Micro-fluidic applications of telephone cord delamination blisters
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Micro-fluidic applications of telephone cord delamination blisters Alex A. Volinsky, Patrick Waters, Gregory Wright University of South Florida, Dept. of Mechanical Engineering, Tampa FL 33620 USA [email protected]; http://www.eng.usf.edu/~volinsky ABSTRACT Argon pressure significantly affects the residual stress in sputter deposited thin films and coatings. In case of W thin films, high residual stresses on the order of 1-2 GPa are quite common. With the rest of sputtering parameters being equal, argon pressure determines the sign and the value of residual stress. When the amount of stored elastic energy in the film due to the residual stress exceeds the interfacial toughness, fracture normally occurs. Telephone cord buckling delamination blisters are commonly observed in compressed thin films. These mechanically active features form by a loss of adhesion between the film and the substrate due to residual stress relief, and exhibit directional growth under certain conditions. This paper considers telephone cord delamination channels for micro-fluidics applications, as this could to be a valuable, reliable, and inexpensive method of forming open channels. INTRODUCTION Microfluidics as a field has been growing with the new advances in nanotechnology [1]. This field has been estimated to grow at a near exponential rate in the decades to come. This relatively new technology has different possible applications, including drug delivery. Pharmacological agents could be successfully delivered directly to the wound sites [2]. The technology relies on the ability to transfer fluids through micro-channels. These channels are commonly etched in silicon by means of standard methods of lithography, although there are multiple steps involved [3], and the whole process is cumbersome, expensive, and labor intensive. Here, we present an alternative in the form of thin film delamination blisters forming open micro-channels. Telephone cord deamination morphology is commonly observed as a result of the thin film residual compressive stress relief by interfacial debonding (Figure 1 a). Here, biaxial film stress is partially relieved by film buckling in the direction perpendicular to the telephone cord propagation, and by “secondary” blister buckling in the direction of telephone cord propagation, which results in the sinusoidal fracture patterns [4, 5]. Detailed dynamics of real time telephone cord blister propagation can be observed online [6]. Normally telephone cord blisters “run out of steam” and stop once the interfacial toughness exceeds the strain energy release rate. It is possible to make blisters propagate further by either putting mechanical energy into the system [5], or by introducing liquids at the crack tip [7]. Liquids can also provide the mechanical energy (through surface tension and capillary forces) needed to continue blisters propagation, as well as reduce the film interfacial toughness. We attribute this to environmentally-assisted cracking causing thin film delamination, similar to a commonly known exa
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