Gaining Momentum


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An accidental discovery 40 years ago led to an improvement in firefighting techniques: When polymers (long chains of molecules) were added to the water pumped through firehoses, that water projected over greater distances - of critical importance to those living and working in high-rise buildings.

But the why behind this finding remained a mystery until it was recently solved by a group of scientists of the Weizmann Institute’s Chemical Physics Department that included Prof. Itamar Procaccia, Dr. Victor L’vov, Dr. Anna Pomyalov and postdoctoral fellow Dr. Vasyl Tyberkevych.

Intuition told them that adding polymers should slow the flow of water through the hose by raising the water’s viscosity. The scientists noted that the normally folded up polymer molecules became stretched out like long strings of beads in the rushing stream, which did, indeed increase water viscosity.

However, a stronger counter-effect was also noted. The pressure produced in the pumps creates momentum, some of which carries water out of the end of the hose and some of which flows to the pipe walls. The polymers interfere with the flow of momentum to the pipe walls, and since the total momentum in the hose remains constant, a reduction in flow of momentum in one direction increases it in the other. The effect, therefore, is to raise the momentum, and thus the speed, of the water exiting the hose.

Institute scientists are currently testing the possibility of replacing the long-chain polymers with tiny air bubbles, an approach that may have relevance for transporting oil in pipelines and reducing drag on the bows of ships.
Prof. Procaccia’s research is supported by the Minerva Center for Nonlinear Physics of Complex Systems; the Naftali and Anna Backenroth-Broniki Fund for Complexity established by Mr. Yehuda Broniki of Israel; and the late Mr. Simon Pupko. He is the incumbent of the Barbara and Morris L. Levinson Professorial Chair in Chemical Physics.