A New Mechanism of Light-Induced Bubble Growth to Propel Microbubble Piston Engine

Radiation pressure refers to the momentum transfer of photons during light "particles" impacting a surface. The force is too small to drive microengines. Different from the classical radiation pressure, the indirect radiation pressure (F_m ) is introduced, coming from the momentum change of light-induced bubble expansion. F_m is shown to obey F_m ∼ (I·r_b )² , behaving faster growth of indirect radiation pressure versus light intensity I and bubble radius r_b . An effective bubble size range is identified for F_m to suppress other forces for bubble in liquid. The top laser irradiation on nanofluid is used in this experiment. A well-defined bubble pulsating flow, being a new principle of bubble piston engine, is demonstrated. During pulse on (≈ns scale), F_m exceeding other forces generates an extremely large acceleration, which is three to four orders larger than the gravity acceleration, propelling the bubble traveling downward. During pulse off, the bubble is floating upward due to the nonexistence of F_m . In such a way, the piston engine sustains the oscillating ranges of 38-347 µm for bubble diameters and 2.7-457.9 µm for traveling distances of piston. This work is useful to manipulate bubble dynamics in solar energy systems, and can find various applications in optofluidics.