The search for the “slipperiest thing in the world” leads directly into the scientific study of friction, a field known as tribology. Slipperiness is quantified using precise physical measurements rather than common perceptions of smoothness. Understanding which materials hold the record for lowest friction requires defining how scientists quantify this property, examining the champion materials, and exploring the fundamental physics that allow for near-zero resistance.
How Scientists Measure Slipperiness
Slipperiness is the inverse of friction, the force that resists the relative motion of two surfaces in contact. Scientists quantify this resistance using a dimensionless value called the Coefficient of Friction (COF). The COF is mathematically defined as the ratio of the force required to slide one object across another to the force pressing the two surfaces together.
A lower COF value indicates less friction and greater slipperiness. Most common dry materials have a COF between 0.3 and 0.6. A high value like 1.0 means the frictional force equals the weight pressing the objects together. A value approaching zero signifies almost no friction, a state known as superlubricity. The COF is measured using a specialized instrument called a tribometer, which determines both the static COF (to initiate motion) and the kinetic COF (to maintain motion).
The Materials That Hold the Record
For decades, the most recognized contender for the slipperiest solid material has been Polytetrafluoroethylene (PTFE), commonly known by the brand name Teflon. This synthetic fluoropolymer exhibits a kinetic COF ranging from 0.04 to 0.1, making it one of the lowest-friction solid materials known. PTFE’s molecular structure consists of a carbon chain completely shielded by fluorine atoms. This structure gives it an extremely low surface energy, which minimizes the attractive forces between its surface and other materials.
Ice is another substance known for its slipperiness, though its mechanism is different and its COF is variable. Ice on ice, or ice against a material like steel, can achieve a COF as low as 0.01 to 0.05, particularly around an optimal temperature of -7°C. The low friction is attributed to a microscopic, semi-liquid layer of water on the surface, which acts as a lubricant. This water layer is a result of the ice surface structure and is often enhanced by frictional heating or pressure from an object sliding across it.
In controlled laboratory environments, certain engineered systems achieve a state of superlubricity, where the COF drops below 0.01 and approaches zero. Specific carbon-based materials like tetrahedral amorphous carbon, or two-dimensional materials like graphene and molybdenum disulfide, have demonstrated ultra-low friction coefficients, sometimes as low as 0.003 under ideal conditions. Biological lubricants, such as hyaluronic acid found in human joints, can also achieve a COF near 0.003.
The Physics Behind Ultra-Low Friction
The ability of these materials to exhibit ultra-low friction is rooted in their unique molecular and atomic interactions, primarily by minimizing adhesive forces. Friction is not just about surfaces being rough; a large component comes from the adhesion, or attractive forces, between molecules of the two surfaces in contact. Materials like PTFE are non-polar and chemically inert, meaning they resist the weak van der Waals forces that cause surfaces to stick together.
The pursuit of superlubricity focuses on eliminating the atomic-scale resistance to sliding. One primary mechanism is structural superlubricity, which occurs when two crystalline surfaces slide against each other in an “incommensurate” contact. In this state, the atomic lattices are misaligned and cannot lock into each other’s atomic “valleys,” allowing them to glide with virtually no resistance. This is often observed in layered materials like graphite or misaligned graphene sheets.
Another path to ultra-low friction involves generating a separating layer, as seen with ice or certain liquid lubricants. In these cases, a thin film of molecules prevents solid-to-solid contact, replacing the high shear resistance of the solids with the much lower shear resistance of the liquid or semi-liquid layer. This boundary lubrication allows engineered systems like ceramic bearings or water-lubricated surfaces to achieve a COF as low as 0.002.