# Laser Induced Aggregation of Light Absorbing Particles by Marangoni Convection
## 3 Results and Discussion
### AP Accumulation Assisted by Bubbles  
When the laser power reaches 104 mW, bubbles will be generated in the sample. The generation of bubbles was not affected by the thickness of suspension layer. Previous studies have shown that this photothermal bubble can be used to trap micron sized particles [41; 43; 44]. In our experiments, the bubbles were used to accumulate absorbing particles. The bubbles were generated by laser heating, and the bubbles acted as the liquid phase to form the liquid-air interface. The laser heating on APs caused a convective flow to migrate the particles, which achieved the AP accumulation. Figure 6 shows a bubble-assisted accumulation process of the APs. The laser power was 120 mW. When a bubble is generated, the APs quickly adhere to the bubble-water interface, forming an AP aggregate. The mechanism of bubble-assisted AP aggregation can be attributed to the Marangoni convection. When the vapor bubble is generated, heat from strong absorption of laser irradiation on the APs induces a thermal gradient simultaneously. This thermal gradient then generates a local convective flow around the bubble. As the laser irradiation time increases, the area of the AP aggregate increases, as shown in Figure 6d. In comparison with Figure 2f, it can be seen that the accumulation speed of APs assisted by the bubble is much higher than that without the bubble. The acceleration of AP accumulation can be attributed to the heating position being close to the bubble, which caused a large thermal gradient at the gas-liquid interface on the bubble's surface. The large thermal gradient induced a stronger Marangoni flow. The strong Marangoni flow accumulated the APs quickly. The location at which the particles accumulate is related to the bubble's growth direction. The bubbles in Figure 6a-c floated up in the suspension layer, and the heating location was just at the bottom of the bubble. Then the APs accumulated at the bottom of bubble and were arranged around the laser heated point. If the bubble is grown in x-y view plane along the coverslip, and the heating location is on one side of the bubble, the particles will be accumulated preferentially on the side of bubble close to the heating location, as shown in Figure 7f. Figure 6: (**a–c**) Images of bubble-assisted forming of AP aggregates. The red circle indicates the laser focus. Scale bar, 10 μm. (**d**) Areas of AP aggregates as a function of laser irradiation time. The laser power was 120 mW at the pupil of the objective. The thickness of suspension layer was 70 μm. The direction of the convective flow can be observed in real time by tracking the positional motion of APs close to the laser spot, as shown in Figure 7. For tracking the motion of APs, the suspension was diluted to 4.1 \(\times\) 10\({}^{16}\)/cm\({}^{3}\) and the particles were few in the observation view. The motion of particles will not be interfered with by the other particles. The laser power was decreased to 80 mW to reduce the particle velocity for observation. When a bubble is generated, as shown in Figure 7a, an AP near the bubble begins to move circularly, as shown in Figure 7b-e. Finally, the AP adheres at the bubble's surface in Figure 7f. The APs are accumulated on the side of the bubble close to the laser focus, not distributed along the bubble uniformly. Laser heating can accumulate the APs, no matter whether there is any bubble generation. The bubble-assisted method accumulates the APs faster than that without the bubbles. However, the bubbles are very unstable, and easily broken under laser heating. The rupture of a bubble is violent. The bubble rupture releases lots of energy and destroys the AP aggregates, forcing the particles to move intensely. By analyzing the trajectory of particles after a bubble explosion, one may analyze the dynamics of bubble explosion. In the liquid-air interface, the optical trapping force does not dominate in the motion of APs because the drag force induced by opto-thermal convection is strong and exceeds the optical force.