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Microscopic Image-Based Particle Analysis: Particle Size and Morphology Made Clear at a Glance

In industries such as pharmaceuticals, battery materials, cement, pigments, abrasives, geology, and environmental dust, 'particle size' often directly determines product quality and performance. Accurately measuring and understanding a particle's size distribution and morphology is the basic homework of quality control. This article systematically introduces common methods of particle-size analysis, and focuses on the principles, workflow, and metrics of the microscopic image method.

1. What Is Particle-Size Analysis

Particle-size analysis is the measurement of 'how big' and 'what shape' a batch of particles is:

Particle-size distribution: how many particles fall into which size intervals, commonly expressed by characteristic values such as D10, D50, and D90;

Morphological characteristics: roundness, aspect ratio, specific surface area, whether they are agglomerated, and whether there are edges and surface defects.

A single 'particle size' number often conceals a great deal of information. Two batches of powder with the same D50 = 10 μm might be uniform spheres or agglomerated flakes—with completely different performance.

2. Several Common Measurement Methods and Their Applicable Boundaries

1. Sieving

Passing material through a set of standard sieves of decreasing mesh size is simple, intuitive, and low-cost, but it can only give size fractions of coarse particles (generally larger than 38 μm), cannot give a continuous distribution curve, and cannot measure fine powders.

2. Sedimentation (gravitational/centrifugal)

Based on Stokes' law, particle size is derived from the settling velocity of particles in a liquid. It suits submicron to a few tens of microns, but it likewise assumes particles are spherical and is strongly affected by the state of dispersion.

3. Laser diffraction

A laser irradiates a group of particles, and the particle-size distribution is inverted from the scattered-light intensity distribution. It is fast, has a wide range (covering 0.01–3500 μm), and offers good reproducibility, making it the mainstream in industry. But it 'treats particles as spheres,' so it easily distorts for flake-like, needle-like, or agglomerated particles.

4. The microscopic image method

A microscope (or particle analysis system) images particles one by one, then measures and tabulates them individually. It can measure both size and morphology—it is the 'seeing is believing' method.

Each method has its boundaries, and in practice cross-verification is often used: the laser method for overall trends, and the image method for individual truth.

3. The Principle and Advantages of the Microscopic Image Method

The core of the microscopic image method is to turn 'one particle' into 'a set of computable pixels':

Imaging: the microscope (brightfield/darkfield/polarized, chosen per particle properties) projects particles onto the camera sensor;

Segmentation: software separates the particles from the background in the image (threshold segmentation, morphological processing);

Measurement: for each particle, it computes area, perimeter, and major/minor axes;

Statistics: it converts the results into equivalent circle diameters, particle-size distribution, and morphology parameters.

Its unique value lies in:

Directly measuring size and size distribution (D10/D50/D90);

Directly seeing morphology: roundness, aspect ratio, concavity, and edges are clear at a glance;

Identifying 'fake particles': the laser method often mistakes agglomerates, bubbles, and impurities for large particles, whereas the image method can separate or remove them;

Locating anomalous particles: it can look back at the original image to judge whether an anomaly comes from a real particle or a sample-preparation problem.

4. A Complete Image Analysis Workflow

1. Sampling

Sampling must be representative. Powder sampling should follow the principle of coning and quartering (riffling) to avoid 'picking up coarser or finer material.'

2. Dispersion

Whether dispersion is sufficient directly affects the result. Dispersion methods fall into dry and wet:

Dry method: blow the powder apart with compressed air or a disperser, suited to samples that disperse easily and are sensitive to solvents;

Wet method: disperse the sample in a suitable liquid (considering wettability, whether it dissolves, and whether it agglomerates), possibly adding a small amount of dispersant and aided by ultrasound.

The key is to avoid agglomeration—once particles clump together, what you measure is the 'size of the clump,' not the size of individual particles.

3. Slide preparation

Place the well-dispersed sample on a slide or dedicated sample stage, controlling an appropriate concentration and thickness—too dense and particles overlap, too sparse and there is insufficient statistical data.

4. Image acquisition

Choose an appropriate objective and illumination to ensure the depth of field covers the particle thickness; keep focus sharp and exposure moderate, and use image stitching and autofocus if needed.

5. Software tabulation

Set segmentation parameters, remove touching and edge particles, and output particle-size distribution and morphology statistics.

5. Understanding a Few Key Metrics

D50: the median diameter, at which half the particles are smaller and half are larger;

D10 / D90: represent the fine end and the coarse end, respectively;

Distribution width: (D90 − D10) / D50; the smaller it is, the more uniform the particles;

Equivalent diameter: converting an irregular particle into a circle of the same area, commonly the area-equivalent diameter and the perimeter-equivalent diameter;

Roundness: related to area and perimeter; the closer to 1, the rounder;

Aspect ratio: the ratio of the longest diameter to the shortest, describing the degree of needle-like/flake-like character.

6. Why These Numbers Matter

Take pharmaceuticals: the particle size of an active pharmaceutical ingredient affects dissolution rate and bioavailability—larger particles dissolve more slowly;

Take lithium batteries: the particle-size distribution of cathode materials affects compaction density, the conductive network, and cycling performance;

Take cement: particle size determines the rate of hydration and strength development;

Take environmental monitoring: the morphology and size of PM2.5 and microplastics are key evidence for source tracing.

If you cannot measure particles accurately, the downstream process becomes 'blind men feeling an elephant.'

7. Common Issues and Precautions

Sample agglomeration: solve it by optimizing dispersion, not by having software 'force it apart';

Unrepresentative sampling: compare multiple samplings, or increase the sample amount;

Touching and overlapping: control concentration, or use the software's watershed segmentation;

Improper threshold selection: affects particle boundaries, and should be checked against the original image;

Static interference (dry method): noticeably affects fine powders; an ionizing air blower can be used to eliminate static.

Conclusion

The laser method looks at 'overall trends,' and the microscopic image method looks at 'individual truth,' and the two are often used together. For scenarios that require evaluating both particle size and morphology, microscopic image analysis is an irreplaceable link.

(For microscopic particle analysis and selection advice, feel free to contact us.)