# Laser Induced Aggregation of Light Absorbing Particles by Marangoni Convection
## 3 Results and Discussion
### Effect of Thickness on AP Aggregation  
When the laser power was larger than 50 mW, the APs were pushed away and could not aggregate. When the laser power was smaller than 50 mW, the thickness of the suspension layer affected whether APs could accumulate or not. The experiments were performed with different thicknesses of suspension layer at the same laser power (P = 30 mW). The thicknesses of the layer were 35, 75, and 110 \(\upmu\)m respectively. The APs could not be accumulated when the thickness was 110 \(\upmu\)m. The changes in area for AP aggregates under the two thicknesses are shown in Figure 4. It can be seen that the area of AP aggregate of the suspension layer with a thickness 75 \(\upmu\)m exceeded that of the suspension layer with a thickness 35 \(\upmu\)m after 6.5 s. The closer the particle is to the liquid-air interface, the greater the laser induced surface tension gradient and the faster the particle's movement speed. The aggregation speed was larger in the suspension layer with 35 \(\upmu\)m thickness than that of the suspension layer with 75 \(\upmu\)m thickness at first. The particles in the suspension were quickly aggregated together in a short time (6 s) in the suspension layer with 35 \(\upmu\)m thickness. However, the number of particles in the suspension layer with 35 \(\upmu\)m thickness was smaller than number of particles in the suspension layer with thickness 75 \(\upmu\)m. The aggregation speed decreased after 6 seconds for the suspension layer with 35 \(\upmu\)m thickness. The experiments with different thicknesses show that the thickness ofthe suspension layer plays a crucial role in the APs' accumulation.  
When the thickness of the suspension layer was larger than 110 \(\upmu\)m, AP aggregates were not observed in the experiments. When the thickness of the suspension layer was large, the thermal gradient between the center and the edge of the water-air interface was weak, and the Marangoni convection at this thickness was insufficient to drive the particles to aggregate.  
Figure 3: Areas of AP aggregates as a function of illumination time at different laser powers. The laser power was measured at the pupil of objective.  
As the irradiation time increases, the thickness of suspension layer gradually decreases. The decrease can be attributed to water evaporation, which is induced by the heat transfer from the light irradiation on the absorbing particles. We measured the variation of thickness of the suspension layer with irradiation time. The results are shown in Figure 5. As the laser begins to irradiate, the AP particles absorb the light and transfer the energy into heat quickly, resulting in the suspension heating up and water evaporation, and the thickness of suspension layer decreases. As shown in Figure 5, the thickness of the liquid phase is reduced from 93 \(\upmu\)m to 22 \(\upmu\)m in 300 s for P = 37 mW. However, in the particle aggregation experiments, the particle aggregation was completed within 10 s. During this short time, the decrease in thickness of the liquid phase can be negligible; the process of AP aggregation was not affected by the change in thickness of the liquid phase.  
If someone wants to aggregate the nanometer absorbing particles, the particles can be aggregated with the laser induced Marangoni convection first, and then form the deposition with water evaporation, as exhibited in the previous research [42]. The water evaporation can lead to cluster aggregation and deposition on the substrate's surface.  
Figure 4: Areas of AP aggregates as a function of illumination time. P = 30 mW.  
Figure 5: Thickness of the suspension layer as a function of illumination time.