In this paper, the amphoteric character of cartilage surface is used to study friction between charged (+/+), (-/-) and uncharged (±/±) biological surfaces. The surface of the bovine articular cartilage at varying pH solutions is either charged positively or negatively. When examining the coefficient of friction of the (cartilage/cartilage) pair, depending on the pH, we observe surfaces charged positively, negatively and without charge. The uncharged surface refers to the isoelectric point (IEP) and shows greater friction than the charged surfaces of the cartilage. The measurement of surface energy versus pH of the spherical lipid bilayer will reveal the amphoteric nature of the membrane surface. The friction coefficient of the (cartilage/cartilage) pair and the surface energy of the spherical lipid bilayer versus pH are expressed by a bell-shaped curve. This is an interesting case that confirms the fact that the phospholipid bi-layer is embedded on the surface cartilage. This experimental confirmation of negative surface cartilage is insufficiently highlighted in the literature on natural lubrication.

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