**2. Electrochemical method for measurements of frictional force and bubble size**

This part involves the measurement character of electrochemical methods that lead to measure the velocity gradient and frictional force on a particle in a bubble column and in a fluidized bed. The possibility of measuring bubble volume is also discussed. Thus, an electrochemical probe may be proposed to measure in the same time the frictional force and the bubble volume. The application of such electrochemical method is very important in biochemical engineering.
