A diffusiophoretic particle deposition force is the net force on a particle that comes from a concentration gradient present in the gas in which the particle is suspended.

The best way to understand this is by taking a binary gas mixture as an example; a mixture with two species of gasses; 'A' and 'B'.

If species 'A' has a gradient in one direction, there is also an equal but opposing gradient of the species 'B' in the opposite direction. Let's take gas 'A' to have a higher molecular weight than gas 'B'. Then, on an average count, collisions of molecules from 'A' with the aforementioned particle, will transfer more momentum than collisions with the 'B' molecules.

There will be a resulting net force on the particle along the gradient of 'A'; the heavier molecule. This brings about the deposition of particles, towards or away from surfaces.