# Micro-Actuators with Light and Magnetism Dual-Control for Maneuverable Movements
## 3 Results and Discussion
### Rotational Movement of the Drives  
Furthermore, in addition to planar linear motion, rotational motion can be achieved using efficient photothermal conversion films. Unlike conventional gear units with asymmetrical blades, the symmetrical triangular structure also allows for rotational motion. For example, a positive triangular device with the center point fixed is designed by a staple, as shown in Figure 5. Driven by the laser beam, the triangular device rotated in a counterclockwise direction \(\approx\) 0.05 rad s\({}^{-1}\), as shown by the arrows in Figure 5c. The drive did not rotate at a very high speed, and the friction from the center point counteracted some of the driving force. Interestingly, it denoted that the actuator would quickly rotate back in the opposite direction after removing the laser, possibly indicating the stress releasing after the light-driven forced rotation; the flexible film created torsional deformation in the rotation. Similarly to the turning of the boat, which can flexibly choose different driving sides to change the forward direction, the rotation device can also change the laser tracking irradiation point to achieve flexible rotation movement. In the design, a full 360 degree rotation of the actuator was divided equally into three rotational movements of 120 degrees each (Figure 5a). In each segment, the laser beam was controlled to illuminate point B and rotated 120 degrees (angle), the actuator was then driven to rotate the same degrees (angle). Repeating this process three times achieved a complete rotational motion of the drive. And repeating the process over and over again achieved a combination of the reciprocating motion of the laser and the continuous rotational motion of the drive. Such a rotational motion is achieved by combining segmental motions, which is more flexible and practical for achieving a circular circumference in some cases where the laser is obscured. To further understand the above rotational motion, the mechanical model of the triangular rotational structure is studied and shown in Figure 5b. Surface tension plays a critical role in interfacial motion and is influenced by the temperature, so we analyzed the temperature distribution and the forces on the rotating device. According to the Harkins formula [35], the surface tension decreases with increasing temperature, and it decreases more significantly in the region of higher relative temperature, so the temperature gradient between the liquid surface regions is sufficient to generate surface tension difference. The forces generated by the Marangoni effect at the interface (liquid/air) can be calculated as follows:  
\[\eta\frac{\partial u}{\partial y}=\gamma\frac{\partial T}{\partial x} \tag{1}\]  
where the temperature derivative of surface tension (N mK\({}^{-1}\)) and the equation indicates that the shear stress on the surface is proportional to the temperature gradient. In the figure, point A indicated the fixed point of the triangle boat, point B indicated the action point of the laser, and the red area below point B indicated that the water surface was heated to a higher temperature. The direction of the force which generated the counterclockwise rotation was perpendicular to the line connecting the two points AB. Denoted by \(F_{1}\) as the total force applied to the drive, this force was equal to the sum of the Marangoni force and the drag force. Therefore, the decomposition force \(F_{3}\) parallel to the AB axis did not affect the rotational motion and could be ignored. Only the decomposition force \(F_{2}\) perpendicular to the AB line produced counterclockwise angular momentum. The moment of force \(F_{2}\) on point A can be calculated as follows:  
\[M=F_{2}d \tag{2}\]  
where \(d\) is the distance between the laser action point and the drive center point. The equation shows that the Marangoni force on the drive is proportional to the moment at the point of action.