## 3 Results & Discussion
### Factors Controlling Pore Cast Quality
#### 3.2.2 Impregnation Pressure  
Figure 8 shows two silicon micromodels impregnated at low pressure (vacuum) (A) and at high pressure (B). We can clearly identify type 1 defects inside the impregnated model under vacuum (Figure 8A) and with less severity in the model that was impregnated under high pressure (Figure 8B). Hence, a degassing step before injection of the resin at high pressure is needed to reduce the defects by removing fluids from the sample and limit gas bubble formation.  
As discussed earlier, the vacuum level control is crucial in limiting shrinkage defects formation where a higher vacuum level is associated with more abundant shrinkage defects; see Figure 6A. Therefore, we opted to use a lower vacuum level, 120 mbar, for the degassing step before high pressure-assisted impregnation. Figure 9 shows a micromodel impregnated at high pressure preceded by degassing at 120 mbar vacuum. The impregnated model does not feature type 1 defects even at the smallest channel, showing the importance of the degassing step. We still observed minor shrinkage defects, even though we used a low vacuum level. However, pressure-assisted impregnation preceded by degassing at a low vacuum level showed higher quality impregnation compared to other recipes, and hence it was applied to our rock sample impregnation.  
Pressure-assisted impregnation may also play an important role in controlling the curing kinetics [53] and the mechanical properties of the cured epoxy resin [19]. Experimentally, this leads to increasing the curing time by almost 50%. Figure 10 shows an abrupt release of the dissolved gas when the sample was depressurized at the original curing time. On the other hand, resin curing under pressure prevents the volatile components from being released by applying a pressure greater than their vapor pressure.  
This approach yields a denser structure free of porosity and with better mechanical properties [56; 57]. [PERSON] et al. [19], [PERSON] et al. [58], [PERSON] and [PERSON] [59] all reported higher tensile and flexural strength samples in comparison with samples after the vacuum process, implying higher stability of the samples during the preparation phases, e.g., cutting, grinding, and polishing.  
Figure 7: Confocal images of type 2 shrinkage defects: (**A**) The voids in the resin that have 10–40 μm size. (**B**) The rough shrinkage cracks that develop across the structure indicate high residual shrinkage stresses.  
Figure 8: Confocal images of silicon micromodels impregnated by resin mixed with a fluorescent dye. Impregnation is carried out at: vacuum (**A**) and at high pressure (**B**). Type 1 defects (gas bubbles) trapped during the inefficient impregnation are highlighted inside the channels of both micromodels. Type 2 defects were only observed in the high-pressure impregnated model.  
Figure 9: Confocal image of the silicon micromodel impregnated by resin mixed with fluorescent dye under high pressure preceded bya degassing step at a low vacuum level, 120 mbar. No trapped gases were observed even at the smallest channel, which shows the importance of the degassing step, while type 2 shrinkage defects were still persistent but with less severity due to the lower vacuum level used at the degassing step.