Application Note: Unveiling the World of Particles

Particles are the unsung heroes of our world, often wielding an astonishing influence on our lives. Whether it’s curbing atmospheric pollutants, crafting ground-breaking pharmaceuticals, advancing product development, streamlining food production, or revolutionizing construction and electronics materials, particles are at the heart of quality, sustainability, and human well-being. 

In an age of technological leaps and heightened environmental consciousness, the study and manipulation of particles have taken center stage, addressing the most pressing challenges of our era. Before diving into the multitude of scientific techniques used to measure and understand particles, let’s start by demystifying what particles are.

Defining Particles

Particles come in many shapes and sizes, yet they all share a common characteristic: they are small, localized entities with unique physical and chemical attributes, such as mass, volume, and composition. With such broad versatility, it’s no surprise that various scientific disciplines approach particles in their distinct ways. We aim to provide clarity when material scientists talk about particles. The behavior of particles can vary significantly based on their size, chemical composition, and environmental context. 

Particle Size 

• (Sub)Atomic: In the vast universe of particles, we first encounter subatomic particles, the tiniest building blocks of matter. Subatomic particles include quarks, bosons, and electrons. These minuscule entities shape the very fabric of the quantum world, playing a pivotal role in the universe’s structure and dynamics.
  
  
• Molecular: Zooming into the atomic level, individual atoms feature a nucleus made up of protons and neutrons, surrounded by electrons orbiting in energy levels. Molecular particles emerge when atoms bond through chemical interactions, forming molecules with distinct properties, which are essential to understanding substance chemistry. Investigating atomic and molecular particles is paramount in fields like chemistry, physics, and materials science, driving advancements across various industries.
  
  
• Nano- and Micrometer: However, material scientists often focus on particles in the nanometer and larger size ranges. Nanoparticles often have unique properties that differ from bulk materials due to their high surface area-to-volume ratio, quantum effects, and altered mechanical properties. Micrometer-sized particles, like pollen, sand, and debris, gain attention due to their ability to travel great distances and settle on functionally critical surfaces.
  
  
• Collodial: Colloidal particles represent another fascinating category, residing between atomic and bulk materials in size. These particles, commonly found in colloidal suspensions, consist of small clusters of atoms or molecules dispersed in a solvent. Colloidal particles exhibit unique properties influenced by forces like Brownian motion, showcasing distinctive optical, electrical, and mechanical behaviors. These particles find essential applications in fields such as chemistry, biology, and materials science, contributing to areas like drug delivery systems, food science, and nanotechnology.

Particle Sizing Techniques

Today, a variety of techniques are at your disposal to size particles, each with its own set of advantages and limitations

Microscopy techniques

• Transmission Electron Microscopy (TEM): particularly useful for the characterization of nanoscale particles, providing morphological, crystallographic, and compositional information.

• Scanning Electron Microscopy (SEM): provides high-resolution imaging of particles in nanometer to micrometer size range

• Optical Microscopy: widely used for characterization of micrometer-sized particles.  Advanced optical microscopy such as confocal laser scanning microscopy, polarized light microscopy, and dark-field microscopy can be applied to extend the size range of particles characterized and measure additional properties of the particles.

Spectroscopy techniques

• Energy Dispersive X-ray spectroscopy (EDS): often used in conjunction with SEM or TEM to provide elemental composition of the particles

• X-ray Photoelectron Spectroscopy (XPS): useful for analyzing the surface chemistry of particles

• X-ray Diffraction (XRD): provides information about the crystallographic structure, including crystallite size and strain.  As XRD characterizes coherently scattering domains (crystallites) rather than individual particles, XRD is often used alongside other techniques such as electron microscopy or dynamic light scattering to provide a comprehensive understanding of particle size and structure. 

• Dynamic Light Scattering: best suited to determine the size distribution of nanoparticles in a liquid.

• Laser Diffraction and Sieve Analysis: used to measure the size distribution of micrometer-sized particles.

Among these techniques, transmission and scanning electron microscopes stand out for their unparalleled magnification capabilities and the ability to provide insights into local chemistries.

While controlling particle size and morphology is crucial for many applications, understanding the chemistry of individual particles often becomes a necessity to validate their true origin. Depending on particle size, various analytical techniques are available to decipher particle chemistry.

It’s worth noting that in some cases, material properties are determined by chemical reactions occurring at the interface between particles or between particles and matrices, such as surface-sensitive processes like corrosion.

Environmental Context

Particles are also commonly classified according to their environmental context, such as atmospheric, soil, or marine particles. There are many environmental factors used to classify particles.  Most are domain specific resulting in overlapping definitions. The most common of which you probably are already familiar with.

Goldenrod Pollen as seen under a scanning electron microscope (SEM)

• Aerosols: Solid or liquid particles suspended in the air, including dust, smoke, and air pollution.
  
  
• Particulate Matter (PM): Categorized by size, PM is a crucial measure for air quality.
  
  
• Suspended and Dissolved Particles: Including sediments, silt, algae, salts, nutrients, and gases.
  
  
• Mineral Particles: Inorganic particles like sand, silt, and clay. 
  
  
• Organic Particles: Composed of decaying plant and animal matter. 
  
  
• Microplastics: Tiny plastic particles found in oceans and water bodies.
  
  
• Pollen: Particles produced by plants for reproduction.
  
  
• Microorganisms: Bacteria, viruses, and fungi. 
  
  
• Nanoparticles: Man-made particles at the nanoscale. 
  
  
• Foreign Particulate: Unwanted particulate, often generated during the manufacturing process 

Summary

Material scientists need to have a clear definition of particles and understand the proper analytical techniques to size and understand particles. When selecting the most suitable analytical techniques, remember that crucial processes often unfold at the interface between particles and matrix materials, underscoring the significance of accurate analysis for your testing laboratory.