Friction of Wet Skin, Lubrication, and... Probes?
There's plenty of research about the friction of skin, but it all uses test probes. A nice flat test probe isn't exactly a razor's edge, so it's going to be difficult to apply this to shaving. There's a lot in the research, and much of it is confusing and seems contradictory. Forget about answers, I'm having a hard time figuring out the questions.
In Physics 101, F=uN.
where:
F = the friction force (the force that prevent movement), sometimes called drag
u = coefficient of friction
N = normal force, or weight pressing against the surface
When discussing friction, we're usually concerned with kinetic friction, which is the friction once something's already moving. But to initiate that movement, a different, usualy larger, amount of friction (called static friction) has to be overcome. The equation is the same, but the coefficient is different. The different coefficients are responsible for the start-and-stop bouncing effect. The blade slips or bounces over an obstacle, but then gets hung up on another spot where it has to overcome the larger static friction.
For most things, the kinetic coefficient remains constant, regardless of the load or velocity. This is called Amonton's Law, and it holds true for solids with limited elastic properties. Wet skin deviates from Amonton's Law because the coefficient decreases when the load is increased. This is due to the flexible nature of skin. The actual force of friction still increases with heavier load, but it increases slower than linearly because of the change in coefficient.
For skin:
u ~ N^(-1/3) -- where ~ means proportional
Typical coefficients for wet skin are in the range 0.2 - 0.65. The number depends on the condition of skin and location on the body, the material used as a probe, the geometry of the probe, the type of movement (linear vs rotation), and other factors.
Dry skin has less friction than wet skin. Dry skin won't be discussed here except regarding the effects of poor preparation. However, it's interesting to note that the coefficient of friction for dry skin might not vary under load. (i.e. it obeys Amonton's Law.)
Water softens the skin, so it's thought that this allows for greater contact with the probe. Wet skin swells, and becomes plasticized and more pliable. These make the surface of wet skin smoother (geometrically, not in terms of sensation). The smoother surface increases the contact area between skin and probe, which causes increases adhesive forces, which means more fiction. The amount of increased friction (due to the coefficient) seems to depend on the probe material, as well as the level of hydration. The effect of water only lasts for a few minutes, after which the skin returns to normal.
It follows that friction should decrease for dry skin. This is the case, for example, when skin is treated with isopropyl alcohol. Increases in the friction coefficient due to wet skin are much higher than the decreases due to dry skin, compared to normally hydrated skin.
Friction has two components: interfacial (slip, adhesion) and deformation (ploughing, surface roughness, etc.). The irregular surface of dry skin can cause a probe to slip or bounce because it doesn't have as much contact as it would with a smoother surface, such as wet skin. Slipping also occurs for probes on wet skin due to build up of water that gets between the probe and skin, causing the probe to hydroplane over the water. Deformation friction happens when the skin flexes, making it harder for the probe to flow smoothly--it's the energy it takes to move the flexible skin. All of this is similar to a tire going over a road. It turns out that the slip effect, which would lower the overall (bulk) friction, is largely countered by... well, possibly the additional adhesion from the larger surface contact made by the smoother wet skin. A variety of explanations are given.
Many emollients (softeners), moisturizers, and creams have the same effect as water, to increase initial friction, except that the effect lasts for hours. Since wet skin is softer, moisturizers actually increase skin's hydration, and the hydration makes it softer.
As an aside, it follows that many cosmetics products advertised as moisturizers--those that make skin or hair feel smoother--actually reduce the water content. It's natural for people to relate wet to a smooth sensation and less friction. The reality is that skin and hair have less friction when dry, so folks seeking smooth skin and hair really want dry skin or hair. This is particularly true for hair products. The problem works in reverse as well. People think of scaly, rough skin or hair as dry, where in reality, that's what happens when skin or hair gets wet.
The friction from emollients lessens with higher temperature.
Lubricants such as petrolatum, glycerin, and mineral oil, lower the coeficient of friction by 10-30%. The effect lasts for about an hour, after which the friction slowly increases by about 30% and that effect lasts much longer.
The amount of increased or decreased friction follows how greasy the lubricant feels. The greasiest of emollients actually decrease initial friction, even though most emollients will initially increase the friction. Many of these greasy products that lower the initial friction, will cause an increase in friction over time. This is because they prevent water evaporation, so the skin becomes more hydrated over time. Some of these products, through a combination of water and greasy properties, will increase the initial friction slightly, but then lower the friction over time below the initial untreated level.
Most of the tests done on skin lubrication are for purposes of studying cosmetics or normal wear, such as touching skin by the hand or rubbing against clothing. Most of these materials are soft. Hard materials, such as metals, actually have less friction than soft materials. Interestingly, the effect of lubricants is much smaller for a hard, inflexible probe. While lubricants do decrease friction of metal against skin, the effect is nowhere near what it is for soft materials.
It's hard to extend these results to shaving.
The blade is an unusual probe with a very small contact area due to the sharp edge. In addition, the blade is actually a plow, cutting through a certain amount of skin. Fortunately, adequate prep smooths out the skin, lessening the amount of skin removed when shaving. The blade also has to cut through hair, which has to be considered in addition to friction. But the cutting action isn't completely unrelated to friction. When cutting through a hair, the hair will actually push the blade in some direction, sometimes up, sometimes down into the skin. At the very least, this causes the normal force and friction to change constantly.
It's not clear which regime of lubrication is involved in shaving. Skin is soft, so it flexes under the pressure of a probe. This spreads the pressure out over a larger area. It results in elastohydrodynamic lubrication. A liquid boundary layer (of the kind that produces hydroplaning) normally forms only under pressure, with the pressure dictating the thickness of the layer. Since the skin flexes, the contact area increases, the pressure on the fluid is reduced, and the liquid becomes more capable of supporting the load. But it's not clear whether any load bearing lubrication regime applies for a sharp razor blade. Also, shaving cream is an unusual lubricant. It's a "non-newtonian fluid" that becomes less viscous under pressure, so it becomes less capable as a lubricant of supporting a load.
It seems better to think in terms of lubricating for cutting, instead of more traditional part-on-part lubrication. For cutting, the lubricant is often applied to protect the flank that sits against the skin, as well as the rake face that lifts the cut part of the hair, and also the edge that slides through the hair. But shaving isn't exactly planing (shaving is a form of planing), nor is it quite cutting.
There's plenty of research about the friction of skin, but it all uses test probes. A nice flat test probe isn't exactly a razor's edge, so it's going to be difficult to apply this to shaving. There's a lot in the research, and much of it is confusing and seems contradictory. Forget about answers, I'm having a hard time figuring out the questions.
In Physics 101, F=uN.
where:
F = the friction force (the force that prevent movement), sometimes called drag
u = coefficient of friction
N = normal force, or weight pressing against the surface
When discussing friction, we're usually concerned with kinetic friction, which is the friction once something's already moving. But to initiate that movement, a different, usualy larger, amount of friction (called static friction) has to be overcome. The equation is the same, but the coefficient is different. The different coefficients are responsible for the start-and-stop bouncing effect. The blade slips or bounces over an obstacle, but then gets hung up on another spot where it has to overcome the larger static friction.
For most things, the kinetic coefficient remains constant, regardless of the load or velocity. This is called Amonton's Law, and it holds true for solids with limited elastic properties. Wet skin deviates from Amonton's Law because the coefficient decreases when the load is increased. This is due to the flexible nature of skin. The actual force of friction still increases with heavier load, but it increases slower than linearly because of the change in coefficient.
For skin:
u ~ N^(-1/3) -- where ~ means proportional
Typical coefficients for wet skin are in the range 0.2 - 0.65. The number depends on the condition of skin and location on the body, the material used as a probe, the geometry of the probe, the type of movement (linear vs rotation), and other factors.
Dry skin has less friction than wet skin. Dry skin won't be discussed here except regarding the effects of poor preparation. However, it's interesting to note that the coefficient of friction for dry skin might not vary under load. (i.e. it obeys Amonton's Law.)
Water softens the skin, so it's thought that this allows for greater contact with the probe. Wet skin swells, and becomes plasticized and more pliable. These make the surface of wet skin smoother (geometrically, not in terms of sensation). The smoother surface increases the contact area between skin and probe, which causes increases adhesive forces, which means more fiction. The amount of increased friction (due to the coefficient) seems to depend on the probe material, as well as the level of hydration. The effect of water only lasts for a few minutes, after which the skin returns to normal.
It follows that friction should decrease for dry skin. This is the case, for example, when skin is treated with isopropyl alcohol. Increases in the friction coefficient due to wet skin are much higher than the decreases due to dry skin, compared to normally hydrated skin.
Friction has two components: interfacial (slip, adhesion) and deformation (ploughing, surface roughness, etc.). The irregular surface of dry skin can cause a probe to slip or bounce because it doesn't have as much contact as it would with a smoother surface, such as wet skin. Slipping also occurs for probes on wet skin due to build up of water that gets between the probe and skin, causing the probe to hydroplane over the water. Deformation friction happens when the skin flexes, making it harder for the probe to flow smoothly--it's the energy it takes to move the flexible skin. All of this is similar to a tire going over a road. It turns out that the slip effect, which would lower the overall (bulk) friction, is largely countered by... well, possibly the additional adhesion from the larger surface contact made by the smoother wet skin. A variety of explanations are given.
Many emollients (softeners), moisturizers, and creams have the same effect as water, to increase initial friction, except that the effect lasts for hours. Since wet skin is softer, moisturizers actually increase skin's hydration, and the hydration makes it softer.
As an aside, it follows that many cosmetics products advertised as moisturizers--those that make skin or hair feel smoother--actually reduce the water content. It's natural for people to relate wet to a smooth sensation and less friction. The reality is that skin and hair have less friction when dry, so folks seeking smooth skin and hair really want dry skin or hair. This is particularly true for hair products. The problem works in reverse as well. People think of scaly, rough skin or hair as dry, where in reality, that's what happens when skin or hair gets wet.
The friction from emollients lessens with higher temperature.
Lubricants such as petrolatum, glycerin, and mineral oil, lower the coeficient of friction by 10-30%. The effect lasts for about an hour, after which the friction slowly increases by about 30% and that effect lasts much longer.
The amount of increased or decreased friction follows how greasy the lubricant feels. The greasiest of emollients actually decrease initial friction, even though most emollients will initially increase the friction. Many of these greasy products that lower the initial friction, will cause an increase in friction over time. This is because they prevent water evaporation, so the skin becomes more hydrated over time. Some of these products, through a combination of water and greasy properties, will increase the initial friction slightly, but then lower the friction over time below the initial untreated level.
Most of the tests done on skin lubrication are for purposes of studying cosmetics or normal wear, such as touching skin by the hand or rubbing against clothing. Most of these materials are soft. Hard materials, such as metals, actually have less friction than soft materials. Interestingly, the effect of lubricants is much smaller for a hard, inflexible probe. While lubricants do decrease friction of metal against skin, the effect is nowhere near what it is for soft materials.
It's hard to extend these results to shaving.
The blade is an unusual probe with a very small contact area due to the sharp edge. In addition, the blade is actually a plow, cutting through a certain amount of skin. Fortunately, adequate prep smooths out the skin, lessening the amount of skin removed when shaving. The blade also has to cut through hair, which has to be considered in addition to friction. But the cutting action isn't completely unrelated to friction. When cutting through a hair, the hair will actually push the blade in some direction, sometimes up, sometimes down into the skin. At the very least, this causes the normal force and friction to change constantly.
It's not clear which regime of lubrication is involved in shaving. Skin is soft, so it flexes under the pressure of a probe. This spreads the pressure out over a larger area. It results in elastohydrodynamic lubrication. A liquid boundary layer (of the kind that produces hydroplaning) normally forms only under pressure, with the pressure dictating the thickness of the layer. Since the skin flexes, the contact area increases, the pressure on the fluid is reduced, and the liquid becomes more capable of supporting the load. But it's not clear whether any load bearing lubrication regime applies for a sharp razor blade. Also, shaving cream is an unusual lubricant. It's a "non-newtonian fluid" that becomes less viscous under pressure, so it becomes less capable as a lubricant of supporting a load.
It seems better to think in terms of lubricating for cutting, instead of more traditional part-on-part lubrication. For cutting, the lubricant is often applied to protect the flank that sits against the skin, as well as the rake face that lifts the cut part of the hair, and also the edge that slides through the hair. But shaving isn't exactly planing (shaving is a form of planing), nor is it quite cutting.
