Several mechanisms have been proposed to explain flow through a membrane.   These are:

Bulk pressure driven flow      The entire flow field is forced through a membrane (filter).   Larger particles are trapped in the filter.   The cleaned fluid is normally reused in the system. Trapped impurities remain in the filter.  Special filters can remove submicroscopic particles.   They are used for air, water, and oil systems.   Fans or pumps are used to develop the pressure required for the bulk flow through the filter.     The required pressure loss across the filter increases with volume flow and smaller particles removed.  

Electrical fields    Ionized particles are driven by an electrical potential.    The power required increases with increasing volume flow and decreasing opening size in the membrane.

DIFFUSION    has been proposed as the mechanism for osmotic transport across a membrane.  Flow proceeds from an area of high concentration to an area of low concentration. 

DETAILED MOLECULAR MODELS

FRANKEL, J, Kinetic Theory of Liquids; 1943, 1955, proposed that liquids were basically crystal formations held in place by weaker intermolecular forces.  Intermolecular bonding forces are stronger in solids.    Molecules are held at the nodal points.  Energy is stored as molecular vibrations about the nodes and increases with temperature.     High energy molecules can jump to adjacent vacant notes or away from the surface.     Molecular transport results from molecules jumping to adjacent nodes.        Vapor pressure results from molecules jumping from the liquid or solid surface.

 

HOWLETT, LD, Osmosis: The Molecular Theory; 2003, 2013 proposed that free (unconstrained) molecules are present in liquids and solids.  These result in transport through the substance and membranes.  They also produce the vapor pressure of a substance.     He used Frenkel€™s model for the crystal structure in both liquids and solids.   Then, he considered condensation in a kinetic theory gas.    He suggested that high speed molecules would not be effected by intermolecular bonding forces.     He showed that the cut off speed was approximately 3 times the root mean square speed of the gas molecules.    The cutoff speed was calculated from the enthalpy of evaporation and by comparing the kinetic energy of the high-speed molecules to the published values of vapor pressure at a specific temperature.   The proof is presented for water and for several other substances.