thermal expansion. [75]

Fig.5.5 (a) Scanning force microscopy (SFM) images of the lithographic pattern produced by oxygen plasma treatment of PDMS after removing polystyrene (PS) latex microspheres.

(a) SFM images and (c) optical microscopy images of wrinkle patterns coupled to lithographically patterned substrates. The left, middle, and right columns indicate the results for PS spheres having diameters (w) of 1.03, 1.59 and 3.06 μm, respectively. [76]

Fig.5.6 Scanning force microscopy images of wrinkled samples prepared by evaporating a thin layer of platinum onto a thick PDMS substrate. The images illustrate the rearrangement of the original disordered wrinkling pattern upon imposing a small uniaxial stress (the corresponding strains are indicated below each image) and a subsequent return to the original pattern upon stress removal. [78]

Fig.5.7 Characterization of the nested hierarchy of buckles (a) Scanning electron microscopy image of a buckle on PDMS substrate (b) Optical microscopy image in the transmission mode of generations of buckles (c) Topography profile collected with profilometry on generations of buckles. (d) Scanning force microscopy image revealing the structure of buckles. [81]

Fig.5.8. (a–f) Showing the SEM images for the surface structures of the PDMS substrates covered with a thin gold film at different surface configurations: (a) on a flat substrate; (b) on a substrate with a square, step-like bumped pattern; (c) on a substrate with a circular, step-like bumped pattern; (d) on a substrate with 5 by 5 square, step-like bumped patterns;

(e) a compound structures combing longitudinal slender patterns and transverse wavy structures; and (f) porous structures on the top surface of patterns; (g) the AFM image showing the surface topography for (a).

Fig.5.9 SEM images showing the surface structures of the PDMS substrates covered with a thin gold film and having different surface patterns: (a) an array of 3 by 3 lens-like bumped patterns; (b) a single lens-like bumped patterns; and (c) an array of 3 by 2 circular, concaved patterns.

Fig.5.10 SEM images showing the surface structures of the PDMS substrates with a circular, step-like bumped pattern covered with a gold thin film with different thicknesses.

(a) 50 nm; (b) 100 nm; (c) 150 nm; and (d) 200 nm.

Fig.5.11 AFM images showing heights of the surface structures. (a), (b), (c), and (d) correspond to figures (a), (b), (c), and (d) in Fig. 5.10, respectively.

Fig.5.12 Schematic diagrams showing the fabrication process of optical gratings on a PDMS substrate.

Fig.5.13 SEM surface images showing the ordered structures from a pre-stretched substrate

Fig.5.14 Effects of strength of the stretched strain and thickness of the metal film on the pitch and the depth of resulting surface wavy structures. (a) The pitch as a function of the strain when film thickness was fixed at 100 nm; (b) The corresponding AFM cross-section images for (a) showing the depths of the wavy structures at strains of 5% (upper) and 30%

(lower), respectively

Fig.5.15 Effects of strength of the stretched strain and thickness of the metal film on the pitch and the depth of resulting surface wavy structures. (a) The pitch as a function of the thickness of metal film when the stretched strain was kept at 30%; (b) The corresponding AFM cross-section images for (a) showing the depth of the wavy structures for the film thickness of 50 nm (upper) and 200 nm (lower), respectively.

Fig.5.16 CCD images showing the diffractive patterns from the single-sided optical grating at differential fabrication conditions, (a) and (b), and from the double-sided optical gratings, (c).

Fig.5.17 FTIR spectra of (a) the oxidized PDMS surface modified by dipped H2SO4/ HNO3

liquids (b) the pristine PDMS surface

Fig.5.18 Photo of water drop on (a) the pristine PDMS surface (b) the oxidized PDMS surface

Fig.5.19 (a) OM image showing wrinkles formed by dipped into H2SO4/ HNO3 liquids during five seconds on the PDMS surface (b) The dipped time as a function of the periodicity of wavy structures

Fig.5.20 OM image showing wrinkles (a) on a two by two square step-like patterns array (b) on a circular step-like pattern (c) on the surface of a glass tube covered a layer of PDMS films (d) on the surface of a prestretched PDMS substrate