Browsing by Author "Souza , Emerson Jose de"

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Open Access
The effect of capillary forces on adhesion of biological and artificial attachment devices
(2007) Souza , Emerson Jose de; Arzt, Eduard (Prof. Dr. phil.)
The presence of a liquid meniscus can cause far greater adhesion between a particle and a surface than occurs under dry conditions. Recent studies on biological attachment systems have highlighted the unique and important effect of liquid capillarity at the micro- and nanometer scale. The results demonstrate that macroscopic considerations of the classic meniscus theory must be modified to take into account new scaling laws and geometric relationships. A general description of wetting and capillary condensation as it applies to interfaces of small scales and to arbitrary substrates is clearly desirable but remains an unsolved challenge. In this work, I have performed numerical simulations of wet adhesion under less restrictive conditions then has been done before. In particular, I calculated the capillary force as a function of the distance between two substrates for the general case of different properties and different geometries of the substrates. The results are in excellent agreement with analytical results and with measurements of the capillary force. They allow us to propose a novel, effective method to evaluate the contact angle hysteresis of a liquid bridge between arbitrary substrates. The numerical calculations also include the effect of contact splitting which has proven to be a powerful mechanism in many biological attachment systems that are based on dry adhesion. Our results show that this mechanism does in principle also apply to wet adhesion and that splitting of one large bridge into many smaller ones enhances the capillary forces for all possible contact angles. This results in new scaling laws of the capillary force as a function of the number of liquid bridges. They predict, for example, an unexpected maximal force for moderately hydrophilic surfaces (i.e. contact angles around 70 degrees) and a maximal force per contact area for cylindrical bridges. These novel scaling laws lead to a deeper basic understanding of wet adhesion and can also serve as an important guideline as to how artificial attachment devices can be engineered to have specific properties.