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Graphene is the first 2D material consisting of carbon atoms densely packed into planar structures. Graphene oxide (GO) is the intermediate derivative of chemically-produced graphene, which retains 2D basal plane structures but is also decorated with functional groups along the basal plane and edges. This functionality allows self-assembly of planar sheets into a paper-like material. However, formations of both intrinsic defects within the sheet structures as well as larger scale extrinsic defects in the paper are expected to significantly degrade mechanical performance. Strength provides the most direct evidence of defect related mechanical behaviour and is therefore targeted for understanding defect effects in GO paper. Such evaluations are crucial both from a technological perspective of realizing designed functions and from a fundamental interest in understanding structure-mechanics in 2D nanomaterials. A complete strategy of performing mechanical testing at different length scales is thus reported to provide a comprehensive description of GO strength. Both conventional larger scale mechanical testing as well as novel smaller length scale evaluations, using in situ atomic force microscopy (AFM) combined with scanning electron microscopy (SEM) and optical microscopy as well as structural probing using synchrotron FT-IR microspectroscopy, were applied to GO materials. Results showed that large structural defects determined mechanical properties of GO papers due to stress concentration effects whereas smaller scale intrinsic effects were defined by interfacial defects and stress concentrations within sheets. Synchrotron FT-IR microspectroscopy provided molecular deformation mechanisms in GO paper, which highlighted the interaction between in-plane C=C and cross-linking C=O bonds. A comprehensive description of macroscopic GO paper using evaluations of strength at the range of length scales studied was attempted, with a good correlation between predictions and experimental observations. This thesis therefore provides a hierarchical understanding of the defects impact on the strength of graphene-based materials from the macroscale to the nanoscale.

Understanding the contact line dynamics during droplet impact is critically important for industrial processes such as coating, the spraying of pesticides and for the design of anti-icing surfaces or impermeable clothing. It is known that, upon impacting on a solid, a droplet can spread, bounce off the substrate or splash depending on the liquid characteristics, the solid properties, the impact speed and the ambient pressure. In this thesis, we explore droplet impact in terms of some of these variables. Consequently, this work is focused on the experimental study of the contact line dynamics of impacting droplets on substrates ranging from wettable to non-wettable and from porous to non-porous. In particular, we focus on the parameters affecting: i) the splashing threshold of impacting droplets on solid substrates and ii) the penetration of impacting droplets through textiles. Furthermore, we apply our findings to the development of a liquid latex droplet-on-demand printing system. Most of the experiments in this thesis consist of the visualisation, by high speed imaging, of the impact of ethanol, water and aqueous glycerol droplets on solid and textile substrates. In addition, we present a custom-made Matlab algorithm that uses a polynomial fitting approach to extract the dynamic contact angle as a function of the contact line velocity. Moreover, we analyse the effect of droplet shape, the order of the fitting polynomial and the fitting domain, on the measurement of the contact angle on various stages following droplet impact. We use our experimental setup to demonstrate the importance of wettability and substrate roughness on the contact line dynamics and the impacting outcome. For smooth surfaces, we show that the maximum advancing contact angle ( max) is greater than 87 degrees for all the liquid/substrates. Moreover, we show that splashing depends on the substrate wettability and its threshold can be parameterised by max and the splashing ratio. Correspondingly, for rough surfaces, we determine that max increases with increasing substrate roughness. Furthermore, we establish that the ratio of the peak to peak roughness to the surface feature mean width, in conjunction with max and the splashing ratio, adequately predict the splashing threshold. Similarly, for the droplet impact dynamics on textiles, we find that the textile characteristics, such as the pore size and solid fraction, are critical for the impact outcome. Correspondingly, we find three different impact regimes, namely, 'no penetration', 'capture' and 'complete penetration'. Additionally, by balancing the kinematic pressure with the capillary pressure, we find a critical pore size for the transition from capture to complete droplet penetration in terms of the Weber number. Finally, we present a setup that permits the printing, by droplet impact, of liquid latex on paper with a high solid content (60 wt %). The process is controllable and reliable, making the printing of patterns possible. With this setup, multilayer objects were created from pure liquid latex, as well as from micronized rubber powder and latex suspensions. These results demonstrate the potential of droplet-based additive manufacturing processes to produce prints of liquid latex and tire rubber reuse.

There have been several studies on the tearing of rubber materials since the seminal paper on rupture of rubber was published by Rivlin and Thomas (1953).The behaviour is typically characterised using a fracture mechanics approach whereby the rubber has a geometrically independent relationship between crack growth rate during tearing versus strain energy release rate. This approach works well under conditions of steady tearing as the crack growth rate is easy to measure. However, this approach is much harder to interpret under the condition where the rubber exhibits discontinuous crack growth behaviour such as knotty tearing or stick slip tearing. Unfortunately, these are common tearing conditions observed in practice for filled rubbers as well as for some unfilled rubbers, especially those such as natural rubber that are capable of strain-induced crystallisation. Under these conditions it is not clear what the actual crack growth rate is as the value typically given results from the average of a very rapid tearing rate and a zero velocity tearing rate. The aim of this work is to develop a new approach to characterise the unsteady tearing behaviour of rubber in terms of the relationship between the rate of increase in the strain energy at the crack tip just immediately prior to the onset of the tearing which is quantified directly as the time derivative of the strain energy release rate · T , and the critical strain energy release rate T* required to propagate the crack. The approach adopted in this study is then evaluated using a range of different crystallising and non-crystallising rubbers as well as crystallising rubbers that have been modified to alter their crystallisation over a range of different test temperatures. Additionally, a new elastic-viscous transition diagram in association with the rate of change in the strain energy release rate at the tip of the crack is presented.