Public PhD defence of Nikhil DAVE on "Multiscale modelling of dimensional stability and through-thickness liquid transport in paper sheets"

Paper, a porous-fibrous network, is made up of micro-scale hydrophilic fibres, which are notably susceptible to deformations due to variations in moisture content. On interaction with water, the dimensional change of a single fibre is transmitted to the fibre network via the inter-fibre bonds. At the sheet scale, this could result in out-of-plane displacements in the form of curling, fluting, cockling. In professional inkjet printing systems, this inadvertent behaviour is attributed to the time-dependent through-thickness liquid penetration. This process generally consists of a fast liquid penetration through the porous structure and subsequently a comparatively slow absorption by the fibres. At this point, the fibres start to swell and as a result of their collective, time-dependent swelling the sheet deforms as well. The deformations which occur depend on the hygro-mechanical properties of the fibres and their geometrical arrangement in the network. Additionally, the inter-fibre bonds and their mechanical and hygro-expansive properties play an important part in the overall expansion of the thin paper sheet.
To understand these macro-scale dimensional instabilities of paper, it is essential to study the properties of the fibrous network at the microscale. This thesis proposes an analytical and computational multiscale multi-physics modelling approach to acquire an enhanced comprehension of the factors affecting the water-induced swelling response and response of a paper sheet. The research enables to capture the complex behaviour of a paper fibrous network using analytical and idealised modelling techniques. To predict the hygro-expansive response at the sheet scale, the role of pores, fibres, inter-fibre bonds and their distribution is examined and the devised modelling framework is used to gain an understanding of the contribution of the microscale properties to the sheet swelling and out-of-plane deformation.