3.2. The Magneto Responsible Adhesion of 2l-MAEFigure 5 gives the relationship between the magnetic field and critical pull-off load when the materials contact a dry acrylic substrate. As shown in Figure 5, when the detach process is without any magnetic fields, the adhesion of 2L-MAE is much stronger than the normal one. Moreover, the pull-off load of 2L-MAE material with a 20% particle volume ratio is as high as 79.5 mN, which is 3.8 times higher than that of normal MAE. The above results fully demonstrate that the dry adhesion property of the material is greatly enhanced by the newt-like microstructure array.Besides, the experimental results also show that the initial adhesion force increased substantially with the CIPs’ volume fraction. We believe that the reason for this phenomenon could be explained by the JKR model [30]; the pull-off load Pc in the JKR model usually can be expressed as follows: where r represents the equivalent contact radius and Δγ is the work of cohesion which is related to the surface energy of those contacted materials.Recent experiments and analytic research [31,32] show that the elastic modulus of MAE E could be divided into two parts: the magneto-induced modulus Em and the pure modulus Ep. When the MAE is without any magnetic field, the MAE could be considered as a typical two-phase composite material; hence the raising of CIPs’ volume fraction would lead to the increase of the MAE’s elastic modulus. Consequently, the lower structure would be generated a larger press-induced deformation when the MAE contacts the substrate (see Figure 6). As a result, the equivalent contact radius r will be increased, and the pull-off load Pc will increase correspondingly according to Equation (1).In addition, different from the adhesion of normal MAE, the adhesive force of 2L-MAE will increase dramatically with an external magnetic field. When the magnetic field increases from 0 mT to 300 mT, the critical pull-off load of 2L-MAE material with a 10% particle volume ratio increases from 49 mN by 165% to 81 mN. Nevertheless, the adhesion of 2L-MAE at 15 vol.% and 20 vol.% increased to 91 mN and 115 mN, respectively, by 152% and 144%. This phenomenon of magnetic field-enhanced adhesion was also found in reference [33]. We believe that this phenomenon also causes by the magneto-induced modulus Em, which could be expressed as [31]: where ε is the average strain of the MAE. and vm represents the magnetic energy intensity. It relates to the particle volume fraction and external magnetic field. Generally, the Em is positively correlated to the particle concentrations and the magnetic intensity. Therefore, The MAE with a higher volume fraction and greater magnetic field would exhibit a stronger adhesion.Figure 7 gives the correlation between critical pull-off load and magnetic field strength of 2L-MAE material on a wet acrylic plate. It can be found that the normal MAE can almost not attach the substrates in the wetting environment. Whereas the 2L-MAE samples still exhibit a strong adhesion. It illustrates that The Newt-inspired 2-level structure can effectively discharge the liquid on the material surface during detaching. Moreover, 2L-MAE samples with different CIPs’ volume fractions exhibit a closed adhesion when not under a magnetic field. It illustrates that the elastic modulus of the material has little effect on the adhesion performance of the 2L-MAE in the wet environment.As shown in Figure 7, the Pull-off load of 2L-MAE material with 10 vol.%, 15 vol.%, and 20 vol.% is increased to 49 mN, 53 mN, and 55 mN, respectively, under the action of 300 mT magnetic field. 2L-MAE materials also exhibit the characteristics that the adhesion force increases with the external magnetic field in the wet adhesion state. As shown in Figure 8, the capillary bridge model (CBM) [34] could explain this phenomenon. According to the CBM model, the critical pull-off load is related to the apparent contact angle (ACA). Furthermore, when the magnetic field is applied to the 2L-MAE, the magnetized particles would interact with each other and cause uniform magneto-induced deformation. Then, the surface roughness of the dome structure would change with the external magnetic fields [35]. Therefore, the ACA of the dome structure would vary with magnetic fields. As a result, the pull-off load is increased.Figure 9 shows changes in the critical pull-off load with the magnetic field when 2L-MAE contacts the iron sheet surface. As displayed in Figure 7, 2L-MAE materials with different volume fractions have similar critical pull-off loads, about 121 mN, in the absence of a magnetic field, which is much larger than normal MAE with a value of only 62 mN. This demonstrates that on the smooth metal surface, the bionic newt microstructure array also makes the adhesion of materials tremendously competitive.Moreover, the surface adhesion of normal MAE samples increased with the external magnetic field. On the contrary, when 2L-MAE materials are subjected to a magnetic field, their critical pull-off load will decrease sharply with the magnetic field. Among them, the adhesive force of 20 vol.% 2L-MAE material decreases the most under the action of 300 mT magnetic field, from about 120 mN to 37 mN, while the critical pull-off load of 10 vol.% and 15 vol.% 2L-MAE will decrease to 51 mN and 43 mN respectively. The mechanism of the decline in adhesion needs to be further studied. However, this behavior matches the experiment result in references [36,37]. They found that MAEs with random surfaces would decrease their friction coefficient when applied to an external magnetic field. Therefore, we have reason to believe that the detachment force between the demo structure and the iron sheet plays a primary role in the pull-off load. As shown in Figure 10, due to the magnetization and interaction of magnetic particles under the action of a magnetic field, non-uniform deformation will be generated on the demo structure’s surface and increase its surface roughness. As a result, hence the equivalent contact area will be further reduced, resulting in the reduction of the adhesive force.