FIG. 4.

(Color online) Geometry of system. The midplane shows two bubbles sitting against a spherical cell of radius a. An arm (black line) bisects the midpoint of the line connecting the bubbles. The arm length is a · cos (γ /2), where angle γ is the angle between the lines that connect the center of each bubble with the center of the cell. Both bubbles experience the same radiation force F shown by red arrow. The corresponding torque acting the cell from each bubble is expressed as a product of the force projection F · sinφ and the arm length, which results in the net torque T = −2 Fa· sinφ· cos (γ /2).