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• The latent heat of water evaporation is particularly high, reaching 2260 J/g. Compared to organic solvents like toluene, which have a close boiling point but only require 367 J/g to evaporate, water needs over six times more energy to evaporate.
• During water evaporation, there is already water vapor present in the atmosphere, creating an existing water vapor pressure that affects the rate of water evaporation. When the external water vapor pressure is saturated, water evaporation will cease.

Vanderhoff et al. (1973) discovered that the moisture evaporation in emulsion gels can be divided into three stages: an initial constant rate period by a decelerating period and finally a gradual progression to a slow evaporation phase with zero velocity. A "skinning" phenomenon occurs.Croll et al. (1986) found that some emulsion drying only involves two stages: a constant rate period and a slow rate period, without any skinning.Both three-stage and two-stage drying phenomena exist in the drying process of dispersions, with the key difference being whether a "skin" forms on the surface during drying.

In the drying process of dispersions, since moisture can only evaporate from the surface, the evaporation will inevitably cause certain areas to concentrate unevenly.Experiments on emulsion drying have found that particles distribute unevenly during drying, appearing both vertically and horizontally.Uneven phenomena occurring in the vertical direction are known as vertical drying.Uneven phenomena occurring in the horizontal direction are known as horizontal drying.

Vertical drying is an inevitable phenomenon in emulsion drying:

• As water evaporates from the surface, particles in the emulsion surface become concentrated.
• Alongside particle concentration on the surface, there is a trend for particles to diffuse from high to low concentrations, hence two competing timescales emerge during drying: the drying time tevap of an emulsion film with wet thickness H, and the time tdiff required for surface particles to diffuse to the substrate.
• If particles tend to stay concentrated on the surface (tevap < tdiff), then a skinning phenomenon occurs—characteristic of the three-stage drying phenomenon;
• If particles tend to diffuse towards the substrate (tdiff < tevap), then a basic uniform concentration is maintained without skinning—characteristic of the two-stage drying phenomenon.

The factors affecting the drying process include:

• Increasing viscosity μ， increasing emulsion particle size R, increasing film thickness H, and accelerating drying speed E will all increase the Peclet number (Pe), exacerbating the skinning phenomenon during drying.

Stress generation during dispersion drying and stress-induced cracking are common phenomena. There are two reasons for stress production:

1. Capillary pressure: Capillary pressure-induced stress occurs during the particle deformation stage.
2. Volume shrinkage: Volume shrinkage is more severe in aqueous polyurethane dispersions and generally occurs later in the particle deformation stage.

The biggest structural difference between waterborne polyurethane dispersions and other dispersions or emulsions is that PUD particles contain a large amount of combined water.

1. The impact of combined water in waterborne polyurethane dispersion particles on cracking caused by capillary pressure
2. Cracking caused by volume shrinkage due to the evaporation of combined water

The characteristics of hierarchical cracking:

• Cracks form sequentially, and upon forming, they develop in a particular direction until they encounter previously formed cracks and terminate, resulting in a series of spatially segmented patterns with hierarchical properties.

Hierarchical properties mean that cracks exhibit differences in length/width and scale:

• Early formed cracks are long and wide;
• Later formed cracks are shorter because they are terminated by earlier cracks; although they release some stress, their limited length means they create narrower cracks;
• The angles at which cracks intersect are mostly very large. Volume shrinkage stress is significant and cannot be compared with capillary pressure-induced stress.