Contact adhesion hysteresis

In a separate study using the JKR technique, Chaudhury and Owen [48,49] attempted to understand the correlation between the contact adhesion hysteresis and the phase state of the monolayers films. In these studies, Chaudhury and Owen prepared self-assembled layers of hydrolyzed hexadecyltrichlorosilane (HTS) on oxidized PDMS surfaces at varying degrees of coverage by vapor phase adsorption. The phase state of the monolayers changes from crystalline (solidlike) to amoiphous (liquid-like) as the surface coverage (0s) decreases. It was found that contact adhesion hysteresis was the highest for the most closely packed... 

As hierarchical, multiscale and complex properties in nature, as shown inO Fig. 52.1, adhesion is an interdisciplinary subject, which undergoes vast experimental, numerical and theoretical investigations from microscopic to macroscopic levels. As an example, adhesion in micro- and nano-electromechanical systems (MEMS/NEMS) is one of the outstanding issues in this field including the micromechanical process of making and breaking of adhesion contact, the coupling of physical interactions, the trans-scale (nano-micro-macro) mechanisms of adhesion contact, adhesion hysteresis, and new effective ways of adhesion control (Zhao et al. 2003). 

Fig. 13. Measurement of surface energies of PS and PMMA. It can be seen that there was a finite adhesion hysteresis. At a given load, the contact radius during loading was less than the contact radius during unloading. From the unloading data, we get yi>s = 45 1 mJ/nr, and yi),viMA = 53 1 mj/m . These number are in good agreement with the values of surface energies determined from the pull-off force measured using the SFA.

Fig. 14. Measurement of adhesion between PDMS and PS, and PDMS and PMMA. It can be seen that there was no adhesion hysteresis. The contact radius at a given load, during loading and unloading was the same. From these data, we get WpoMs-ps = 49 3 mJ/m-, and Tfdm.s-fm.ma = 57 1 mJ/m-.

She et al. [128] used rolling contact to estimate the adhesion hysteresis at polymer/oxide interfaces. By plasma oxidation of the cylinders of crosslinked PDMS, silica-like surfaces were generated which could hydrogen bond to PDMS r olecules. In contrast to unmodified surfaces, the adhesion hysteresis was shown to be larger and proportional to the molecular weight of grafted polymer on the substrate. The observed hysteresis was interpreted in terms of the orientation and relaxation of polymer chains known as Lake-Thomas effect. 

The effect of thermal aging on polyethylene and isotactic polypropylene have been studied by Konar et al. [49]. They used contact angle, contact angle hysteresis, and XPS to characterize the modified surfaces of the polymers. Hysteresis increased with aging temperature. In the case of polyethylene, thermal aging led to a significant increase in adhesion strength of polyethylene with aluminium, but the increase in the case of polypropylene was much less marked. 

Amongst different channels of the energy dissipation, the most relevant are the sample viscosity, the contact friction, and the adhesion hysteresis. When the drive frequency is chosen to be CQ0, the power that is dissipated when the tip periodically interacts with the sample can be written as... 

These surface modifications were performed in "pure" micro-wave (2.45 GHz, "single-mode") or in combined microwave/ radio frequency (2.45 GHz/13.56 MHz, "dual-frequency") plasma. Important systematic changes of the surface composition, wettability, and adhesion of thin metal films were observed for different substrate bias values, and for the different gases. The modified surface-chemical structure is correlated with contact angle hysteresis of water drops this helps to identify which surface characteristics are connected with the wettability heterogeneity and with adhesive bonding properties, and how they are influenced by plasma-surface interactions. 

Chen, Y. L., Helm, C. A., and Israelachvili, J. N. Molecular mechanisms associated w ith adhesion and contact angle hysteresis of monolayer surfaces. ]. Phys. Chem., 95, 10736 (1991). 

Dettre, R.H., Johnson, R.E.J., 1964. Contact angle hysteresis n. Contact angle measurements on rough surfaces Contact SI32. The dream of sta3ing clean lotus and biomi-metic surfaces. In Fowkes, F.M. (Ed.), Angle, Wettability, and Adhesion. American Chemical Society, Washington, DC, pp. 136—144. 

When particles make elastic contact with a surface, equilibrium is not attained immediately. Time is required for the contact spot to enlarge, and more time is needed for the contact to separate when a tensile force is imposed. This is adhesive drag. Indeed, equilibrium may never be attained. On making the contact, the spot size has a certain diameter at a given load. When breaking the contact at an identical load, the contact spot is bigger. This is known as adhesive hysteresis, which was observed by Drutowski in 1969. These effects may be studied systematically with smooth elastomer spheres at zero load as shown in Fig. 9.23." ... 

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