## 4 Discussion
### Link between Mechanical Properties and Surface Properties
#### 4.3.2 Weakening Mechanism Induced by Molecular Repulsive Forces  
The low surface energy values acquired through AFM on Obourg Chalk (Table 8) could be caused by the possible presence of a very thin layer of water adsorbed onto the surface due to humidity. While for contact angle the Owens-Wendt method allows one to get the intrinsic surface energy of chalk or calcite, which is not dependent on any fluid interaction, the presence of even a very small amount of water may have resulted in a partial saturation of the atomic bonds. This latter scenario could also affect the adhesion force in dry conditions. Indeed, while the contrast in adhesion pressures is perfectly in line with those obtained in the different mechanical tests conducted in dry and wet conditions, the measurements conducted with AFM are very sensitive to humidity, tip radius, and shape, which can cause variations in the measured values. Hence, it would be wise to consider that even in dry conditions, a very thin layer of water exists on the grain surface, and for this reason, the capillary force acting on the probe needs to be subtracted from the measured adhesion force. However, as we do not know if such effects, which depend on the relative humidity in the testing room, really occurred during our measurements, we will discard them in the following. We here compute the adhesion pressure in dry (P\({}_{\text{adh}}\)(dry)) and wet (P\({}_{\text{adh}}\)(wet)) conditions by dividing the forces by the area of a half sphere with a radius corresponding to the probe tip radius R [26]:  
\[\text{P}_{\text{adh}}^{\text{(dry, wet)}}=\frac{\text{F}_{\text{adh}}^{\text {(dry, wet)}}}{2\pi\text{R}^{2}} \tag{7}\]  
Using for R, the values given in Section 2.6.2 after probe calibration, a pressure of 5.06 MPa and 0.99 MPa was obtained in dry and wet conditions respectively, for Obourg Chalk, and for Ciply Chalk, we found 4.79 MPa and 0.84 MPa, respectively  
Regardless of the effect caused by the relative humidity, in all the measurements taken, the adhesion pressure was always higher when the sample was nominally dry with respect to full water saturation in the liquid cell. To go further, the difference \(\text{P}_{\text{adh}}\)(dry) - \(\text{P}_{\text{adh}}\)(wet) between the adhesion pressure in dry and wet conditions can be considered as a repulsive pressure acting between the chalk grains in the water-saturated chalks. This repulsive pressure would contribute to reduce the rock cohesion. Many authors have argued that the water adsorbed onto the grain surface can induce repulsive effects, referred to as disjoining pressure [31; 32; 33; 34]. This pressure may arise from the dipolar nature of the water molecules: if water is adsorbed onto two close layers of the same composition and with the same surface species (with a negative net charge [35; 36]), repulsive effects can arise from this structure (Figure 17a). By adding salts, the ions could replace the water molecules or balance the repulsion in between the two layers, reducing the disjoining pressure (Figure 17b). The ionic strength of the pore fluid is then the parameter that can quantify the amount of weakening [4; 31]. Nevertheless, more complexities could be introduced into the simplified scenario in Figure 17; such as, the calcite surface, which actually has calcium and carbonate ions with partial positive and negative unsaturated charge (thus, not totally negative); or the hydration layers/shell on the dissolved ions, such features could themselves set up repulsion forces between two nearby surfaces [37]. To quantify the effect of repulsive forces, we estimated the value of the repulsive pressure \(\mathrm{P_{adh}}^{\mathrm{(dry)}}-\mathrm{P_{adh}}^{\mathrm{(wet)}}\) and found 4.1 MPa for Obourg Chalk and 3.9 MPa for Ciply Chalk. For Obourg Chalk the repulsive pressure value can be compared to the contrast in cohesion given by the Mohr-Coulomb approach between dry and wet conditions \(\mathrm{S_{0}}^{\mathrm{(dry)}}-\mathrm{S_{0}}^{\mathrm{(sat)}}=0.78\) MPa (Table 7).