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Macroscopic single contact properties of cohesive silanized glass beads

Werf, MSc S.IJ. van der (2019) Macroscopic single contact properties of cohesive silanized glass beads.

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Abstract:The flowing behaviour of granular materials is determined by the contacts between individual solid grains. This contact behaviour is highly dependent on the particle properties; in particular we are interested in the effect of cohesion. This research experimentally investigates the contact of a single pair of macroscopic cohesive glass beads. The surface of the glass beads were functionalised via chemical silanization, attaching a silane group to the particle surface by a covalent bond. Ideally, a monolayer around the bead should be observed that increases the dry adhesivity of the surface. However, findings indicate the silanization procedure to be moisture sensitive, especial when a trichlorosilane is used. Agglomerates of polymerized silanes are unequally divided over the surfaces, as shown on AFM measurements and SEM images. To measure the adhesion, a CMS Nano-Tribometer was used. With this device the contact force and displacement of the particle can be measured, from which the adhesivity can be extracted. The force-displacement curves of unmodified glass were compared with the Hertzian contact model and showed a good agreement. For polymerised particles, a broad variance is noticed in the results, which we attribute to agglomeration of polymerised silanes on the surface. Some of the force-displacement curves show highly irregular behaviour. A possible explanation is that the contact occurred on a high polymerized spot, resulting in a force-displacement graph which is comparable to the model “stiff particle with soft contacts” of Tomas [1] for highly adhesive surfaces. In most cases, however, a contact behaviour of the functionalised particles was observed in which the loading stiffness is slightly smaller than the Hertzian behaviour of untreated glass. When unloading, the displacement increases while the normal force decreases to a point where there is still a positive normal force, thereafter, to return almost back to the previous loading curve, in such a way that the adhesion is not depended on the applied load, see figure. The point where this phenomenon occurs is dependent on the load and the location of contact but does not seem to depend on velocity. The gradual change observed during cyclic loading and unloading is attributed to wear of the silane layer. An explanation why the displacement increases when the normal forces decreases, is still a matter to be researched.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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