3.1. Electrical Mobility

Electrical mobility is a method used to classify nanoparticles based on their charge and size. Particles are charged as they enter an analyzer, and their movement is influenced by an electric field.

Electrical mobility refers to how quickly and easily a particle moves through an electric field, which can be related to its size and charge. When particles enter the differential mobility analyzer, they acquire a charge. Because each particle can hold a different amount of charge based on its size, this creates varying movement patterns. The analyzer uses these patterns to separate particles by their size.

Imagine a crowded room (the analyzer) where everyone carries balloons of different sizes (the particles). When a fan (the electric field) is turned on, the larger balloons may move slowly while the smaller ones float away quickly. Similarly, in the analyzer, the size and charge of the particles determine how they respond to the electric field.

The differential mobility analyzer charges incoming particles and applies a potential difference between concentric cylinders, causing particles to follow different trajectories based on their size and charge.

The differential mobility analyzer consists of two cylinders placed one inside the other. When particles are introduced, they get charged. The applied potential difference creates an electric field that influences the path that these charged particles take. Depending on their size and charge, particles will take different paths through the cylinder. The analyzer selectively allows specific particle sizes to pass through while others are deflected and removed.

Think of it as a racetrack with curves (the cylinders). Each car (a particle) is of a different size and speed (charge). Depending on how tightly the curves are made (the strength of the electric field), some cars can navigate the turns more easily than others. The smaller cars will tend to zip through, while the larger ones might struggle and get stuck.

Different particles will follow different paths based on changes in the applied voltage, allowing the analyzer to identify and separate particle sizes effectively.

By varying the voltage between the cylinders, the analyzer systematically changes the strength of the electric field. This allows it to selectively focus on different particle sizes one at a time. As the voltage changes, different sized particles will be attracted or repelled by the electric field in unique ways. By mapping these movements, researchers can gather data about the size distribution of the particles present.

Imagine a bouncer at a club who allows only people of certain heights to enter based on a marked height. When the bouncer raises the height limit (the voltage), a different group of people (particles) can enter. This selection process is similar to how the analyzer identifies different sized particles by adjusting the conditions.

Measuring nanoparticles is challenging, but essential. New techniques like the condensation particle counter allow for tracking the size and concentration of ultra-fine particles.

Nanoparticles, which can be smaller than 100 nanometers, are often difficult to measure using traditional methods. Gravimetric techniques, which rely on weighing particles, are ineffective because the particles are too small. Instead, condensation particle counters (CPC) use vapor to help the nanoparticles grow larger, making them easier to measure. The CPC turns vapor into liquid that condenses onto the nanoparticles, effectively enlarging them.

Imagine trying to see tiny specks of dust in the air. Instead of looking for the dust directly, you could spray mist into the air (the vapor). As the mist condenses onto the dust, the dots become larger and more visible. This is similar to how CPC helps in measuring nanoparticles by allowing them to grow before counting.

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