Surface Tension-Induced Gel Fracture. Part 1. Fracture of Agar Gels
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- 作者：Constantinos Spandagos ; Thomas B. Goudoulas ; Paul F. Luckham ; Omar K. Matar
- 刊名：Langmuir
- 出版年：2012
- 出版时间：May 8, 2012
- 年：2012
- 卷：28
- 期：18
- 页码：7197-7211
- 全文大小：759K
- 年卷期：v.28,no.18(May 8, 2012)
- ISSN：1520-5827
文摘
This work involves an experimental investigation of the spreading of liquids on gel layers in the presence of surfactants. Of primary interest is the instability that accompanies the cracking of gels through the deposition and subsequent spreading of a drop of surfactant solution on their surfaces. This instability manifests itself via the shaping of crack-like spreading 鈥渁rms鈥? in formations that resemble starbursts. The main aim of this study is to elucidate the complex interactions between spreading surfactants and underlying gels and to achieve a fundamental understanding of the mechanism behind the observed phenomenon of the cracking pattern formation. By spreading SDS and Silwet L-77 surfactant solutions on the surfaces of agar gels, the different ways that system parameters such as the surfactant chemistry and concentration and the gel strength can affect the morphology and dynamics of the starburst patterns are explored. The crack propagation dynamics is fitted to a power law by measuring the temporal evolution of the length of the spreading arms that form each one of the observed patterns. The values of the exponent of the power law are within the predicted limits for Marangoni-driven spreading on thick layers. Therefore, Marangoni stresses, induced by surface tension gradients between the spreading surfactant and the underlying gel layer, are identified to be the main driving force behind these phenomena, whereas gravitational forces were also found to play an important role. A mechanism that involves the 鈥渦nzipping鈥?of the gel in a manner perpendicular to the direction of the largest surface tension gradient is proposed. This mechanism highlights the important role of the width of the arms in the process; it is demonstrated that a cracking pattern is formed only within the experimental conditions that allow S/螖w to be greater than G鈥? where S is the spreading coefficient, 螖w is the change in the width of the crack, and G鈥?is the storage modulus of the substrate.