Based on the market research conducted in this study, it was found that commercially available socks are designed for different end use applications (athletics, dress, everyday use, military, outdoor activities, etc.) and are typically made from cotton, polyester, polyamide, or blends of these fibers. Further, the most common fabric structures found in socks are single jersey and terry knit structures. Socks are also made from both filament and spun yarn, and linear density of fibers can vary between socks. In this current study, the influence of these four different textile parameters was investigated by measuring the friction profile of twelve different sock variations. It was found that, three of the four fabric parameters: knit structure, linear density, and yarn type all affect the friction seen at the sock-skin interface. However, it was determined that fiber composition plays no noticeable role in the friction measured across the bottom inside of a sock.

Knit structure was found to have the largest influence with regards to measured dynamic frictional force at the sock-skin interface which is consistent with the study completed by Van Amber et al. [3] which found fabric structure to be the dominate mechanism of friction in socks. In Van Amber’s study, terry knit structures also consistently produced a higher frictional force than the single jersey knit structures. However, both these findings partially oppose the findings from a study completed by Baussan et al. [2]. Baussen’s study found that terry knit socks have a lower coefficient of friction than the single jersey knit socks, and thus lower frictional force, in the direction oriented along the terry knit structure. However, Baussen’s study did find that in the direction against the piles (or terries), the terry knit socks produced a higher coefficient of friction than single jersey knit socks. While this current study used a similar method of friction measurement with cyclic motion to that of Baussen’s study, the findings from this current study still corroborate Van Amber’s findings. One possible explanation for why the terry socks produced the higher frictional force is due to the piles that are protruding from the bottom of the sock. These piles are a characteristic of a terry knit structure and are used in socks to provide cushioning for the wearer to stand on. All piles in the socks were the same size however, when these piles compress it causes an increase in the contact angle between the probe/foot and the sock. Increasing the contact angle between the probe/foot at this interface will increase the amount of resistance it takes to slide along the bottom of the sock. Consequently, this would create an increase in the measured frictional force between the sock and foot. Further, because these terries have a preferred orientation (the positive direction), this would explain the sporadic nature seen in the frictional profile when sliding against the terries (the negative direction).

To our knowledge, no previous study found has examined the influence of linear density on the friction measured at the sock-skin interface. In this study it was found that the 30/1 Ne terry knit socks consistently measured higher frictional forces than the 18/1 Ne socks. The likely cause of this is due to the contact area between the probe/Lorica and the terry knit structure. Since the socks made from the 30/1 Ne fibers have a smaller diameter than the 18/1 Ne socks, there are more piles/loops per inch (pile density) to cover the same area. This in turn would mean that there are more fibers in one area resulting in an increased contact area with the probe or lorica for the terry knit socks. The relationship between friction and contact area for textile structures is known as the Howell-Mazur relationship (eq. 1) and is widely accepted as true when measuring dynamic friction at an interface [16]

Where F is the frictional force, R is the normal force, a is the coefficient of friction (equal to u only when n = 1) and n is the friction index which varies based on material and depends on the materials geometry and its surface roughness. From this relationship we can determine that if pile density is increased, then the geometric contact area is also increased magnifying the effect of measured frictional force at the interface. When examining why the single jersey knit strucutures all have similar friction profiles, one justification would be to assume the lack of piles seen in single jersey knit structure. When there are no piles to increase the potential geometric contact area, the measured friction between the probe and Lorica will be very similar. Even if there are more loops per inch in the 30/1 Ne sock, the increased diameter of the 18/1 Ne sock will counterbalance these. In turn, this means the frictional force values measured in all the single jersey knit structures would be very similar between each sock variation across their entire friction profile which is what was observed across all results.

Yarn type, for terry knit socks, also played a role in the frictional force experience at the sock-skin interface. Against the probe, all the spun terry knit socks measured a higher frictional force than their corresponding filament socks. While against the Lorica, in the positive direction, again, all the spun terry knit socks produced a higher frictional force than their corresponding filament socks. However, against the Lorica in the negative direction the 2/150/34 filament socks measured the higher frictional force. As for single jersey knit samples, it was seen that yarn type may have a small effect on the friction force experienced. In the positive direction, both the spun single jersey knit samples had similar friction profiles, while in the negative direction the spun samples measured a higher frictional force than the filament samples. Yarn type has previously been studied and it is known that fabrics produced from spun yarn typically measure a higher friction than fabrics made from filament yarn. This is due to the short fibers (yarn hairiness) protruding from the spun yarn surface compared to the smooth surface of the filament yarns. On a microscopic level the short fibers protruding from the spun yarn, interact with the opposing surface increasing the resistance to motion thus increasing the frictional force between the two objects. From this experiment, it is unknown why the 2/150/34 filament yarn measured a higher frictional force in the negative direction than the spun yarn. More analysis would need to be completed to understand this deviation.

Fiber composition is (disciplinarily) defined in the textile discipline/industry as the weight percentage of each fiber type that encompasses the textile; this study utilizes socks of 100% Cotton and 100% Polyester fiber composition [17]. As mentioned previously, this study focused on understanding the frictional forces between socks and skin using various sock compositions and the Lorica in-vitro skin simulant only in the dry condition. In the dry state, according to the graphical and statistical analyses conducted, the fiber composition did not significantly affect the frictional force imparted onto the Lorica. This result is supported by existing literature; the study conducted by Van Amber et al. agrees that other factors such as fiber structure, fiber type, and weight had more significant effects on frictional coefficients and values than fiber composition [3]. In addition, because the Coefficient of Friction between polyester and cotton yarn are of similar intensities, frictional force among these two materials will be similar.

Van Amber et al. also suggests that as moisture in the sock-skin interface increases, the frictional force coefficient measured will also significantly change. Hes et al.’s study on how moisture affects friction shows that as the fabric becomes increasingly damp, skin and its underlying layers are increasingly displaced. According to the results of the Hes et al. study, up until the moisture regain value of 40%, increasing moisture in the fabric will increase the friction coefficient measured in the fabric-skin interface. This is because the surface film created at higher moisture levels does not significantly increase the friction coefficient in this environment [18]. This study was conducted solely in a dry environment; thus, the fiber composition did not have a significant effect on frictional force measured between the sock and Lorica skin simulant.