# Laser Induced Aggregation of Light Absorbing Particles by Marangoni Convection
## 3 Results and Discussion
### AP Aggregation  
When the AP suspension was injected into the sample cell with a thickness of 1 mm, there was a water-air interface in the sealed cell. The thickness of liquid phase was about 70 \(\upmu\)m. The laser induced thermal gradient resulted in the generation of a flow that drove the APs to migrate and accumulate, forming an aggregate. The process of laser induced AP aggregation is shown in Figure 2. The laser power was 18 mW. When there was no laser illuminating the APs, the particles do not move, as shown in Figure 2a. When the laser began to work (t = 0.6 s) and focused on a select, small aggregate in the substrate, as in Figure 2b, the thermal gradient generated induced a thermal convective flow. The flow transported the APs to the heat center, and the APs formed a large AP aggregate finally. The number of APs accumulated at the center of focus increases with the increasing illumination time, as shown in Figure 2c-f. When the laser illumination was terminated, most of the APs were still adhering to the aggregate. The threshold power that could induce APs aggregation was 13 mW for the suspension layer with 70 \(\upmu\)m thickness. The accumulation was not seen when the laser beam was focused on top of the layer except when the thickness of suspension layer was decreased to about 20 \(\upmu\)m.  
The movement of absorbing particles in the experiments could be controlled by three mechanisms, including Marangoni convection, thermophoresis, and photophoresis [40]. Among them, photophoresis originated from the uneven heat distribution on the surfaces of the tiny objects irradiated by the light beam. According to the degree of absorption of incident light by the particle material, it can form positive photophoresis (strong absorption) and negative photophoresis (weak absorption). Photophoresis can produce a pulling force or a pushing force on the particles, which drives the particles to accumulate along the direction of beam propagation. Photophoresis can trap a large number of particles, but the trapped particles arrange along the beam propagation direction mainly. Furthermore, the photophoretic effect in a liquid environment is much weaker compared with that in air [40]. In our experiments, the particle aggregates arranged in the view plane of the microscope, which was perpendicular to beam propagation. Therefore, the APs' aggregation is not dominated by photophoresis. Thermophoresis describes the directed migration of different components in the solution along a temperature gradient. The drift velocity of colloidal particles can be given by \(u=-D_{T}\
abla T\), where \(D_{T}\) is the thermophoretic mobility and \(\
abla T\) is the temperature gradient [40]. When the laser power was 50 to 104 mW in our experiments, the absorbing particles were pushed away from the laser focus when there was a liquid-air interface. Moreover, experiments were repeated for examination with the sample cell with a thickness of 70 \(\upmu\)m. The thickness of the suspension layer was also 70 \(\upmu\)m. There was no water-air interface in the sample cell. The particles were also pushed away when the laser power was 50 to 104 mW. Therefore, the sign of \(D_{T}\) of the absorbing particles is positive and the drag force of thermal flow is dominant when the laser power is 50-104 mW. However, the particles aggregated at the liquid-air interface when the laser power was 13 to 50 mW. Therefore, the APs' aggregation can not be dominated by lateral thermophoresis. In addition, AP aggregation was not seen in the experiments when the laser power was 13 to 104 mW. The examination results confirm that the APs' aggregation in the suspension layer is not induced by thermal convective flow from the bottom to the top and circulating inside the layer. The previous research [41] has shown that laser pulses can create a temperature gradient, which induces Marangoni flow that can drag particles towards aggregation. The particles are released from a location close to a bubble and aggregate at its boundary.