### 1.3 Mechanical friction in the modern world

Delving into history books, we can learn that already thousands of years ago the Egyptians realized that lubricants like water and oil helped reduce the friction of sledges that moved the heavy stones in construction of the pyramids.
Leonardo da Vinci (1452-1519) was the first to study sliding friction systematically. To move a block of wood across a table it is necessary to pull it. The bigger the block, the more you need to pull. When he examined this deceptively simple relationship systematically more than 500 years ago, he discovered the basic laws of sliding friction.
Already in 1493, da Vince understood two fundamentals of friction: He observed that the force of friction acting between two sliding surfaces is proportional to the load pressing the surfaces together; and that friction is independent of the apparent area of contact between the two surfaces (see: Wear, "Leonardo da Vinci's studies of friction").
Today, these laws of friction are credited to French physicist Guillaume Amontons (1663-1705), who formulated them 200 years after da Vinci.
Similar observations were made by French physicist Charles-Augustin de Coulomb (1736-1806), best known for developing what is now called Coulomb's law, the description of the electrostatic force of attraction and repulsion.
The first reliable test on frictional wear was carried out by British chemist Charles Hatchett (1765-1847), using a simple reciprocating machine to evaluate wear on gold coins. He found that coins with grits between them wore at a faster rate than those without.
The development of this branch of science – today called tribology – is the science and technology of friction, lubrication, and wear, derived from the Greek word tribo meaning I rub.
Tribology applies to all length scales. The relatively new field of nanotribology extends the study of friction and wear processes down to the nanometer scale.
The field of biotribology aims to explore friction, adhesion, lubrication and wear of biological friction systems (as we've seen with above examples of friction designs in Nature) and to apply this bioinspired knowledge to innovate technology.
And yet, given all these scientific explorations conducted over the past 500 years, the precise mechanisms underlying friction at the nanoscale are still not completely understood. As it turns out, there appears to be no universal model suitable to explain friction in all the different materials and length scales that researchers experimented with.
The reason for this is that the macroscopic behavior of friction is the result of many microscopic interactions acting at different scales of the touching surfaces.
Friction is not just a physical phenomenon, it also causes huge economic costs: Friction is the primary source of energy dissipation (i.e. the loss of energy that results from friction, usually in the form of heat pollution), which represents wasted energy. When a motor runs hot, that's friction at work.
Friction is responsible for about 20-30 percent of the world energy consumption! Lubrication reduces the effects of friction but, of course, lubricants are a cost factor, too.
Another significant cost factor of friction is material deterioration through wear and tear, which causes progressive damage between working machine parts.
The cost of undesirable (i.e. too much) friction in modern society is truly staggering. Take automobiles: a whopping one-third of the energy of gasoline is used to overcome friction in their engine, transmission, tires, and brakes.
On average, a single passenger car uses 340 liters of fuel per year just to overcome friction. And that car hasn't moved an inch yet. If you combine the effects of friction from all the cars in the world, more than 200 billion (200,000,000,000) liters of fuel (gasoline and diesel) is used just to overcome this friction (see: Tribology International, "Global energy consumption due to friction in passenger cars").
Another example is the huge amount of fuel used by the approximately 90,000 ocean-going cargo ships that roam the seas (international shipping uses about 300 million metric tons of fuel and it is estimated it is responsible for 3.5% to 4% of all climate change emissions). Most of the energy in shipping is used to overcome surface friction in the water. Therefore an effective way to reduce frictional drag underwater could significantly reduce marine fuel consumption, thus making the shipping industry more efficient and environmental friendly.

### 1.4 The role of lubricants

We use lubricants to reduce friction in machines with moving parts and with it reduce abrasion and the waste of energy. A lubricant is a substance that is introduced between two moving surfaces to reduce friction and reduce wear.
Take water. Water is a good lubricant as witnessed by the "Wet Floor" signs that warn you to be extra careful. One of the reasons water isn't used as a lubricant in machinery is that it evaporates quickly.
A lubricant may also serve the function of dissolving or transporting foreign particles, carrying away contaminations and debris, preventing corrosion or rust, sealing clearances, and dissipating heat.
In general, there are three categories of lubricants: liquid, solid (dry), and gaseous.
Nearly all lubricants used in the automotive (engine oils and transmission fluids) and manufacturing (hydraulic fluids and gear oils) industries are oil or grease based.
Solid lubricants include grease and powders. Some of the commonly known powder lubricant materials are graphite, molybdenum disulphide (MoS2), tungsten disulphide (WS2), and titanium dioxide (TiO2), some in nanoscale form. Solid lubricants offer lubrication at temperatures (up to 350°C) that are higher than many liquid and oil-based lubricants can manage. Solid lubricants such as Teflon are typically used as a coating layer to provide a non-stick surface.
Gaseous lubricants have a much lower viscosity (thickness) than liquid or solid lubricants. They also exhibit lower heat capacity and higher compressibility than liquid or solid lubricants. Some examples for gaseous lubricants are air, technical gases, steam or liquidñmetal vapors.
During millions of years of evolution, Nature has produced lubricant systems with water as a base stock and biomolecules as additives. Generally, these natural lubricants by far outclass the best oil-based lubricants of most man-made devices.
Even today, researchers are far from being able to replicate Nature's lubrication recipes.
Science notes: friction basics
Key takeaways
• There are four types of friction: static, sliding, rolling, and fluid.
• Friction is not just a physical phenomenon, it also causes huge economic cost and is a major use of fossil fuels.
• Evolutionary processes have led to various designs used by animals and plants to increase or reduce friction.
• Nanotribology extends the study of friction and wear processes down to the nanoscale.
Vocabulary
Adhesion: When the friction between two surfaces is so strong that it provides a bonding force.
Tribology: The science and technology of friction, wear, lubrication, and the design of bearings; the science of interacting surfaces in relative motion.
Kinetic friction: Also known as sliding friction or moving friction, this is the amount of opposing force between two objects that are moving relative to each other.
Static friction acts on objects when they are resting on a surface.
Sliding friction acts on objects when they are sliding over a surface.
Rolling friction acts on objects when they are rolling over a surface.
Fluid friction acts on objects that are moving through a fluid.
Friction coefficient: The ratio between friction and applied load.