hydrodynamic diameter

Hydrodynamic diameter refers to the effective size of a particle or molecule in a fluid, accounting for the surrounding layer of liquid it carries with it as it moves. Imagine a tiny particle moving through water: it doesn’t just have its core size, but also drags along nearby molecules, making it act larger than its actual physical size. This combined apparent size—core plus the surrounding fluid layer—is what we call the hydrodynamic diameter. It's an important measurement in fields like chemistry and biology for understanding how particles behave in liquids.