Why do red blood cells settle faster when they clump together?

In the context of blood analysis, red blood cells settle faster when they clump together because their effective size increases.

According to the principles of sedimentation, the velocity at which particles fall through a fluid is influenced by their size and density. When inflammatory factors or coagulation proteins—such as fibrinogen—are present in the blood, they cause red blood cells to adhere to one another, forming larger clumps. Because these clumps have a greater mass and effective radius than individual cells, they fall out of the plasma more rapidly than they would if they remained in suspension as separate cells.1

This phenomenon is measured by the erythrocyte sedimentation rate (ESR), which serves as a nonspecific indicator of inflammatory disease or anemia; the faster the cells settle, the higher the sedimentation rate.1