When a person describes a liquid as “thin,” they are typically comparing how easily it pours, such as the difference between water and thick molasses. This resistance to flow has a precise scientific name: viscosity. Viscosity is a fundamental property of fluids that dictates their behavior and is widely measured in science and industry. A liquid’s thinness is directly related to having low viscosity. The search for the thinnest liquid takes us from everyday observations into the extreme cold of quantum physics, where matter can exhibit unusual behavior.
Defining Fluid Thinness
Viscosity is a measure of a fluid’s internal friction, representing its resistance to being deformed or sheared by stress. Fluids with high viscosity, like honey or motor oil, flow slowly because their molecules create a drag force on one another. Conversely, low-viscosity fluids, such as rubbing alcohol or water, flow quickly and easily because this internal friction is minimal.
This property is quantified using scientific units. The standard SI unit for dynamic viscosity is the pascal-second (Pa·s). A common older unit is the centipoise (cP). Dynamic viscosity measures the force required to make a fluid flow at a certain rate and is the direct metric for determining a liquid’s thinness.
Dynamic viscosity is often distinguished from kinematic viscosity, which is dynamic viscosity divided by the fluid’s density. Kinematic viscosity is relevant when the fluid’s flow is driven solely by gravity, such as in a simple pour test. However, dynamic viscosity is the more fundamental property, as it directly describes the molecular interactions and internal resistance within the fluid itself. For example, water at room temperature has a dynamic viscosity of approximately 0.001 Pa·s, or 1 centipoise.
The Role of Temperature and Molecular Forces
The viscosity of any conventional liquid is determined by two main factors: the strength of cohesive forces between its molecules and the fluid’s temperature. Intermolecular forces, such as hydrogen bonds or van der Waals forces, cause molecules to “stick” together. This molecular cohesion creates the internal friction that resists flow, meaning liquids with stronger intermolecular forces tend to be more viscous.
Temperature has an inverse effect on the viscosity of most liquids. As a liquid is heated, the thermal energy of its molecules increases, causing them to move faster and further apart. This increased kinetic energy helps molecules overcome the attractive cohesive forces, reducing internal friction and allowing the liquid to flow more easily. For instance, the viscosity of water decreases by more than half when heated from room temperature to nearly boiling.
The path to lower viscosity in normal liquids is achieved by reducing intermolecular forces and increasing temperature. However, this contrasts with gases, whose viscosity increases with rising temperature because higher kinetic energy leads to more frequent molecular collisions.
Identifying the Thinnest Liquid
To find the thinnest liquid possible, one must seek a state of matter where internal friction is completely absent, a condition known as zero viscosity. This state is not found in conventional liquids but in a quantum phenomenon called superfluidity. The liquid that holds the record for the lowest viscosity is Helium-4 (\(\text{}^4\text{He}\)), which becomes a superfluid when cooled to extremely low temperatures.
This transformation occurs at a precise point known as the Lambda Point, which is 2.17 Kelvin (K), or about -270.98 degrees Celsius. Below this temperature, liquid helium-4 undergoes a phase transition into a liquid with bizarre properties, designated as Helium II. The transition to this superfluid state is a direct consequence of quantum mechanics becoming visible on a macroscopic scale.
The atoms of Helium-4 are bosons, meaning they can all occupy the same quantum state. When the liquid is cooled sufficiently, these atoms transition into the lowest available energy state, forming what is closely related to a Bose-Einstein Condensate. This collective state of matter allows the fluid to flow without any mechanism for energy dissipation, giving it a dynamic viscosity of absolute zero. While Helium-4 is the most famous example, the rare isotope Helium-3 (\(\text{}^3\text{He}\)) can also become a superfluid, but it requires much colder temperatures, around 2.5 millikelvin (mK).
Characteristics of Zero Viscosity
The complete absence of viscosity in superfluid helium results in several physical behaviors that defy classical fluid dynamics. One of the most counterintuitive is the ability of the liquid to flow without friction, meaning any swirling motion, or vortex, created in the liquid will continue to rotate indefinitely. The superfluid component can also effortlessly pass through microscopic channels and pores that would completely block a normal liquid.
The Fountain Effect
Superfluid helium also exhibits the “fountain effect,” where the liquid flows upward against gravity when heat is applied. If a small tube with a porous plug is submerged in the superfluid and a light source warms the liquid inside the tube, the superfluid rushes through the plug and shoots out the top like a fountain. This effect is driven by the superfluid’s infinite thermal conductivity, which creates a pressure difference between the warmer and cooler parts of the liquid.
The Rollin Film
Another phenomenon is the Rollin film, a thin, mobile film of superfluid that is only a few atoms thick and coats all surfaces of its container. Because it has zero viscosity, this film will creep up and over the rim of the container, causing the superfluid to self-siphon until the liquid levels inside and outside the container are equal. These unique characteristics make superfluid helium a tool for studying the fundamental laws of quantum mechanics and have practical applications in the cooling systems of high-precision scientific instruments.