Mass transfer
Mass transfer is mass in transit due to a species concentration gradient in a mixture.
By concentration gradient, we mean a spatial difference in the abundance of the chemical species.
Modes of mass transfer
diffusion
- Diffusion is the macroscopic result of random molecular motion on a microscopic scale.
convection
- Mass transfer by convection involves the transport of material between a boundary surface (such as solid or liquid surface) and a moving fluid or between two relatively immiscible, moving fluids.
- Don’t confuse this phenomenon with the movement of mass caused by a chemical species simply being carried along in a fluid stream (advection).
Example: The story of engineering the contact lens
The cornea of the eye needs oxygen from the air
Mindful of mass transfer, here is the evolution of this important product.
‘Hard lenses’ (polymethylmethacrylate)
- physically uncomfortable
- inadequate oxygen diffusion (irritation, inflammation)
‘Soft lenses’ (hydrocarbon hydrogels)
- physically more comfortable
- inadequate oxygen diffusion
‘Oxygen permeable’ (siloxane)
- physically uncomfortable
- better oxygen diffusion
Latest generation (siloxane hydrogels)
- physically more comfortable
- better oxygen diffusion
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