At short times after the beginning of the sedimentation,a sharp decrease in absorbance are observed for the dimer acid content larger than 2 g/L,which can be a result of fast aggregation induced by polar attractions between iron particles. Further,the sedimentation seems to slow down when the content of dimer acid increases to 8 g/L,and the inset photo shows the final result of the sedimentation after 2 h. As shown in this image, visible big aggregates suspend or adhere to the cell walls,which provoke a loss of light intensity in the direction of the detector, thus explaining the slow absorbance changes.The result illustrates that the inter-particle attraction exists even in dilute suspension.