Mechanism of Tubular Pinch Effect of Dilute Suspension in a Pipe Flow
  Experiments have shown that in dilute suspension flow at laminar state through a circular tube particles migrate towards a concentric annular region with a mean radius of about 0.6 of the tube radius. This phenomenon is well-known as the tubular pinch effect, which is still not fully understood. In this study, the energy gradient method is used to study this phenomenon. It is found that at low Reynolds number particles will move to the position of r/R=0.58. Based on the result, the mechanism of this phenomenon is well explained. 
Hua-Shu Dou, Boo Cheong Khoo