# Density of a Virus/Virion

by Aero51
Tags: density, virus or virion
 P: 546 I am doing a project for my fluids class pertaining to particle tracking. As a sample application, I would like to simulate a virus traveling through a filter or (most simply) a very narrow tube in laminar flow conditions. I need the density of an individual virus particle (a virion as Wikipedia says) to resolve the buoyant forces. Right now I have found very low density viruses to be about 1.03 g/ml or about 1030 kg/m^3 (just under the density of water). I have no physical intuition, but that number seems very large for a virus. I would think if their density was almost as high as water that there would be no such thing as an "airborne virus" simply because it would sink to the ground. Does anyone have a good reference?
 Mentor P: 10,511 The density of water is a good approximation for all cells and cell-like structures, usually water is the main component. Virus particles are so small that buoyancy (and gravity in general) is negligible in realistic setups - sure, if your air is perfectly motionless, on average the particle will sink, but if that happens on a timescale of hours (or even longer) you can neglect it. Brownian motion is relevant for those particles, too.
 Other Sci Sci Advisor P: 1,317 Viruses have about the same density as water. They can be airborne when they become aerosolized small water droplets. A typical way to purify viruses is by density gradient centrifugation. Essentially, you set up a gradient in centrifuge tube with a high concentration of a salt or sugar solution at the bottom of the tube and a lower concentration on top. You then add the virus in and centrifuge overnight so that the virus migrates to where it's density matches the density of the surrounding solution. Here's a paper that estimates the density of hepatitis C virus using this method and also discusses the density of other viruses: http://vir.sgmjournals.org/content/73/3/715.full.pdf
P: 546

## Density of a Virus/Virion

Thank you for the information. According to my calculations the Brownian motion is generally small, but may not be negligible. For instance, I would like to model a filtration system. We have a fully developed laminar flow in a tube of a diameter D. What diameter would be required to say, stop 90% of virions traveling through a filter channel of 100 microns in length? I am not planning on including the wall attraction just yet, but I have included the drag forces, buoyant forces and the Saffman lift force. The Brownian motion of the particle will help enhance the catching frequency. If this is not a good application, what would you suggest?

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