pubs.acs.org/Langmuir © 2010 American Chemical Society
Morphological Transformation of a Liquid Micropattern on Dynamically Tunable Microwrinkles Takuya Ohzono* and Hirosato Monobe Nanotechnology Research Institute (NRI), National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Received February 11, 2010. Revised Manuscript Received March 22, 2010 When a very thin, hard layer supported by a soft substrate is laterally compressed beyond a critical strain, then buckling of the hard layer occurs, leading to the formation of small sinusoidal surface undulations (microwrinkles). The orientation of the wrinkle grooves can be reversibly altered by simply adding a strain externally. Using this nonlinear microtopological change, we demonstrate that the morphology of an array of liquid filaments formed on microwrinkle grooves is dramatically and reversibly transformed into liquid filaments with a different orientation or a regular array of microdroplets, “dots”, depending on the predefined contact angle. The novel liquid transformation at nanomicrometer scales will find unique applications, such as switchable light diffraction grating, laboratories-on-a-chip systems as well as a simple liquid micropatterning technique.
Introduction Methods of shaping and manipulating liquids on small length scales are important for micropatterning,1,2 microfluidics,3,4 and biosensing5,6 and may provide fundamental insights into phenomena in small confined spaces. At equilibrium, the curvature of the liquid droplet on a smooth solid surface must be constant in accordance with Laplace’s equation. Thus, the droplet takes the shape of a spherical cap whose edges intersect the substrate at an angle, called the contact angle, that is determined by the force balance with respect to interfacial tension.7 If the solid surface has chemical heterogeneity8-10 or microtopography,11-15 then the liquid shape can be strongly affected and tuned. Some low-cost methods for liquid micropatterning that exploit transient interfacial instability, such as spinodal dewetting16 and the fingering instability of the receding three-phase line,17 have also been reported. Most of these methods have focused on the formation of micropatterns of solutes after solvent evaporation or liquid *Corresponding author. E-mail:
[email protected]. (1) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550–575. (2) Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya, T. Nat. Mater. 2009, 8, 494–499. (3) Squires, T. M.; Quake, S. R. Rev. Mod. Phys. 2005, 77, 977–1025. (4) Dittrich, P. S.; Manz, A. Nature Rev. 2006, 5, 210–218. (5) van den Heuvel, M. G. L.; de Graaff, M. P.; Dekker, C. Science 2006, 312, 910–914. (6) Service, R. F. Science 1998, 282, 399–401. (7) de Gennes, P.-G.; Brochard-Wyart, F.; Quere, D. Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves; Springer: NewYork, 2004. Here, the effects of gravity on larger length scales (> mm) and van ver Waals forces on the very small scales ( 0 and ∂c/∂θ < 0 for 0 < θ < π/2; a higher contact angle results in a higher liquid thickness and a smaller wetted area. (Here, we neglect the line tension at the three phase interface.) Whereas for the wrinkled surface it is difficult to obtain analytical expressions for the lengths, a and b, and for the thickness profile, a similar argument can be applied other than the anisotropy due to the wrinkle grooves. That is, it is plausible that the length a increases and that the thickness decreases as the contact angle decreases. This is confirmed by the 3D profiles of the typical isolated droplets captured by tapping-mode atomic force microscopy29 shown in Figure 2g. The cross sections of the droplet along the x axis show that the thickness decreases and a increases as the contact angle decreases. The contact angles estimated from the cross-sectional profiles (4 ( 3°, 12 ( 5°, and 19 ( 7° for the low, intermediate, and high contact angles, respectively) also qualitatively correspond to those on longer length scales (8.7 ( 1°, 13.5 ( 1°, and 31.5 ( 1°). Periodic microstructures can generally act as optical diffraction gratings.30,31 To demonstrate the potential of the liquid microdot array formed using a liquid with a high contact angle, we observed the diffraction pattern of laser light (He-Ne laser with a wavelength of Λ = 633 nm and the diameter of 1 mm) transmitted through the structure and screened on white paper above. The inset in Figure 1n shows two sets of first-order diffraction spots that correspond to two periodicities with directions perpendicular to each other: the wrinkle grooves and the periodic dots in the wrinkle groove direction. Both diffraction angles are about 18°, which corresponds to the estimated value from the microscopic observation: sin-1(Λ/λ0) ≈ 17.7°. Meanwhile, the pattern under the strain of -0.12, where the dot array was transformed to the LF array, showed no clear diffraction spots (inset in Figure l). Because the wrinkle grooves are filled with the liquid (higher refractive index >1: air) with the width of ca. 0.8λ90 on the LF array, it is assumed that the optically active amplitude of the periodic structure on the solid-liquid complex surface becomes much smaller (10-20 nm, Figure 1p) in comparison with the bare wrinkles (∼200 nm). Consequently, the dramatic switching between the square-lattice grating and the nongrating is repeatedly possible by simply straining the sample (Supporting Information, Movie 1). To the best of our knowledge, this is the first optical application of the transformability of a liquid micropattern triggered by structural changes at the surface. In summary, we have demonstrated the dramatic morphological transformation of liquid micropatterns on dynamically tunable microwrinkles. The transformation was based on the nonlinear change in the surface microstructures, which affected the liquid morphology as the physical boundary condition. Two processes were involved in the transformation: the division of liquid filaments into dot arrays and the fusion of dots in the newly formed grooves. As a result, for a liquid with a relatively low contact angle, the direction of the liquid filament array is changed as a whole. For a liquid with a relatively high contact angle, the fusion process was suppressed because the neighboring droplets remained separated as a result of the higher contact angle. Consequently, a microdot array forms over a large area. These transformations are reversible and repeatable by simply straining the sample. We believe that novel liquid transformations on (30) Rittenhouse, D. Trans. Amer. Phil. Soc. 1786, 2, 37–42. (31) Fraunhofer, J. Ann. Phys. 1823, 74, 337–378.
DOI: 10.1021/la1006204
6131
Letter
the nano/micrometer scale will find unique applications, such as in switchable light diffraction gratings, laboratories-on-a-chip systems,3-6 and simple liquid micropatterning.1,2 Acknowledgment. We thank Dr. H. Kitahata for fruitful discussions. This work was carried out under the auspices of the New Energy and Industrial Technology Department Organization (NEDO) of Japan under the Industrial Technology Research Grant Program in 2008.
6132 DOI: 10.1021/la1006204
Ohzono and Monobe
Supporting Information Available: Optical microscopy (OM) images of a morphologically transformed micropattern of a liquid on microwrinkles with a shorter wavelength, an OM image of a morphologically transformed micropattern of liquid on microwrinkles after a change in wrinkle orientation in the oblique direction, and a movie showing the reversible change in the laser diffraction pattern of laser light transmitted through the liquid micropattern on microwrinkles. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(9), 6127–6132
