Abstract
In this work, the control of the droplet generation by a focused infrared (IR) laser with a wavelength of 1550 nm was studied, in which the liquid water and the oil with the surfactant of Span 80 were employed as the disperse and continuous phases, respectively. The characteristics of the droplet generation controlled by the laser was explored under various flow rates, laser powers and spot positions and the comparison between the cases with/without the laser was also performed. The results showed that when the laser was focused on the region away from the outlet of the liquid water inflow channel, the droplet shedding was blocked due to the IR laser heating induced thermocapillary flow, leading to the increase of the droplet volume and the cycle time of the droplet generation as compared to the case without the laser. Decreasing the continuous phase flow rate led to the increase of the droplet volume, cycle time of the droplet generation and the volume increase ratio, while increasing the disperse phase flow rate led to the increase of the droplet volume and the decrease of the cycle time and volume increase ratio. For a given flow rate ratio between the continuous and disperse phases, the increase of the flow rates decreased the volume increase ratio. In addition, it is also found that the droplet volume, the cycle time and the volume increase ratio all increased with the laser power. When the laser was focused at the inlet of the downstream channel, the droplet volume, the cycle time and the volume increase ratio were the largest. Moving the laser spot to the downstream or upstream led to the decrease of them. When the laser was focused on the outlet of the liquid water inflow channel, the generated droplet volume and cycle time of the droplet generation were even lower than the case without the laser because of the lowered viscosity. This works provides a comprehensive understanding of the characteristics of the droplet generation controlled by the laser, which is beneficial to promote the application of this optical method in the droplet based microfluidics.
Highlights
It is found that when the laser was focused on the region away from the outlet of the liquid water inflow channel, the droplet shedding was blocked, leading to the increase of the droplet volume and the cycle time of the droplet generation as compared to the case without the laser
This is because the non-uniform temperature distribution and the resultant temperature gradient across the interface arising from the IR laser localized heating led to the thermocapillary flow, which blocked the droplet shedding
It is found that when the laser was focused on the cross junction, with the increase of the continuous phase flow rate and the simultaneous increase of the continuous & disperse phase flow rates with fixed flow rate ratio, the volume increase ratio was reduced because the strength of thermocapillary force became weak
Summary
The droplet generations by these methods are realized by the hydrodynamic means, which mainly relies on the flow field to deform the interface and to promote the interfacial instabilities. These measures are classified as passive methods. Just because they are passive ways, it is difficult to achieve the goal of the on-demand droplet generation in these microfluidic devices.[17]. This work provides a comprehensive understanding of the IR laser controlled droplet generation and promotes its applications in the droplet based microfluidics
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