Research On Friction Characteristics Between Camouflage Clothing And Human Elbow Skin3

1,The Surface Characteristics of Camouflage Clothing Fabrics

        Figures 2(a~e) show surface morphology images of four types of camouflage clothing fabrics and one type of camouflage clothing underwear. It can be observed that Fabrics 1" and 5" appear relatively loose due to their twill and loop knitting structures, with larger pores. Conversely, Fabrics 2, 3, and 4" exhibit relatively smooth and dense surfaces due to their plain knitting structures, with smaller pores.

         The surface hydrophilicity/hydrophobicity is one of the important factors influencing the frictional properties of the skin. Measuring the hydrophobic/hydrophilic properties of camouflage clothing fabrics helps to understand the tribological behavior between different camouflage clothing fabrics and the skin. Due to differences in the fabric knitting structure, there is a significant difference in hydrophilicity between twill and plain structures. For twill and loop fabrics with strong water absorption (completely absorbed within 2 seconds), the hydrophobicity is characterized by the time the water droplet stays on the surface, where a shorter time indicates stronger water absorption. As for plain fabrics, the contact angle after a 2-second test is measured, where a larger contact angle indicates a smaller surface energy, making the material more hydrophobic. Figure 3 shows contact angle measurement photos of camouflage fabrics 2", 3", and 4".

         Table 2 lists the results of the contact angle tests, showing that Fabric 2" exhibits the strongest hydrophobicity, followed by Fabric 4". The disappearance time of water droplets on the surfaces of Fabrics 1" and 5" is listed in Table 3, indicating that Fabric 5" completely absorbs the water droplet in a shorter time, demonstrating better hydrophilicity than Fabric 1". Overall, in terms of comparison, the hydrophilicity of the fabrics ranks as follows: 5" > 1" > 3" > 4" > 2", indicating that fabrics made of pure cotton and with a twill knitting structure exhibit better hydrophilicity.

2,Frictional Behavior between Camouflage Clothing Fabrics and Skin

          To investigate the adhesive and sliding behavior between camouflage clothing fabrics and skin, Figure 4 illustrates typical friction coefficient curves and friction force displacement (F-D) curves of Fabric 1" with skin under normal loads of 1 N and 5 N, in both dry and sweaty conditions. Based on our previous research results, skin under different reciprocating sliding friction conditions exhibits three types of F-D curves: parallelogram, circular, and semi-closed shapes. According to the different shapes of the F-D curves, the friction behavior of the skin can be classified into relative sliding state, intermediate state where relative sliding and adhesion coexist, and adhesive state.

         As shown in Figure 4, under a normal load of 1 N and in a dry state, the static friction coefficient and static friction force are small, indicating relative sliding between the skin and the fabric during friction. After adding artificial sweat, interlayer forces form between the skin and the fabric due to the presence of sweat, leading to adhesion between the skin and the fabric. Initially, they are in an adhesive state. As the displacement increases, the skin wrinkles and accumulates in front of the spherical probe, increasing the surface tension of the skin. When the sum of the pushing force and surface tension exceeds the static friction force, relative sliding occurs between the skin and the camouflage clothing fabric. Therefore, after adding artificial sweat, the skin initially adheres to the camouflage clothing fabric and then slides.

        When the normal load is 5 N, due to the higher load, the adhesive force between the skin and the fabric increases. The interface between the camouflage clothing fabric and the skin is in an intermediate state of relative sliding and adhesion in both dry and wet environments, with a reduced sliding distance compared to when the normal load is 1 N.

         Figure 5 illustrates the distribution of average friction coefficients between different camouflage clothing fabrics and skin. Under a normal load of 1 N, all fabrics exhibit higher friction coefficients in sweaty conditions compared to dry conditions. This is because the addition of artificial sweat increases the hydration of the skin, leading to an increase in adhesion between the camouflage clothing fabric and the skin.

         As a result, the frictional resistance that needs to be overcome increases, leading to an increase in the friction coefficient. This is consistent with the findings of Naik et al. When the normal load increases to 5 N, there is no significant difference in the average friction coefficients between different fabrics and skin under dry and wet conditions. This is because at higher normal loads, the relative motion between the skin and the fabric requires overcoming the frictional component of skin elastic deformation.

        At the same time, the adhesive force between the skin and the fabric also increases. However, the increment of the frictional component due to skin elastic deformation is much greater than the increment of the adhesive force between the skin and the fabric. Therefore, the material and structure of the fabric, as well as the dry and wet conditions, appear to have less importance in influencing the frictional resistance. Consequently, the differences in frictional performance between different fabrics and skin under dry and wet conditions are not significant.

          Under a normal load of 1 N and in dry conditions, the average friction coefficients are as follows: 3" > 2" > 1" > 5" > 4". These results are related to the material and knitting structure of the fabrics. Analyzing from the perspective of material, Fabrics 1" and 5" are made of 100% cotton, while Fabric 4" is a blend of 50% nylon and 50% cotton. These materials are softer compared to fabrics with polyester (2") and viscose (3"), resulting in lower friction coefficients with the skin. From the perspective of knitting structure, plain structures (2", 3") have more intersections and shorter floats, leading to larger actual contact areas with the skin, hence higher average friction coefficients compared to twill structures (1", 4") and loop structures (5").

         Additionally, the presence of long micro-convex fiber structures on the fabric surfaces also influences the frictional performance. Fabrics 1", 3", and 5" have long micro-convex fibers on their surfaces, which increase resistance during friction. On the other hand, Fabrics 2" and 4" have smoother surfaces. Therefore, Fabric 2" has slightly lower friction coefficients compared to Fabric 3", and Fabric 4" has slightly lower friction coefficients compared to Fabrics 1" and 5".

        Under a normal load of 1 N and in wet conditions, the average friction coefficients increase significantly compared to dry conditions. Comparing the average friction coefficients of different fabrics under wet conditions, they are as follows: 2" > 4" > 3" > 1" > 5". These results are consistent with the hydrophilicity of the fabrics. The more hydrophobic the fabric, the more water it repels, resulting in stronger hydration of the skin and greater adhesion at the interface between the camouflage clothing fabric and the skin, thus leading to higher friction coefficients. This is because skin hydration softens the stratum corneum and forms capillary bridges, increasing the actual contact area and adhesion [3-14]. In actual military operations and training, sweating is inevitable, and most of the time the skin is in contact with sweat.

Therefore, when selecting camouflage clothing that comes into contact with the skin, it is best to choose fabrics with strong moisture absorption (hydrophilicity), such as cotton, and fabrics with twill or loop knitting structures. This helps to reduce friction with the human skin.

3,Skin Surface Morphology

          To better observe the skin surface damage caused by friction with different camouflage clothing fabrics, Table 4 lists the changes in surface roughness before and after skin friction. Compared to the control group skin, the surface roughness slightly increases after skin friction under a normal load of 1 N in both dry and wet conditions. However, the increase is small, which is due to the lower load resulting in less frictional force and less peeling of the skin surface stratum corneum, leading to minimal changes in skin surface roughness.

        However, under a normal load of 5 N in both dry and wet conditions, the changes in skin surface roughness before and after friction are significant. The order of increase in skin surface roughness in both dry and wet conditions is as follows: 3 > 2 > 1 > 4.

         To gain a more intuitive and detailed understanding of the changes in skin surface morphology, Figure 6 illustrates 3D images of the skin in friction with different camouflage clothing fabrics under a normal load of 5 N and in dry conditions.

        From Figure 6, it can be observed that the surface of the control group skin (without friction) is relatively smooth (Figure 6(a)), showing distinct texture structures with minimal surface roughness.

       However, after friction with Fabrics 2 (Figure 6(c)) and 3 (Figure 6(d)), the skin surface becomes uneven with some textures disappearing, resulting in increased surface roughness. This is attributed to the long protruding fibers of Fabrics 2 and 3, which have a plain weave structure and a relatively hard texture, resulting in higher friction coefficients during friction. Consequently, they are prone to scratching the skin, causing separation of the stratum corneum and epidermal tearing, leading to the leakage of tissue fluid.

       In contrast, after friction with Fabrics 1, 4, and 5, the skin surface remains relatively smooth, with minimal disruption to the texture structure and less trauma to the skin. Despite no significant difference in the average friction coefficients between fabrics and skin under dry and wet conditions when the normal load increases to 5 N, the material and structure of the fabrics still influence the extent of skin surface damage.

       Therefore, in the selection of camouflage clothing, it is advisable to choose fabrics that are soft in texture, such as cotton, and fabrics with a twill knitting structure to reduce frictional damage to the skin. Additionally, efforts should be made to minimize the contact load between the skin and the fabric, thereby reducing the frictional force on the surface and mitigating the shear effect between the epidermal layers, thereby lowering the risk of epidermal tearing.

Conclusion:
          a. Under a normal load of 1 N, the wet environment enhances the adhesion between fabrics and skin, resulting in higher friction coefficients between fabrics and skin in the wet state compared to the dry state. When the normal load increases to 5 N, the dry/wet state of the interface has no significant effect on the friction coefficients between fabrics and skin.

          b. In the dry state, the frictional performance between different camouflage clothing fabrics and skin is related to the material and knitting structure of the fabrics. Fabrics made of pure cotton and with a twill structure exhibit lower friction coefficients with the skin. In the wet state, the frictional performance between different camouflage clothing fabrics and skin is related to the hydrophilicity of the fabrics. Fabrics with stronger hydrophobicity increase the friction coefficients between fabrics and skin due to enhanced skin hydration and interfacial adhesion.

          c. Fabrics with a relatively hard plain weave structure are prone to causing rough skin surfaces after friction, resulting in varying degrees of stratum corneum separation and epidermal tearing, thereby increasing the risk of skin surface damage.