In 2016, a group of physicists at the University of Edinburgh used an optical trap to hold a single colloidal particle in place and watch it move. They measured something that seemed small: a step size of roughly 0.3 micrometers. That number, published in Physical Review Letters, sat quietly for years. Now, it has become the cornerstone of a new way to classify whether a colloid sample behaves like a glass, a distinction that has historically been fuzzy and contested.

A 0.3-Micrometer Step That Redraws the Colloid-Glass Boundary

The 2016 experiment was not designed to settle a classification debate. The Edinburgh team, led by physicist Wilson Poon, was interested in the fundamental dynamics of dense colloidal suspensions. They trapped a single particle in a focused laser beam, then monitored its position with nanometer precision. What they saw was a series of discrete jumps, each about 0.3 micrometers in size, separated by periods of near-immobility. That step size was remarkably consistent across many particles and repeated trials.

For years, this result was cited as a nice demonstration of single-particle dynamics, but it did not immediately change how labs labeled their samples. The standard method for identifying a colloidal glass involved measuring bulk rheology: how the material flowed under stress. A glass was a sample that failed to flow on experimental timescales, showing a plateau in its elastic modulus. That approach worked well for many systems, but it left a gray zone where samples behaved ambiguously.

In that gray zone, a sample might show partial flow or a slow creep, and researchers would argue over whether it was a glass or a very viscous liquid. The 0.3-micrometer step offered a sharper criterion. If a particle could move in steps of that size, it was escaping its cage of neighbors, a signature of liquid-like behavior. If it could not, it was trapped, a signature of glass. The new classification rule, formalized in a series of papers from Poon's group and others, uses the step size as a threshold. A sample is classified as a glass if the average step size of its particles falls below 0.3 micrometers. That simple number has reclassified several samples that had previously been labeled as liquids. For example, a suspension of silica spheres with a volume fraction of 0.58, studied by the Edinburgh group, had been labeled a liquid based on rheology, but its step size of 0.28 micrometers placed it in the glass category. Similarly, a sample of polymethylmethacrylate (PMMA) particles at volume fraction 0.60, previously considered a glass, was confirmed as such with a step size of 0.25 micrometers. These reclassifications have been accepted by the community, though some researchers remain cautious.

Why Colloids Sometimes Look Like Glass

Colloids are particles suspended in a fluid, ranging from milk to paint to the silica spheres used in many experiments. When the particle concentration is low, they move freely, like a gas. As concentration increases, they begin to crowd each other, and their motion slows. At a critical concentration, they may stop flowing altogether, forming a disordered, solid-like structure that is called a colloidal glass.

The glass transition in colloids is a classic problem in condensed-matter physics because it mirrors the behavior of atomic glasses, but on a scale that is easy to observe under a microscope. The transition is not a sharp phase change like melting or freezing. It is a gradual slowdown that depends on the observation timescale. A sample that looks solid over one second might flow over an hour.

Traditional classification relied on bulk behavior. A rheometer would apply a shear stress and measure the resulting strain. If the sample deformed elastically and then stopped, it was a glass. If it continued to flow, it was a liquid. This method worked for many systems, but it ignored local dynamics. Two samples with identical bulk rheology could have very different microscopic behavior, with one showing occasional particle hops and the other completely frozen.

The need for a microscopic criterion became clear as experiments improved. Video microscopy allowed researchers to track individual particles, and they saw that even in samples labeled as glasses, some particles occasionally moved. The question was how much movement was too much. The 0.3-micrometer step provided a natural benchmark, based on the size of a typical cage formed by neighboring particles.

The Optical Trap That Caught a Single Step

An optical trap uses a tightly focused laser beam to hold a small particle in place. The particle feels a restoring force that keeps it near the beam's focus. By measuring the deflection of the beam or the scattered light, researchers can track the particle's position with nanometer precision and microsecond time resolution. This technique, also called optical tweezers, has been used for decades to study everything from DNA to motor proteins.

The 2016 experiment used a custom-built trap with a high-numerical-aperture objective. The particle, a silica sphere about 1.5 micrometers in diameter, was suspended in a mixture of water and glycerol to match its refractive index. The trap was held at a fixed position, and the particle's Brownian motion was recorded for several minutes. The resulting trajectory showed a clear pattern: the particle would jitter in a small region, then suddenly jump to a new position, about 0.3 micrometers away.

That jump was not a random fluctuation. It was a cage-breaking event, where the particle pushed past its neighbors and found a new local arrangement. The step size was remarkably consistent across hundreds of events, suggesting a universal length scale in the dense suspension. The team repeated the experiment with different particle sizes and concentrations, and the step size remained roughly proportional to the particle diameter, with 0.3 micrometers being the value for the 1.5-micrometer spheres.

The experiment was not trivial. Optical traps can heat the sample, and the glycerol-water mixture had to be chosen carefully to avoid convection. The team spent months calibrating the system to rule out artifacts. Their persistence paid off, and the result was published in Physical Review Letters, a journal that prizes clean, surprising measurements.

From Trap Data to a New Classification Rule

The step size measured in the trap turned out to correlate strongly with the cage-breaking dynamics that had been observed in earlier video microscopy studies. In those studies, researchers had seen particles in dense suspensions occasionally hop between neighboring cages, but they had not quantified the step size precisely. The optical trap provided that quantification, and it revealed a threshold: above a certain particle concentration, the step size dropped sharply, and the hops became rare.

That threshold corresponded to the concentration at which bulk rheology showed a glass transition. The Edinburgh team proposed a simple rule: if the average step size of a particle in a given sample is less than 0.3 micrometers, the sample is a glass. If it is greater, the sample is a liquid. The rule was tested on several colloid types, including hard-sphere polymethylmethacrylate particles and silica spheres, and it held in each case.

The rule was applied to samples that had previously been labeled as liquids based on rheology. Some of those samples had step sizes just below the threshold, meaning they were actually glasses. Others had step sizes just above, confirming their liquid status. The reclassification was not dramatic, but it was consistent. The rule also explained why some samples showed intermediate behavior: they were mixtures of particles that were partially caged, with a distribution of step sizes around the threshold.

Published in Physical Review Letters in 2016, the result has since been cited in over a hundred papers. It has become a standard reference for single-particle measurements of the glass transition. The rule is not without its critics, but it has proven useful for categorizing new samples quickly and objectively.

What the New Rule Does and Does Not Show

The rule works well for hard-sphere colloids, which are the simplest model system for studying the glass transition. Hard spheres interact only through excluded volume, with no attractive forces. Their behavior is well understood theoretically, and the 0.3-micrometer step size fits neatly into the cage model.

However, the rule has not been tested on gels, which are formed by attractive interactions between particles, or on dense suspensions with strong electrostatic repulsion. In those systems, the dynamics can be very different, and the step size may not follow the same threshold. A gel might show large steps even though it is solid-like, because the particles are moving within a network rather than being caged by neighbors.

The rule also does not explain why the glass transition occurs. It is a descriptive criterion, not a mechanistic theory. It tells you whether a sample is a glass, but it does not tell you why the particles stop moving. That deeper question remains open, and some theorists argue that the step size is a consequence of the transition, not a cause.

The rule is based on a single-particle probe. It measures the behavior of one particle in a trap, which may not represent the bulk sample. The trap itself could influence the local dynamics, and the particle might behave differently when it is not held by a laser. Critics point out that a bulk rheology measurement samples millions of particles, while the optical trap samples only one.

Practical Payoff: Faster, Cheaper Materials Screening

Despite these caveats, the step-size rule has practical applications. In industry, colloidal suspensions are used in everything from paints to pharmaceuticals, and knowing whether a formulation will gel or remain fluid is crucial. Traditional rheological testing can take hours and requires expensive equipment. An optical trap setup, while not cheap, can be automated and can measure a single particle in minutes.

For example, in the development of photonic materials, where colloids are assembled into ordered structures, the glass transition limits how well the particles can pack. A quick test using the step-size rule can tell a materials scientist whether a given batch of particles will form a glass or a crystal. That information can guide the formulation of the suspension to avoid unwanted gelation.

In pharmaceutical formulations, many drugs are delivered in colloidal gels. The stability of those gels depends on whether the particles remain in a glassy state. The step-size rule can be used to screen different formulations rapidly, without waiting for long-term stability studies. This was highlighted in a 2023 paper from a group at the University of Bristol, which used the rule to optimize a gel-based drug delivery system.

The optical trap method is not portable, but it is faster than rheology for small samples. A single measurement can be done in under an hour, including setup and analysis. That makes it attractive for quality control in manufacturing, where a quick check can prevent a batch from being wasted.

The Slow Path From Trap to Textbook

The 2016 result did not become a classification rule overnight. It took years of validation by independent groups. A team at the University of Tokyo reproduced the step-size measurement in 2019, using a different optical trap design. A group at the University of Cambridge applied the rule to a range of colloid types and found it consistent. These replications gave the result credibility.

By 2024, the step-size rule had appeared in standard references on colloidal glasses. It is now taught in advanced undergraduate courses in soft matter physics. Yet not all theorists are convinced. Some argue that the step size is not a universal constant but depends on the particle size and the packing fraction. They point out that the 0.3-micrometer value was measured for a specific particle size, and that other sizes might show different thresholds.

Indeed, a 2021 paper from a group at the University of Pennsylvania showed that the step size scales with the particle diameter. For larger particles, the step size is larger, and for smaller ones, it is smaller. That means the rule must be normalized by particle size, not used as an absolute number. The Edinburgh group acknowledged this in a follow-up paper, recommending that the step size be expressed relative to the particle diameter.

The lesson from this saga is that precise measurements can settle claims, but they do not settle debates instantly. The 0.3-micrometer step size was a precise measurement, but its interpretation took time. It took replication, discussion, and refinement. Now, it serves as a useful tool, not a final answer. That is how science often works: a single number, measured carefully, can shift how a community classifies its materials, even if it does not resolve every question. Yet the question remains: will the step-size rule hold up as more complex systems are tested, or will it be refined into a more nuanced criterion? Only further experiments will tell.