The relationship between water content and capillary pressure and the microstructural changes in apples (Granny Smith) during drying was studied. Apples were dried in a pilot tray dryer and samples, with different moisture contents, were put in liquid nitrogen. The remaining water was eliminated by the critical point method to avoid structural changes and samples were then observed by scanning electron microscopy (SEM). It was possible to characterize the internal space of the apples during drying and find a relationship between the drying rate and microstructural changes. We postulate that shrinkage during drying is mainly due to changes in capillary pressure.

Numerical simulations of isothermal drying of non-hygroscopic liquid-wet rigid porous media are performed. Two- and three-dimensional pore networks represent pore spaces. Two types of mechanisms are considered: evaporation and hydraulic flow. The drying is considered to be a modified form of invasion percolation. Liquid in pore corners allows for a hydraulic connection throughout the network at all times. As drying progresses, liquid is replaced by vapor by two fundamental mechanisms: evaporation and pressure gradient–driven liquid flow. Using a Monte Carlo simulation, evaporation and drainage times are computed. The controlling mechanism is indicated by the shorter calculated time. Initially, the drying is governed by liquid flow, then by a combination of liquid flow and evaporation and finally by local evaporation. Reported here are the distributions of liquid and vapor with drying time, capillary pressure curves, liquid film saturation curves, and liquid diffusivity and vapor diffusivity as a function of liquid saturation.

This work presents results of Monte Carlo simulations of isothermal drying of a nonhygroscopic porous media initially saturated with a sugar solution. The porous media is represented by a two-dimensional network of cubic pores connected by throats with a given radius distribution. The considered network had just one open side (the three other sides were sealed) from which water evaporation occurred. Water evaporation, hydraulic flow, and diffusivity of sucrose in water are considered in the physical model. It was considered that drying occurred under isothermal conditions (low drying rates) and that the capillary forces surpass the viscous forces, as in invasion percolation. It was also considered that water evaporation inside the network of pores and throats causes solution concentration, which remains at the corners, allowing hydraulic connection throughout the whole network. At each simulation step, a single meniscus moves through a particular pore segment with the higher displacing force. As drying progresses, air replaces the solution. Determination of the mechanism prevailing at any given drying stage requires calculation of evaporation. In other words, each step of the simulation involves finding the solution to three systems of equations: the vapor pressure field in the vapor phase, the pressure field in the liquid phase, and the solutes' concentration in the liquid phase. Herein, we report results of drying curves calculated as a function of the sucrose and water saturation and of the distribution of liquid, sucrose, and vapor as drying advances. The results presented in this work showed that network models are a powerful tool for investigating the influence of the main mechanisms controlling drying at its different stages; that is, from liquid saturation condition to very low saturation (end of drying). Despite the applied simplifications, the model can capture the main aspects of drying of liquids and solutions present in porous media.

Formulating wood drying models with predictive capabilities is still a challenge. Traditionally, the wood drying process is studied macroscopically, with pore-level physics lumped into effective transport properties. However, microvisual experiments in 2-D transparent porous media (known as micromodels) can improve our understanding of the drying process and add to current theories. Here, we make etched glass micromodels with the pore morphology of softwoods and carry out simple drying experiments, thereby proving the potential of this experimental tool on the wood science field. We develop a procedure to generate the random morphology of softwood with a computer program based on known probability distribution laws. In particular, we generate the tracheid cell microfeatures of a softwood longitudinal-tangential section. The wood morphology is etched onto glass using a photolithographic technique, creating softwood glass micromodels. Good agreement is seen between the morphology of microtome sections and our computer-generated softwood. During wood drying, liquid flow is driven by capillary instabilities. Sudden liquid redistribution, know as a Haines jump, spontaneously drains the liquid contained in cell cavities. Viscous flow occurs mainly through liquid films in partially dry cells.