自由和约束干燥纸张的瞬时湿膨胀和湿膨胀：纤维网络耦合
Transient hygro- and hydro-expansion of freely and restrained dried paper: the fiber-network coupling
摘要：
监测自由和受约束的干燥软木和硬木片材和纤维在\textit{hygro}膨胀和\textit{hydro}膨胀过程中的瞬时尺寸变化，以解开在逐渐水饱和过程中发生的控制微观机制。单个纤维的响应测量使用全场全球数字高度相关方法，该方法已被扩展到监测的瞬态\textit{hydro} -膨胀的纤维从干燥到完全饱和。对于自由干燥的手抄纸和受限干燥的手抄纸以及软木手抄纸和硬木手抄纸，\textit{hygro} -和\textit{hydro} -膨胀更大。瞬态片状\textit{hydro}膨胀揭示了突然的应变和水分含量阶跃。据推测，驱动机制是在纤维的纤维素微纤丝中的所谓的“错位区域”的水分诱导软化，解锁进一步的纤维溶胀。对于受约束的干燥手抄纸，应变阶跃是可忽略的，这归因于“错位的纤维素区域”在受约束的干燥期间被锁定在它们的拉伸构型中，这由单纤维\textit{hydro} -膨胀测量支持。最后，利用纤维间键合模型，并从纤维水平特征预测片材尺度的\textit{hygro}膨胀。该模型正确地预测了自由与约束干燥和软木与硬木手抄纸之间的定性差异，然而，其简化的几何形状不允许更定量地预测纸张规模的\textit{hydro}膨胀。
英文摘要：
The transient dimensional changes during \textit{hygro}-expansion and \textit{hydro}-expansion of freely and restrained dried, softwood and hardwood sheets and fibers is monitored, to unravel the governing micro-mechanisms occurring during gradual water saturation. The response of individual fibers is measured using a full-field global digital height correlation method, which has been extended to monitor the transient \textit{hydro}-expansion of fibers from dry to fully saturated. The \textit{hygro}- and \textit{hydro}-expansion is larger for freely versus restrained dried and softwood versus hardwood handsheets. The transient sheet-scale \textit{hydro}-expansion reveals a sudden strain and moisture content step. It is postulated that the driving mechanism is the moisture-induced softening of the so-called "dislocated regions" in the fiber's cellulose micro-fibrils, unlocking further fiber swelling. The strain step is negligible for restrained dried handsheets, which is attributed to the "dislocated cellulose regions" being locked in their stretched configuration during restrained drying, which is supported by the single fiber \textit{hydro}-expansion measurements. Finally, an inter-fiber bond model is exploited and adapted to predict the sheet-scale \textit{hygro}-expansion from the fiber level characteristics. The model correctly predicts the qualitative differences between freely versus restrained dried and softwood versus hardwood handsheets, yet, its simplified geometry does not allow for more quantitative predictions of the sheet-scale \textit{hydro}-expansion.
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