Colloid

SEM image of a colloid.

Not milk, but flour suspended in water

A colloid is a special kind of mixture where tiny particles of one substance are spread evenly throughout another substance. These particles are very small, bigger than the ones in a regular solution, but still too small to settle at the bottom like in a suspension. Instead, they stay floating or scattered in the mixture without sinking. The size of these particles is usually between 1 and 1000 nanometers (which is much smaller than anything humans can see). Because of their size, colloids have some unusual properties that make them behave differently than regular mixtures. Colloids can be found everywhere in milk, fog, jelly, whipped cream, and even blood. Scientists study colloids in chemistry, physics, biology, and materials science because they are important in both nature and industry.

Colloids can be made from any combination of gases, liquids, or solids. The tiny particles (called the dispersed phase) are spread throughout another substance (called the dispersion medium). Depending on what phases are involved, there are different types of colloids. For example, aerosols are when liquid or solid particles are mixed into a gas, like fog (tiny water droplets in air) or smoke (tiny solid particles in air). Emulsions are liquids mixed into other liquids, like milk or mayonnaise. Foams are gases trapped in a liquid or solid, like whipped cream or Styrofoam. Sols have solid particles in a liquid, such as in paint or ink. Another type is a gel, which has a solid network spread through a liquid, like jelly or gelatin. What makes colloids interesting is how the tiny particles behave. Their movement and stability are controlled by things like Brownian motion (random jiggling of particles), electrical charges that keep them apart, and surface tension between the different materials, not by the usual weight or size of the materials themselves.

The study of colloids, called colloid chemistry, became more important in the late 1800s and early 1900s. A scientist named Thomas Graham was one of the first to explain the difference between colloids and crystalloids by looking at how they spread or diffuse through materials. Later, scientists developed special tools like the ultramicroscope, which made it possible to actually see the tiny particles in colloids. This helped researchers learn more about how colloids behave. Today, the study of colloids connects many areas of science, like heat and energy (thermodynamics), motion and reaction speed (kinetics), how fluids flow (fluid dynamics), and even nanotechnology. It helps scientists understand important things like how particles stick together or stay apart (coagulation and stability), how they self-organize, and how mixtures flow or change shape (rheology).

Colloids are found everywhere in our daily lives and in many industries. In nature, common examples of colloids include blood, milk, latex (like from rubber trees), and even clouds. Scientists also make special colloids for use in things like paint, makeup, medicine, food, cleaning products, and tiny materials called nanomaterials. In medicine, colloids help with targeted drug delivery (getting medicine exactly where it is needed), medical imaging (like special scans), and healing wounds. In environmental science, tiny colloid particles affect how soil works, how pollution moves, and how water is cleaned. In technology and materials science, colloids are used to make smart gels (that respond to things like heat or light), nanocomposites (strong, tiny materials), and even photonic crystals, which are special materials that can control light.

One way to tell if something is a colloid is by looking at how it interacts with light. Colloids can show a special effect called Tyndall scattering, where the tiny particles inside scatter light, making the mixture look cloudy or even a little shiny or milky. This is different from true solutions like salt water, which usually look completely clear because the particles are too small to scatter light. Colloids do not easily separate or settle out, and that’s because they are stable. Their stability depends on things like particle size, electric charge on the particles, the pH (acidity or basicity) of the mixture, and whether there are helper substances like surfactants or polymers added. These helpers can stop the tiny particles from clumping together. Two main ways particles stay apart are electrostatic repulsion, where the particles have like charges and push each other away, and, steric hindrance, where large molecules or coatings block the particles from coming close. These help keep the colloid mixed and stable over time.