However, it is very evident from a wealth of experimental data that solid core materials do provide a benefit in terms of the ‘A’ term. Since this parameter is dependent primarily on the particle size and the packing efficiency, the logical conclusion is that the packing is better with solid core materials than with fully porous materials.Solid core materials intrinsically have a rougher surface than fully porous media. As a consequence, there is considerably more shear stress applied to the particles when they are packed. Once the column is packed, it then undergoes a period of consolidation where the pressure is slowly released and the frits and other end fittings are attached. Although it is harder to pack the material, once the material is packed the amount of shear stress required to over-come the frictional forces associated with the roughened surfaces is so great that bed expansion is virtually eliminated, whereas the situation with fully porous materials is very different. Fully porous materials have a much smoother topography resulting in particles that are easier to pack than compared to solid core materials. How-ever, on consolidation of the bed, the lack of a roughened surface means that these particles slide over each other with relative ease compared to the solid core materials, which results in the creation of bed heterogeneity.