High-frequency micro-droplet generation method based on periodic cavitation bubbles

Abstract

The invention relates to the field of microfluid, in particular to a method and a technical means for periodically generating liquid drops by periodically generating micron-sized cavitation bubbles generated on the basis of a laser photothermal effect near a gas-liquid interface in a micro-channel and generating strong jet flow and breaking the jet flow towards a gas phase direction to form the liquid drops. The method is characterized by comprising the following steps: irradiating focused continuous wave laser onto a heat production substrate of the micro-fluidic chip to generate cavitation bubbles; meanwhile, the generation position of the cavitation bubbles is controlled to be close to a gas-liquid interface by controlling the focusing position of laser spots; when the gas-liquid interface serves as a flow field boundary, the cavitation bubbles can move away from the gas-liquid interface in the collapsing process, a light spot area is covered with liquid again, and therefore the cavitation bubbles can be nucleated periodically, meanwhile, a jet flow liquid column in the gas phase direction is formed in the bubble asymmetric collapsing process, and the cavitation bubbles are formed in the gas phase direction. And the jet flow liquid column is broken due to Rayleigh-Praxert instability to form liquid drops, and finally periodic liquid drop generation is realized.

Description

Technical Field

[0001] The present invention relates to the field of microfluidics, and particularly to a method and technical means for periodically generating micro-droplets by periodically generating micron-sized cavitation bubbles based on the laser photothermal effect near the gas-liquid interface inside a microchannel, generating strong jets, and the jets breaking to form droplets. Background Art

[0002] With the rapid development of microfluidics technology and nanomaterial science, micro-scale droplet technology has shown broad application prospects in multiple fields. In the biomedical field, micro-droplet technology is widely used in single-cell analysis, drug screening, gene detection, etc. By encapsulating single cells or molecules in micro-droplets, high-throughput single-cell sequencing and gene expression analysis can be achieved, thus deeply studying cell heterogeneity and functions. In the field of chemistry and materials science, micro-droplet technology is used as a micro-reactor for chemical synthesis, catalytic reactions, and nanomaterial preparation. By precisely controlling the reaction conditions, micro-droplet technology can improve reaction efficiency and product purity, and at the same time prepare nanoparticles and microspheres with uniform sizes. These materials have important applications in fields such as drug delivery, sensors, and optical devices.

[0003] Micro-droplet technology can divide liquids into tiny droplets with uniform sizes, usually in the micron to nanometer scale range, and has the characteristics of high precision, high throughput, and strong controllability. This technology can not only achieve efficient liquid processing but also provide new solutions for a variety of complex applications. Micro-droplet generation technology can be divided into two categories: active generation and passive generation according to its working principle. Active generation methods drive droplet generation through external energy input (such as electricity, heat, sound waves, etc.), while passive generation methods rely on the kinetic characteristics of the fluid itself (such as shear force, surface tension, etc.) to achieve droplet generation. There are many active droplet generation methods, mainly including magnetic control, mechanical control, thermal control method, and electrocontrol method, etc.

[0004] The passive generation method has the characteristics of lower precision, and the droplet size and frequency are affected by the fluid characteristics; compared with the passive generation method, the active generation method has the characteristics of high precision, controllable droplet size and frequency, and is suitable for high-precision and high-frequency application scenarios such as lithography, inkjet printing, and bioprinting. However, it needs to face the problems of complex equipment and high cost. Based on the cavitation bubbles generated by the laser photothermal effect near the gas-liquid interface, relying on the liquid in the microchannel, it rapidly expands and collapses in about ten microseconds, with an explosive growth dynamic. At the same time, the asymmetric fluid environment causes the cavitation bubbles to collapse asymmetrically and generate periodically, and a jet liquid column is formed in each cycle. The jet liquid column breaks to form droplets. This non-invasive droplet generation method has a simple structure, low energy consumption, controllable generated droplet size, and can reach a generation frequency of kHz level and above, and has excellent application potential for micro-droplet generation. Summary of the Invention

[0005] The object of the present invention is to propose a method for generating high-frequency micro-droplets based on periodic cavitation bubbles.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows: A continuous-wave laser is used and after being focused by a microscope objective lens, it is irradiated onto the heat-generating substrate of a microfluidic chip to generate cavitation bubbles. At the same time, by controlling the focusing position of the laser spot, the generation position of the cavitation bubbles is controlled to be near the gas-liquid interface. The gas-liquid interface, as the boundary of the flow field, will cause the cavitation bubbles to move away from the gas-liquid interface during the collapse process, and the spot area will be covered by the liquid again, enabling the cavitation bubbles to nucleate periodically. At the same time, during the collapse process, a strong jet is accompanied. The jet liquid column towards the gas phase breaks to form droplets. The generation frequency of the droplets is the same as the generation frequency of the cavitation bubbles. The droplet size is controlled by the maximum size of the cavitation bubbles and the distance between the bubble and the gas-liquid interface. At the same time, the maximum size of the cavitation bubbles can be regulated by the laser power.

[0007] Further, in the present invention, the heat-generating substrate is a photo-thermal conversion substrate, including but not limited to a fused silica sample covered with gold nanoparticles;

[0008] Further, the cavitation bubbles are generated in the liquid, and the types of liquids for generating droplets include but not limited to water;

[0009] Further, in the present invention, the generation position of the cavitation bubbles is adjusted to near the gas-liquid interface. By virtue of the asymmetry of the surrounding flow field environment, the cavitation bubbles are subjected to an asymmetric pressure and move away from the gas-liquid interface, enabling the cavitation bubbles to nucleate periodically. At the same time, the gas-liquid interface will first be sunken towards the liquid phase due to the flow field movement caused by the bubble contraction, and then the liquid near the interface is driven by the high-pressure region to shoot towards the gas phase at a high speed to form a jet liquid column. The jet liquid column moves into the air and breaks due to Rayleigh-Plateau instability to form spherical droplets;

[0010] Further, the surface of the substrate of the microfluidic chip is treated by hydrophobic treatment, and the gas-liquid interface and the substrate plane form an included angle of approximately 90° to control the direction of the jet to be perpendicular to the gas-liquid interface;

[0011] Further, by controlling the focusing position of the laser spot, the generation position of the cavitation bubbles is controlled, and thus the generation position of the droplets is controlled;

[0012] Further, after a period of time, the liquid of the laser spot is superheated and nucleated again, and cavitation bubbles of the same size are generated again. After the above process, droplets of the same size are formed again, thereby realizing the periodic generation of micro-droplets;

[0013] Furthermore, the lower the power of the laser, the larger the size of the bubbles, the stronger the formed jet, and the larger the size of the droplets. Therefore, the regulation of the droplet size is achieved by regulating the laser power;

[0014] Furthermore, the present invention includes two working modes, namely the continuous laser irradiation mode and the pulsed laser irradiation mode: in the continuous laser irradiation mode, changing the laser power can change the frequency of the periodic generation of cavitation bubbles, thereby regulating the generation frequency of droplets; in the pulsed laser irradiation mode, by modulating the on and off times of the laser, the frequency of the periodic generation of cavitation bubbles can be changed, thereby regulating the generation frequency of droplets. Description of the Drawings

[0015] Figure 1 is a schematic diagram of the present invention for realizing the high-frequency generation of micro-droplets by using periodic cavitation bubbles;

[0016] Figure 2 is an example time series diagram of the present invention for realizing the generation of 1.5 kHz droplets by using 1.5 kHz periodic cavitation bubbles in the continuous laser irradiation mode.

[0017] Explanation of the reference numerals in the drawings: 1 - liquid for generating droplets; 2 - heat-generating substrate; 3 - microfluidic chip; 4 - cavitation bubble; 5 - continuous laser; 6 - gas-liquid interface; 7 - droplet. Detailed Embodiments

[0018] The present invention will be further described below with reference to the drawings and examples.

[0019] As Figure 1 shown, the incident continuous laser irradiates the surface of the heat-generating substrate 2 on the surface of the microfluidic chip 3, generating cavitation bubbles 4 in the liquid phase region 1. Next, in combination with Figure 2The experimental diagram is further illustrated. The cavitation bubble 4 starts to generate at t = 0 μs and grows to the maximum at t = 7.4 μs. Due to the existence of the gas-liquid interface, at t = 14.7 μs, the bubble undergoes an asymmetric collapse. The gas-liquid interface will first be sunken into the liquid phase due to the flow field movement caused by the bubble contraction. At t = 22.1 μs and t = 31.9 μs, the high-pressure region between the gas-liquid interface and the bubble drives the formation of a jet liquid column shooting towards the air. At t = 51.5 μs, the jet liquid column moves into the air below the gas-liquid interface 6 (due to the optical path difference caused by the different refractive indices of the chip substrate and air, the liquid column is out of focus and appears as a blurred black shadow), and breaks into droplets due to Rayleigh-Plateau instability. At t = 115.2 μs, the droplet 7 moves onto the surface for observing the droplet, with a size of 22 μm. At t = 178.9 μs, it moves to the line of sight boundary. The heat-generating substrate converts the laser energy into heat energy after a period of time. At t = 715.4 μs, a new cavitation bubble is generated, and then the above-mentioned process is cycled. Finally, micro-droplets with a frequency of 1.5 kHz and a size of 22 μm are periodically generated.

[0020] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A high-frequency micro-droplet generation method based on periodic cavitation bubbles, characterized in that: Irradiate the heat - generating substrate of the microfluidic chip with a focused continuous - wave laser to generate cavitation bubbles. At the same time, by controlling the focusing position of the laser spot, the generation position of the cavitation bubbles is controlled to be near the gas - liquid interface. As the gas - liquid interface serves as the flow - field boundary, the cavitation bubbles will generate a movement away from the gas - liquid interface during the collapse process, causing the spot area to be covered by the liquid again, so that the cavitation bubbles can nucleate periodically. At the same time, during the asymmetric collapse of the bubbles, a jet liquid column towards the gas phase is formed. The jet liquid column breaks due to Rayleigh - Plateau instability to form droplets, ultimately realizing periodic droplet generation.

2. The high-frequency micro-droplet generation method based on periodic cavitation bubbles according to claim 1, characterized in that: The heat - generating substrate is a photothermal conversion substrate, including but not limited to a fused - silica sample covered with gold nanoparticles.

3. The high-frequency micro-droplet generation method based on periodic cavitation bubbles according to claim 1, characterized in that: The cavitation bubbles are generated in the liquid, and the types of liquids for generating droplets include but are not limited to water.

4. The high-frequency micro-droplet generation method based on periodic cavitation bubbles according to claim 1, characterized in that: The cavitation bubbles generated near the solid - liquid - gas three - phase contact line formed by the gas - liquid interface and the heat - generating substrate of the microfluidic chip will generate a strong liquid jet towards the gas phase during the asymmetric collapse process.

5. The high-frequency micro-droplet generation method based on periodic cavitation bubbles according to claim 1, characterized in that: The surface of the substrate of the microfluidic chip is hydrophobic - treated, and the gas - liquid interface and the substrate plane form an angle of approximately 90° to control the direction of the jet to be perpendicular to the gas - liquid interface.

6. The high-frequency micro-droplet generation method based on periodic cavitation bubbles according to claim 1, characterized in that: When the jet liquid column shoots out into the air, it is not affected by the surface tension between the solid and the liquid. Due to Rayleigh - Plateau instability, the jet breaks to form spherical droplets, and the response time for generating droplets is at the microsecond level. The liquid used to generate droplets can be in a static state without the need for directional liquid flow.

7. The high-frequency micro-droplet generation method based on periodic cavitation bubbles according to claim 1, wherein: By controlling the focusing position of the laser spot, the generation position of the cavitation bubbles is controlled, and thus the generation position of the droplets is controlled.

8. The high-frequency micro-droplet generation method based on periodic cavitation bubbles according to claim 1, characterized in that: This method controls the maximum size of the cavitation bubbles by controlling the laser power. The lower the power, the larger the bubble size, the stronger the formed jet, and the larger the droplet size. Based on this principle, the droplet size is regulated by adjusting the laser power.

9. The high-frequency micro-droplet generation method based on periodic cavitation bubbles according to claim 1, characterized in that: This method includes two working modes, namely the continuous - wave laser irradiation mode and the pulsed - wave laser irradiation mode: In the continuous - wave laser irradiation mode, changing the laser power can change the frequency of periodic generation of cavitation bubbles, thereby regulating the generation frequency of droplets; In the pulsed - wave laser irradiation mode, by modulating the laser on - and - off time, the frequency of periodic generation of cavitation bubbles can be changed, thereby regulating the generation frequency of droplets.

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