A device and a method for measuring the radius and the refractive index of a suspended single droplet

Abstract

The application discloses a kind of measuring device and measuring method of suspending single droplet radius and refractive index, belong to aerosol microdroplet spectrum analysis field;Solve the weak signal of existing optical tweezers stimulated Raman method, the technical problem that the radius and refractive index of droplet cannot be measured to PM 2.5 Scale. The device includes a single droplet suspension sample cell, a microdroplet pumping system, an atmosphere control system, an optical trap system, an optical signal detection system, an optical imaging system and an LED beam shaping system;Among them, the LED beam shaping system is communicated with the optical trap system, and the optical trap system is respectively communicated with the single droplet suspension sample cell, the optical signal detection system and the optical imaging system;Single droplet suspension sample cell is respectively communicated with microdroplet pumping system and atmosphere control system. The present application uses LED to light the suspended single droplet, and accurately obtains the radius and refractive index data of the droplet according to the stimulated elastic scattering signal, to provide a key parameterization measurement scheme for revealing the generation mechanism of aerosol in the process of atmospheric fine particulate matter pollution prevention and control.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of aerosol droplet spectrum analysis and detection technology, and particularly relates to a device and method for measuring the radius and refractive index of a suspended single droplet. BACKGROUND

[0002] PM 2.5 The formation mechanism is complex, and there are many chemical components. The key physical and chemical parameters are scarce. As a main pollution form of PM 2.5 , the aerosol droplet involves the saturated vapor pressure of semi-volatile organic compounds, the rate constant of trace gas conversion into particulate matter, the molecular diffusion coefficient of high viscosity, and the critical condition of phase separation. There is a lack of reliable experimental data. The single droplet suspension technology combined with stimulated Raman measurement can measure the radius and refractive index of aerosol droplets in real time, in situ, and dynamically under real atmospheric environmental conditions, and further deduce various physicochemical parameters of aerosol. Laser tweezers suspend single droplets, generate spontaneous Raman spectrum peaks of water molecules in the range of 3000-3700 wave numbers, and form standing waves at certain specific wavelengths to output a set of whispering gallery modes, also known as stimulated Raman spectrum. According to the positions of the stimulated Raman spectrum peaks and the Mie scattering theory, the radius and refractive index of the droplet can be determined, which is a precise direct measurement method and has been used for the measurement of various physicochemical parameters of atmospheric aerosol.

[0003] The existing single droplet suspension combined with stimulated Raman measurement has several limitations. First, the spontaneous Raman signal of the droplet is weak, and the spontaneous Raman signal is 6-8 orders of magnitude lower than the elastic scattering signal, which brings difficulties to the measurement of high-quality spectra. Second, this method is suitable for the measurement of droplets with a radius of 4-10 μm, which belongs to the size range of cloud droplets. If the droplet radius is less than 2 μm, the number of stimulated Raman peaks in the range of 3000-3700 wave numbers (half peak width ~ 25 nm) will be less than 4, and the Raman signal is also very weak, which brings difficulties to the precise fitting of the droplet radius.

[0004] The existing spontaneous-stimulated Raman measurement method is based on the whispering gallery mode formed by the droplet and coherent laser. When measuring aerosol droplets close to real atmospheric environment, when the droplet radius is as low as 1 μm, the signal-to-noise ratio of the measured spectrum is low, and the peak intensity and peak position of the stimulated Raman peak are not obvious. In addition, this method detects the stimulated Raman peak of water in the droplet, i.e. the peak shift in the range of about 620-670 cm -1 , so the detection range of the spectrum is limited.

[0005] The above reasons result in that when the measured droplet radius is as low as 1 μm, the measured spectrum quality is low, the spectrum measurement width is limited, the peak intensity and peak position are not obvious, and the accuracy of the Mie scattering inversion and fitting result is low. SUMMARY

[0006] In view of the above analysis, the present application aims to provide a device and a method for measuring the radius and the refractive index of a single droplet in suspension, so as to solve the technical problem that the radius and the refractive index of a droplet cannot be measured in the prior art. 2.5 In view of the above analysis, the present application aims to provide a device and a method for measuring the radius and the refractive index of a single droplet in suspension, so as to solve the technical problem that the radius and the refractive index of a droplet cannot be measured in the prior art.

[0007] The present application is mainly achieved by the following technical solutions:

[0008] In one aspect, the present application provides a device for measuring the radius and the refractive index of a single droplet in suspension, which comprises a single droplet suspension sample cell, a droplet pumping system, an atmosphere control system, an optical trap system, an optical signal detection system, an optical imaging system and an LED beam shaping system.

[0009] The LED beam shaping system is in communication with the optical trap system, and the optical trap system is in communication with the single droplet suspension sample cell, the optical signal detection system and the optical imaging system, respectively.

[0010] In one possible design, the droplet pumping system is used to pump aerosol into aerosol droplets and pump them into the single droplet suspension sample cell.

[0011] In one possible design, the optical trap system comprises a laser, a first half-transmission half-reflection mirror, a beam expander, a second half-transmission half-reflection mirror, a microscope objective, a third half-transmission half-reflection mirror and a mirror.

[0012] The optical trap system is used to capture and suspend a single droplet.

[0013] In one possible design, the laser is used to emit a laser beam, and the laser beam is split after passing through the first half-transmission half-reflection mirror, the beam expander and the second half-transmission half-reflection mirror, and the upward laser beam is focused by the microscope objective to form an optical trap to capture a single droplet.

[0014] In one possible design, the LED beam shaping system comprises an LED light source.

[0015] The LED beam shaping system can shape the divergent light of its LED light source into parallel light, which enters the optical trap system and can be coupled with the laser-emitted laser beam, and after coupling, the focused light by the microscope objective in the sample cell provides the backscattering signal of the suspended single droplet, and the radius and the refractive index of the droplet are directly measured according to the LED stimulated signal. The LED is lit to suspend the droplet, and the stimulated scattering signal is collected in various ways other than backscattering.

[0016] In a possible design, the optical signal detection system comprises a spectrometer, a spectral CCD detector and a data processing unit, the spectrometer being connected with the spectral CCD detector and the data processing unit respectively.

[0017] In a possible design, the optical imaging system comprises a lens and an imaging CCD;

[0018] The lens is connected with the imaging CCD; after the light beam is focused by the lens, the droplet signal is focused on the imaging CCD, and the macroscopic morphology of the suspended droplet is observed through the imaging CCD.

[0019] In a possible design, the atmosphere control system comprises a gas generating device, a pressure reducing valve and a mass flow meter; the gas generating device, the pressure reducing valve and the mass flow meter are connected in sequence.

[0020] In another aspect, the present application also provides a measurement method for the radius and refractive index of a suspended single droplet, which adopts the measurement device for the radius and refractive index of a suspended single droplet, and comprises the following steps:

[0021] Step 1: pumping the aerosol into a single droplet suspension sample pool by using a microdroplet pump-in system;

[0022] Step 2: simultaneously, starting the optical trap system and the atmosphere control system, using the laser emitted by the optical trap system as a trapping light beam to trap and suspend a single microdroplet of the aerosol in the single droplet suspension sample pool; simultaneously, introducing the required gas into the single droplet suspension sample pool through the atmosphere control system;

[0023] Step 3: starting the LED light beam shaping system and obtaining the stimulated elastic scattering signal of the microdroplet by using the LED light beam shaping system.

[0024] Further, in step 1, the single droplet diameter is ≤2.5 μm.

[0025] Compared with the prior art, the present application can at least achieve one of the following beneficial effects:

[0026] (1) The present application can autonomously change or combine the elastic scattering wavelength range by using the LED light source, and can simultaneously measure the spontaneous and stimulated elastic scattering peak value changes of the microdroplet of a specific chemical component in a wider wavelength range, so as to obtain the real-time changes of the droplet component concentration and the droplet radius.

[0027] (2) The existing measurement method is not only limited to the spontaneous and stimulated Raman peak position change of water in a single droplet. When a microdroplet with a larger radius forms a whispering gallery mode with a laser, the number of stimulated Raman peaks of water in the droplet tends to be 4-5 groups of peaks. However, the number of stimulated Raman peaks of water measured by a microdroplet with a smaller radius tends to be 1-2 groups of peaks, which greatly affects the accuracy of the Mie scattering theory inversion of the droplet radius. The present application not only widens the spectrum collection range, but also has the advantages of a large number of stimulated elastic scattering peaks and high intensity. Therefore, more clear fitting parameters can be obtained in the subsequent inversion calculation.

[0028] (3) The present application suspends the aerosol microdroplet of micron- submicron size by the optical tweezers device, and uses the stimulated elastic scattering peak (the peak displacement trajectory is obtained by identifying the stimulated elastic scattering peak, and then the radius r and the refractive index are inverted) of the stimulated elastic scattering whispering gallery mode. The peak intensity is 6 to 8 orders of magnitude higher than that of stimulated Raman scattering, has a higher signal-to-noise ratio, and the droplet radius change (i.e. dr / dt, the real-time change of radius with time) obtained by Mie theory inversion and fitting is clear and has high accuracy. Therefore, the method of measuring the droplet radius and refractive index by stimulated elastic scattering can effectively overcome the problem of low accuracy of subsequent inversion and fitting caused by the small radius of micron- submicron microdroplet, poor optical signal and poor signal-to-noise ratio of spectrum.

[0029] (4) The present application measures the radius and refractive index of the suspended droplet by using the stimulated elastic scattering signal, breaks through the limitation of the prior art, and realizes the accurate measurement of the physicochemical parameters of the droplet with PM 2.5 According to the actual needs of the experiment, different waveband LED light sources can be selected to light the suspended droplet. The elastic scattering signal is strong, which solves the problem of weak spontaneous Raman signal. The spectrum peak range of the LED light source is wide (half peak width ~ 50 nm), so that more groups of stimulated elastic scattering peaks can appear for the droplet with a radius less than 2 um, and the accurate measurement of the radius can be realized.

[0030] (5) The present application provides a new concept and method with high sensitivity and high accuracy for measuring the micron- submicron size microdroplet of atmospheric aerosol.

[0031] In the present application, the above technical solutions can be combined with each other to realize more preferred combination schemes. Other features and advantages of the present application will be described in the subsequent specification, and some advantages will become apparent from the specification, or will be understood by implementing the present application. The purpose and other advantages of the present application can be achieved and obtained by the contents specifically pointed out in the specification, examples and drawings. BRIEF DESCRIPTION OF DRAWINGS

[0032] The accompanying drawings are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments contemplated for carrying out the present application, and together with the description serve to explain the principles of the application. In the drawings:

[0033] Figure 1 Schematic diagram of the composition of the measuring device for the radius and refractive index of the droplet of the present application;

[0034] Figure 2 Morphology of the suspended single droplet of Example 2 in the microscopic imaging camera;

[0035] Figure 3 Spectrogram of the stimulated elastic scattering of the suspended single droplet of Example 2;

[0036] Figure 4 Plot of the peak shift of the stimulated elastic scattering of the suspended single droplet of Example 2 as a function of time;

[0037] Figure 5 Plot of the peak shift of the stimulated Raman scattering of the suspended single droplet of Example 2 as a function of time;

[0038] Figure 6 Plot of the radius of the ammonium chloride droplet as a function of time measured by the stimulated elastic scattering method of Comparative Example 1;

[0039] Figure 7 Plot of the radius of the ammonium chloride droplet as a function of time measured by the stimulated Raman scattering method of Comparative Example 1;

[0040] Figure 8 Lower limit value of the radius of the ammonium chloride droplet measured by the stimulated elastic scattering method of Comparative Example 2;

[0041] Figure 9 Lower limit value of the radius of the ammonium chloride droplet measured by the stimulated Raman scattering method of Comparative Example 2;

[0042] Figure 10 Plot of the peak shift and peak intensity of the radius 2.9-2.5 μm droplet measured by the stimulated elastic scattering of Comparative Example 4;

[0043] Figure 11 Plot of the peak shift and peak intensity of the radius 2.9-2.5 μm droplet measured by the stimulated Raman scattering of Comparative Example 4;

[0044] Figure 12 Plot of the peak shift and peak intensity of the radius 2.5-1.5 μm droplet measured by the stimulated elastic scattering of Comparative Example 3;

[0045] Figure 13 Plot of the peak shift and peak intensity of the radius 2.9-1.7 μm droplet measured by the stimulated Raman scattering of Comparative Example 3;

[0046] Figure 14Figure 1 is a schematic diagram of the LED beam shaping system;

[0047] Figure 15 Figure 2 is a schematic diagram of the sample cell structure of the present application;

[0048] Figure 16(a) is a schematic diagram of the structure of the droplet beam device of the present application;

[0049] Figure 16(b) is a schematic diagram of the different diameter diaphragms of the present application;

[0050] Figure 16(c) is a schematic diagram of the different diameter gaskets of the present application;

[0051] Figure 17 Figure 17 is a sectional view of the droplet beam device of the present application;

[0052] Figure 18 Figure 18 is a schematic diagram of the principle of the monodisperse effect of the droplet beam device of the present application.

[0053] Reference signs:

[0054] 1 - single droplet suspension sample cell; 101 - sample cell body; 102 - illumination light source; 1021 - outer ring wall; 1022 - inner ring wall; 1023 - single droplet inlet; 1024 - gas inlet; 1025 - gas outlet; 2 - microdroplet pumping system; 201 - aerosol generator; 2031 - pipeline; 2032 - diaphragm; 2033 - gasket; 2034 - clamp; 2035 - screw cap; 3 - atmosphere control system; 301 - gas generating device; 302 - pressure reducing valve; 303 - flow meter; 4 - optical trap system; 401 - laser; 402 - first half-transmission half-reflection mirror; 403 - beam expander; 404 - microscope objective; 405 - second half-transmission half-reflection mirror; 406 - third half-transmission half-reflection mirror; 407 - reflecting mirror; 5 - optical signal detection system; 501 - spectrometer; 502 - spectral CCD detector; 503 - data processing unit; 6 - optical imaging system; 601 - imaging CCD; 602 - lens; 7 - LED beam shaping system; 701 - LED light source; 702 - diaphragm group; 703 - lens group. DETAILED DESCRIPTION

[0055] The preferred embodiments of the present application will be described in detail below with reference to the drawings, which form a part of this application. The drawings illustrate embodiments of the application and, together with the description, serve to explain the principles of the application, and are not intended to limit the scope of the application.

[0056] In one aspect, the present application provides a device for measuring the radius and refractive index of a single droplet in suspension, as shown in Figure 1As shown, the measuring device includes a single droplet suspension sample cell 1 (optical tweezers device), a microdroplet pumping system 2, an atmosphere control system 3, an optical trap system 4, an optical signal detection system 5, an optical imaging system 6, and an LED light beam shaping system 7; wherein the LED light beam shaping system 7 is in communication with the optical trap system 4, the optical trap system 4 is in communication with the single droplet suspension sample cell 1, the optical signal detection system 5, and the optical imaging system 6, respectively; in addition, the single droplet suspension sample cell 1 is in communication with the microdroplet pumping system 2 and the atmosphere control system 3, respectively.

[0057] Specifically, in the above measuring device, the single droplet suspension sample cell 1 is a container for aerosol microdroplet suspension, which is independent, can pass background gas and reaction gas, and has a specific humidity; the microdroplet pumping system 2 is used for pumping aerosol of specific components into aerosol microdroplets and pumping into the single droplet suspension sample cell 1.

[0058] The single droplet suspension sample cell 1 includes a sample cell body 101, as shown in Figure 15

[0059] The sample cell body 101 includes a concentric outer ring wall 1021 and an inner ring wall 1022, a connecting portion is provided between the outer ring wall 1021 and the inner ring wall 1022, the connecting portion is a part of the side wall connecting the outer ring wall 1021 and the inner ring wall 1022, and a single droplet inlet 1023 penetrating the outer ring wall 1021 and the inner ring wall 1022 is provided on the connecting portion; a plurality of gas inlets 1024 and gas outlets 1025 penetrating the ring wall and spaced apart are provided on the outer ring wall 1021 and the inner ring wall 1022, and the gas inlets 1024 and the gas outlets 1025 on the outer ring wall 1021 are staggered with the gas inlets 1024 and the gas outlets 1025 on the inner ring wall 1022.

[0060] Specifically, the single droplet inlet 1023, the gas inlet 1024, and the gas outlet 1025 are all arranged along the radial direction. Moreover, the diameter of the gas inlet 1024 and the diameter of the gas outlet 1025 are both smaller than the diameter of the single droplet inlet 1023, and the diameter of the gas inlet 1024 is smaller than the diameter of the gas outlet 1025.

[0061] Exemplarily, the diameter of the single droplet inlet 1023 is 3-4 times the diameter of the gas inlet 1024, and the diameter of the single droplet inlet 1023 is 2-2.5 times the diameter of the gas outlet 1025. For example, the diameter of the single droplet inlet 1023 is 3 mm, the diameter of the gas inlet 1024 is 1.0 mm, and the diameter of the gas outlet 1025 is 1.5 mm.

[0062] It should be noted that the outer ring wall 1021 and the inner ring wall 1022 are coaxially arranged and share a bottom surface with the sample cell. Moreover, the spacing between the outer ring wall 1021 and the inner ring wall 1022 is 2-3 mm.​

[0063] In a preferred embodiment, the bottom of the sample cell body 101 is provided with a quartz glass sheet (not shown in the figure) provided with an anionic surfactant layer. The anionic surfactant, while reducing the surface tension of the spheroidal droplets, causes the droplets at the bottom of the sample cell to no longer deposit in the form of spheroids, but to spread directly on the bottom of the sample cell, avoiding measurement errors caused by the curved surface pressure of the droplets containing volatile chemical components at the bottom of the sample cell.

[0064] Illustratively, the thickness of the anionic surfactant layer is 0.1-0.2 mm.

[0065] Illustratively, the anionic surfactant is sodium fatty alcohol ether sulfate and / or sodium dodecyl sulfonate.

[0066] It should be noted that the droplets deposited on the inner wall of the sample cell cannot be directly solved by applying an anionic surfactant, because in addition to the single droplet inlet 1023, the structure of the sample cell body 101 of the present application also has a gas inlet 1024 and a gas outlet 1025. If the anionic surfactant is applied to the inner wall surface, when the humidity in the sample cell body 101 increases, water molecules and the anionic surfactant will emulsify on the inner wall of the sample cell body 101 to form foam, which may cause the hole to be blocked, and when the foam collides and accumulates to a certain weight, it will slide to the quartz glass sheet at the bottom of the sample cell. If it slides near the focusing position, it will affect the macroscopic morphology effect of the suspended droplet imaging, and if it slides to the focusing center position of the microscope objective 404, it will affect the refraction effect and cause the suspended droplet to fall off.

[0067] Compared with the prior art, the sample cell structure of the present application can make the gas uniformly enter and uniformly exit the sample cell, and ensure the stability of the gas pressure in the sample cell, thereby stably suspending the single droplet with the optical tweezers, and reducing the contact between the droplet and the wall, thereby reducing the wall effect; and the diameter of the gas inlet and the diameter of the gas outlet are both smaller than the diameter of the single droplet inlet. When the single droplet is pumped into the sample cell from the single droplet inlet, the turbulent flow effect of the aerosol droplet in the sample cell can be generated and enhanced. The swirling gas flow does not simply go from the inlet to the outlet, but tends to approach the center of the sample cell (i.e. the focusing center of the laser) by virtue of the turbulent flow effect, and the swirling effect also increases the probability of the gas flow outside the focusing center returning to the focal point, thereby increasing the capture efficiency and reducing the probability of the aerosol droplet depositing on the inner wall of the sample cell, and reducing the error influence of the wall effect on the measurement results.

[0068] The microdroplet pumping system 2 is used to pump the aerosol of a specific component into aerosol microdroplets and pump them into the single droplet suspension sample cell 1.

[0069] The atmosphere control system 3 of the present application comprises a dry gas control device and a wet gas control device; the dry gas control device and the wet gas control device both comprise a gas generating device 301, a pressure reducing valve 302 and a mass flow meter 303; wherein the gas generating device 301, the pressure reducing valve 302 and the mass flow meter 303 are connected in sequence, and the working process is as follows: the gas cylinder of the gas generating device 301 outputs the controlled flow of gas through the pressure reducing valve 302, and the controlled flow of gas obtains accurate gas mass and gas volume through the mass flow meter 303. In addition, the wet gas control device further comprises a humidification container.

[0070] The above-mentioned atmosphere control system 3 has the following effects: 1. Humidity control: by adjusting the ratio of dry gas and wet gas, the relative humidity is controlled; 2. Reaction gas control: the demand of trace gas is different for each experimental condition, and the mass flow meter 303 is used to convert the mass flow into the demand of trace gas in ppm / ppb.

[0071] The micro-droplet pumping system 3 of the present application comprises an aerosol generator 0201, a shut-off valve (not shown in the figure) and a droplet beam device (not shown in the figure) connected in sequence, and the droplet beam device is in communication with the sample cell 101. When the aerosol droplets pass through the droplet beam device, the droplet beam device changes the droplets from an aggregated state to single dispersed single string droplets which enter the sample cell one by one.

[0072] Exemplarily, as shown in FIG. 16(a), the droplet beam device 303 comprises a plurality of pipes 2031 with different inner diameters connected in sequence, and the inner diameters of the plurality of pipes 2031 gradually decrease along the aerosol flow direction.

[0073] Further, as shown in FIG. 16(b) and Figure 4 The inside of the pipe 2031 is provided with a plurality of light barriers 2032 with different inner diameters, and the inner diameters of the plurality of light barriers 2032 gradually decrease along the aerosol flow direction.

[0074] Exemplarily, Figure 17 As shown in FIG. 16(c), along the aerosol droplet flow direction, three light barriers with gradually decreasing inner diameters are sequentially arranged in the first pipe, a light barrier with an inner diameter smaller than that of the second pipe is arranged at the outlet of the second pipe, and a light barrier with an inner diameter smaller than that of the third pipe is arranged at the outlet of the third pipe. This embodiment can gradually arrange the aggregated droplets into a string and discharge them from the outlet, as shown in FIG. 16(d). Figure 18

[0075] Exemplarily, the inner diameter of the light barrier at the outlet of the third pipe is greater than or equal to the diameter of a single droplet and less than twice the diameter of a single droplet. That is, the light barrier at the outlet of the third pipe can only pass a single droplet at a time, as shown in FIG. 16(e). Figure 18

[0076] ​​Exemplarily, the diaphragm is coaxially arranged with the pipeline, and a gasket 2033 (Fig. 16(c)) is arranged between the diaphragm and the inner wall of the pipeline.

[0077] Exemplarily, a clamp 2034 is arranged at the inlet end of the first pipeline, which is used to fix the pipeline and adjust the pipeline to be aligned with the light capture point in the sample cell 101.

[0078] Exemplarily, a threaded cap 2035 with a central hole is arranged at the outlet of the third pipeline. The threaded cap 2035 is used to protect the outlet of the third pipeline.

[0079] The aerosol generator of the present application is arranged with a stop valve between the aerosol generator and the droplet beam device. When the droplets enter the droplet beam device, the stop valve can be closed, and ambient background gas is introduced into the droplet beam device to dilute the droplets in the droplet beam device, further promoting the droplet beam device to change the droplets from an aggregated state to single dispersed single string droplets entering the sample cell one by one.

[0080] In the method of the present application, after the single droplet is suspended by the optical tweezer technology, dry gas and / or wet gas is introduced into the sample cell through the gas atmosphere control system, the humidity in the sample cell is reduced to below the weathering point humidity of the chemical components in the droplet, the droplets on the inner wall of the sample cell are weathered and crystallized, and then the humidity in the sample cell is increased to the required humidity for reaction. During the weathering and crystallization process, the volatile components in the droplets deposited on the wall have been volatilized, and when the environmental humidity increases to the reaction humidity, the crystals on the wall adsorb water molecules in the environment to form a solid-liquid mixture, the liquid phase in the solid-liquid mixture is only the adsorbed water molecules, and no longer has the volatile organic components in the aerosol droplets, and the volatile components in the single droplet suspended by the optical tweezer remain unchanged during the process, thereby effectively avoiding the influence of the volatile components in the droplets deposited on the wall on the measurement of the single droplet suspended by the optical tweezer.

[0081] The optical trap system 4 of the present application includes a laser 401, a first semi-transparent half mirror 402, a beam expander 403, a second semi-transparent half mirror 405, a microscope objective 404, a third semi-transparent half mirror 406, and a mirror 407; and the working process is as follows: the laser 401 emits a laser beam, which first passes through the first semi-transparent half mirror 402, the first semi-transparent half mirror 402 is used to change the propagation angle of the laser beam, and the downward light is reflected to become a horizontal light beam, which is then expanded by the beam expander 403 to increase the diameter of the laser beam, and then passes through the second semi-transparent half mirror 405 to change the propagation angle of the laser beam, and the horizontal light is reflected to become upward and downward light beams, the upward light beam is focused to form an optical trap after passing through the microscope objective 404, and the formed optical trap is used to capture droplets.

[0082] In the optical signal detection system 5, it comprises a spectrometer 501, a spectral CCD detector 502 and a data processing unit 503; the spectrometer 501 is connected with the spectral CCD detector 502 and the data processing unit 503 respectively; the working process is as follows: the light beam is focused on the slit of the spectrometer 501, then enters the spectrometer 501, the spectrometer 501 detects the light beam through the spectral CCD detector 502, then returns to the spectrometer 501, and the spectrometer 501 is converted to the data processing unit 503 after signal processing to output the spectrum through software.

[0083] It should be pointed out that the detector for spectral measurement includes other detectors in addition to the spectral CCD detector; hereinafter will not be enumerated one by one.

[0084] In the optical imaging system 6, it comprises a lens 602 and an imaging CCD 601, the lens 602 is connected with the imaging CCD 601; the working process is as follows: the light beam is focused on the lens 602, then the droplet signal is focused on the imaging CCD 601, then the macroscopic morphology of the suspended droplet is observed through the imaging CCD 601 (such as Figure 2

[0085] The LED light beam shaping system 7 comprises an LED light source 701, an aperture group 702 and a lens group 703, which changes the divergent light source of the LED light source 701 into a parallel light beam, the parallel light enters the optical trap system 4 and can be coupled with the laser beam emitted by the laser 401, after coupling, the micro-objective 404 focuses on the single droplet suspended in the sample cell 1 to provide the backscattering elastic scattering signal of the microdroplet, and the radius and refractive index of the droplet are directly measured according to the LED stimulated signal.

[0086] Specifically, it is crucial to determine the uniformity of the light beam profile on its propagation distance, the LED non-uniform light beam and profile emitted by the LED light source 701 are converted into a circular profile and are homogenized by using the lens 602, since the LED has different divergence angles in the x-axis and y-axis without collimating optical elements, therefore, the aperture and the lens group 703 are used to change the circular profile into a parallel light beam with small divergence angle. At the same time, the net aperture of each lens 602 must be greater than the corresponding maximum beam width to avoid cutting off the light beam. The relative position and distance between the aperture and the lens 602 in the LED light beam shaping system 7 can be freely adjusted, Figure 14 For one of the ways, finally it is necessary to ensure that the divergence angle and the spot divergence angle of the LED light beam are smaller than the divergence angle and the spot divergence angle before the light beam shaping.

[0087] ​When the measuring device is implemented, the microdroplet pump-in system 2 pumps the aerosol of a specific component into the single-droplet suspension sample cell 1; the laser emitted by the optical trap system 4 acts as a trapping beam, and a single microdroplet is trapped and suspended in the single-droplet suspension sample cell 1 by constructing an optical trap; after the microdroplet is suspended, the required gas is introduced into the single-droplet suspension sample cell 1 through the atmosphere control system 3; at the same time, the LED beam shaping system 7 is turned on, the LED beam shaping system 7 shapes the divergent light emitted by the LED light source 701 into parallel light through the diaphragm group 702 and the lens group 703, the parallel light enters the optical trap system 4 and is coupled with the laser beam emitted by the laser 401, and after the coupling, the focused light is focused on the single-droplet suspension sample cell 1 through the microscope objective 404 to obtain the stimulated elastic scattering signal of the microdroplet, the stimulated elastic scattering signal returns to the optical trap system 4 in a backscattering manner, and then is focused on the optical signal detection system 5 and the optical imaging system 6, the spectrometer 501 in the optical signal detection system 5 can convert the obtained droplet optical signal (spectrum information) into droplet radius, droplet refractive index and droplet peak position information (as shown in Figure 6 and Figure 7 ) after processing by the data processing unit 503 thereof; the optical imaging system 6 is used for focusing the obtained droplet radius, droplet refractive index and droplet peak position information on the imaging CCD 601, and then observing the macroscopic morphology of the suspended droplet through the imaging CCD 601. The real-time and in-situ change information of the radius and refractive index values of the obtained single droplet of a specific component (the single droplet of a specific component refers to a configured droplet, including an ammonium sulfate droplet, an ammonium sulfate + ammonium bisulfate droplet or an ammonium chloride droplet) before and after reaction is inversed, and then the droplet radius change rate dr / dt is obtained according to the redshift or blueshift of the measured droplet stimulated Raman peak, so that the measurement of the parameters of the mass growth factor of the hygroscopic process of the suspended droplet, the saturated vapor pressure of the volatilization process, the diffusion coefficient of the mass transfer diffusion process, the critical humidity of the liquid-liquid phase separation process and the rate constant of the chemical reaction kinetics process is realized.

[0088] Compared with the prior art, the measuring device of the present application adopts a suspended droplet close to the microdroplet (microdroplet size ≤2.5 μm) of the aerosol size in the real atmospheric environment, the microdroplet radius in the real environment is smaller, PM2.5 is a microdroplet with an equivalent diameter of 2.5 μm, and the method can suspend smaller microdroplets close to the real atmospheric environment. The present application utilizes the resonance peak of the stimulated elastic scattering whispering gallery mode, i.e. the stimulated elastic peak, which is obviously distinguished from the spontaneous elastic scattering, the signal-to-noise ratio of the stimulated elastic scattering signal of the measured single droplet is good, and the droplet radius change obtained through the Mie theory inversion and fitting is clear and has high accuracy. Therefore, the measurement of the droplet radius and refractive index by using the stimulated elastic scattering can effectively overcome the problem of low accuracy in subsequent inversion and fitting caused by the small micro- sub-micron level microdroplet radius, poor optical signal and poor spectrum signal-to-noise ratio.

[0089] It should be noted that when collecting the stimulated elastic scattering signal of the LED-illuminated suspended liquid droplet, it is not limited to backscattering collection, but also includes other various stimulated scattering signal collection methods.

[0090] The application also provides a method for accurately measuring the radius and refractive index of a suspended single liquid droplet, which comprises the following steps:

[0091] Step 1: Pumping the aerosol of a specific component into the single-droplet suspension sample pool 1 by using the microdroplet pump-in system 2.

[0092] Step 2: At the same time, turn on the optical trap system 4 and the atmosphere control system 3, use the laser emitted by the optical trap system 4 as the trapping beam to capture and suspend a single microdroplet of the aerosol in the single-droplet suspension sample pool 1; at the same time, introduce the required gas into the single-droplet suspension sample pool 1 through the atmosphere control system 3.

[0093] In the above step 2, the laser beam emitted by the laser 401 in the optical trap system 4 is focused by the microscope objective 404 to construct an optical potential well with a light gradient force, and the light intensity in the optical potential well is gradiently distributed, so that the single microdroplet of the aerosol is stably captured and suspended by using the light gradient force.

[0094] Compared with the prior art, the method for accurately measuring the radius and refractive index of a suspended single liquid droplet is a direct measurement method. According to the whispering gallery mode of the stimulated elastic scattering of the droplet, the radius and refractive index results can be directly presented on the basis of the Mie scattering theory.

[0095] In the above step 2, the laser beam emitted by the laser 401 of the optical trap system 4 is expanded by the first half-mirror 402 and the expander 403, and then split into two beams of upward and downward laser beams by the second half-mirror 405, wherein the upward laser beam passes through the microscope objective 404 to construct the optical trap.

[0096] The above laser light source can be selected according to requirements, including solid-state laser 401, gas laser 401, semiconductor laser 401, dye laser 401 and optical fiber suitable for the light source required by the optical tweezers suspended liquid droplet.

[0097] In step 2, the environment gas and trace reaction gas are introduced into the suspended single liquid droplet through the pressure reducing valve 302 and the flow meter 30303 by the atmosphere control system 3 to simulate the real atmospheric environment of the external field and control the humidity.

[0098] In step 2, the atmosphere control system 3 controls the proportion of dry gas, wet gas and reaction gas into the single droplet suspension sample cell 1 to simulate the trace reaction gas environment and relative humidity (60%-90% RH) in the actual atmospheric environment, and records the humidity and mass concentration of the mixed gas in real time.

[0099] In step 3, the LED beam shaping system 7 is turned on or the LED beam shaping system 7 is used to obtain the stimulated elastic scattering signal of the microdroplet.

[0100] In the above step 3, the LED beam shaping system 7 is turned on at the same time when the optical trap system 4 is turned on.

[0101] In the above step 3, after a single microdroplet is captured and suspended, the LED beam shaping system 7 is turned on, the LED beam shaping system 7 shapes the divergent light of the LED light source 701 into parallel light into the optical trap system 4 and couples with the laser beam emitted by the laser 401, is focused on the single droplet suspension sample cell 1 through the microscope objective 404 to obtain the stimulated elastic scattering signal of the microdroplet, the stimulated elastic scattering signal returns to the optical trap system 4 in a backscattering manner, and then is focused on the spectrometer 501 of the optical signal detection system 5 (the spectrometer 501 can collect spectral data) and the optical imaging system 6, and the dynamic changes of the droplet radius and refractive index are obtained under the specific atmosphere condition through the stimulated elastic scattering whispering gallery mode and the Mie scattering theory inversion.

[0102] In the above step 3, the LED light source 701 is totally reflected in the droplet cavity to form a standing wave, that is, an optical whispering gallery mode (WGM) resonant cavity, and the LED light source 701 forms a stimulated elastic scattering signal at a specific wavelength after total reflection in the droplet cavity, and the signal intensity is higher than that of the elastic scattering signal.

[0103] In the above step 3, the wavelength coverage range of the light source of the LED beam shaping system 7 is wide, including ultraviolet light, blue light, green light, yellow light, red light, infrared light and white light, the LED light source 701 is in communication with the diaphragm group 702 and the lens group 703 and is coupled with the optical trap light path. The optical axis of the LED light source 701 is coaxial with the optical axis of the laser light path and is incident into the microscope objective 404, and the spot size and intensity can be adjusted according to specific needs.

[0104] Compared with the prior art, by selecting the wavelength band of the LED light source at the top of the LED beam shaping system 7, high-quality stimulated elastic scattering spectrum can be obtained, the stimulated elastic scattering measurement of fine particulate matter (PM 2.5 ) with an aerodynamic diameter less than or equal to 2.5 μm is realized, and the radius and refractive index information thereof is accurately obtained.

[0105] In step 3, the single droplet carries the elastic scattering signal from the LED light source 701, and the standing wave is formed in the droplet by using the whispering gallery mode to produce stimulated elastic scattering signal, and the radius and refractive index of the droplet can be obtained by Mie scattering theory, and the radius change rate dr / dt and the refractive index of the droplet of a specific component in the preset atmosphere environment are obtained through fitting, and then the mass growth factor of the hygroscopic process of the droplet, the saturation vapor pressure of the evaporation process, the diffusion coefficient of the mass transfer diffusion process, and the rate constant of the chemical reaction kinetics process are calculated by the radius change rate dr / dt.

[0106] It should be noted that the optical imaging system 6 can simultaneously complete the spectral detection and optical imaging of the single droplet by using the backscattering stimulated elastic scattering signal formed by the LED light source 701 together with the spectral signal detection system.

[0107] In addition to the backscattering measurement by using the above-mentioned device, the stimulated elastic scattering measurement method of the suspended droplet also includes other stimulated elastic scattering signal collection methods and devices at any angle.

[0108] Compared with the prior art, the single suspended droplet is inversed based on the whispering gallery mode of the stimulated elastic scattering and the Mie scattering theory, and the real-time dynamic changes of the radius and refractive index of the droplet of a specific component are obtained. By obtaining the real-time dynamic changes of the radius and refractive index of the droplet, real-time change references for calculating the droplet dynamics and physicochemical parameters can be provided, and experimental errors can be reduced.

[0109] The measurement principle of the measurement method of the present application is as follows: the single droplet is captured by using the single droplet suspension sample cell 1 (optical tweezers device) and the optical trap system 4, the LED light source 701 is used to light the suspended droplet and measure the elastic scattering signal of the droplet LED, the elastic scattering light of a specific wavelength forms a standing wave in the droplet, that is, a whispering gallery mode, and a group of resonance peaks of the whispering gallery mode, that is, stimulated elastic scattering peaks, are obtained, which are superimposed on the elastic scattering peaks generated by the continuous LED light source. According to the position of the stimulated elastic scattering peak, the radius and refractive index of the droplet are calculated by using the Mie scattering theory. The optical tweezers device captures single droplets containing different chemical components by using the gradient force of laser, and the single droplets and the surrounding atmosphere undergo dynamic processes such as gas-particle distribution, gas-particle equilibrium, and gas-particle transformation. The stimulated elastic scattering signal of the LED can provide real-time and in-situ radius and refractive index change information, so as to realize the measurement of the mass growth factor of the hygroscopic process of the suspended droplet, the saturation vapor pressure of the evaporation process, the diffusion coefficient of the mass transfer diffusion process, the critical humidity of the liquid-liquid phase separation process, and the rate constant of the chemical reaction kinetics process.

[0110] In summary, the aerosol microdroplet of micron-submicron size is suspended by the optical tweezers device, the stimulated elastic scattering peak of the resonant peak of the stimulated elastic scattering echo wall mode is used (the peak displacement trajectory is obtained by identifying the stimulated elastic scattering peak, and the radius r and the refractive index are inversely calculated), the peak intensity is 6 to 8 orders of magnitude higher than that of stimulated Raman scattering, the signal-to-noise ratio is higher, and the droplet radius change (i.e. dr / dt, the real-time change of radius with time) obtained by the Mie theory inversion and fitting is clear and has high accuracy. Therefore, the method of measuring the droplet radius and refractive index by stimulated elastic scattering can effectively overcome the problem of low inversion and fitting accuracy caused by the small micron-submicron microdroplet radius, poor optical signal and poor signal-to-noise ratio of the spectrum.

[0111] Compared with the prior art, the LED light of the present application generates stimulated elastic scattering, and the stimulated elastic scattering signal of the droplet is measured. When the droplet radius is very small, the traditional method cannot measure the optical signal, and the method can also have a good optical signal and can be detected. In addition, the method provided by the present application can directly measure the important physical and chemical parameters (droplet radius and refractive index) of atmospheric aerosol, can realize accurate measurement and joint characterization of key physical and chemical parameters in the evolution process of atmospheric particulate matter, and provides a key parameterized measurement scheme for revealing the generation mechanism of atmospheric fine particulate matter in the process of atmospheric fine particulate matter pollution prevention and control.

[0112] In the following, the method for accurately measuring the droplet radius and refractive index by stimulated elastic scattering of the present application is further described by specific embodiment modes.

[0113] Embodiment 1

[0114] The embodiment provides a stimulated elastic scattering method for accurately measuring the droplet radius and refractive index, including the device of embodiment 1, including:

[0115] The microdroplet pump-in system 2 pumps the aerosol of a specific component into the single-droplet suspension sample cell 1, at this time the laser beam emitted by the laser 401 in the optical trap system 4 is focused by the microscope objective 404 to construct an optical potential well, and the aerosol single microdroplet is stably captured and suspended by using the optical gradient force. The atmosphere control system 3 controls the environment gas and trace reaction gas through the pressure reducing valve 302 and the flow meter 30303, and simulates the external field real atmospheric environment atmosphere and controls the humidity of the suspended single droplet.

[0116] The laser 401 of the optical trap system 4 emits a laser beam, which is expanded by the half-mirror 402 and the beam expander 403, and then split into two beams by the half-mirror 405. One of the two beams is used to construct an optical trap by the microscope objective 404, and the other beam is split by the half-mirror 406 and input into the optical signal detection system 5, and then changes direction by the reflecting mirror 407 and enters the optical imaging system 6.

[0117] The LED beam shaping system 7 shapes the divergent light of the LED light source 701 into parallel light to couple into the optical trap system 4 and the laser beam, and focuses on the single droplet suspension sample cell 1 through the microscope objective 404 to obtain the stimulated elastic signal of the droplet, which returns to the optical trap system 4 in a backscattering manner and is then focused on the optical signal detection system 5. The characteristic peak displacement change of the suspended single droplet is analyzed according to the stimulated elastic scattering signal data, and then the numerical change of the droplet refractive index and radius in the reaction process is obtained by inversion using Mie scattering theory.

[0118] The atmosphere control system 3 controls the proportion of dry gas, wet gas and reaction gas entering the single droplet suspension sample cell 1, so as to simulate the trace reaction gas environment and relative humidity (60%-90% RH) in the actual atmospheric environment, and record the humidity and mass concentration of the mixed gas in real time.

[0119] In implementation, the microdroplet pump-in system 3 pumps the aerosol into the sample cell 101 of the single droplet suspension system 1 in the form of single droplets; the single droplet suspension system 1 includes a sample cell 101, as shown in Figure 2 The sample cell 101 includes a concentric outer ring wall 1021 and an inner ring wall 1022, and a connecting portion is arranged between the outer ring wall 1021 and the inner ring wall 1022, which is part of the side wall connecting the outer ring wall 1021 and the inner ring wall 1022. The connecting portion is provided with a single droplet inlet 1023 penetrating the outer ring wall 1021 and the inner ring wall 1022; the outer ring wall 1021 and the inner ring wall 1022 are both provided with a plurality of gas inlets 1024 and gas outlets 1025 penetrating the ring wall and arranged at intervals, and the gas inlets 1024 and the gas outlets 1025 on the outer ring wall 1021 are staggered with the gas inlets 1024 and the gas outlets 1025 on the inner ring wall 1022.

[0120] Specifically, the single droplet inlet 1023, the gas inlet 1024 and the gas outlet 1025 are all arranged along the radial direction. Moreover, the diameter of the gas inlet 1024 and the diameter of the gas outlet 1025 are both smaller than the diameter of the single droplet inlet 1023, and the diameter of the gas inlet 1024 is smaller than the diameter of the gas outlet 1025.

[0121] Figure 2 The figure is the morphology of the suspended single droplet in Example 1 in the microscopic imaging camera, which shows that a single microdroplet is stably captured at the focal point of the optical tweezer; Figure 3 The figure is the stimulated elastic spectrum of the suspended single droplet in Example 1.

[0122] Example 2

[0123] The method of detecting the radius and refractive index of the droplet by stimulated elastic scattering (Method 1) Figure 4 ) and the detection method by stimulated Raman scattering (Method 2)Figure 5 Compared to the previous method, the relationship between the characteristic peak shifts and time in the acquired spectral data shows significant differences. Figure 4 The shift in peak position can be clearly seen in the middle, while Figure 5 Despite the continuous linear trend, a large number of noise points are still identified, which will make it impossible to accurately calculate dr / dt and droplet radius values ​​in subsequent numerical judgments based on Mie scattering inversion.

[0124] Comparative Example 1

[0125] By finding the peaks, the displacement trajectory of the characteristic peak position is obtained, and the linear relationship between the droplet radius dr / dt-t is obtained (e.g., Figure 6 and Figure 7 As shown in the figure, this set of data can determine the droplet radius growth rate, i.e., the volatilization rate of ammonium chloride, and the numerical value of droplet radius change during the reaction process. Figures 6-7 A comparison of stimulated elastic scattering (SES) and stimulated Raman scattering (SRS) reveals that, under the same ambient humidity of 80%, the SES data points exhibit a continuous and dense pattern of dr / dt, while the SRS data points show discontinuities and sparseness. This result may be because, when detecting droplet optical signals, the elastic scattering spectral bandwidth of the LED light source 701 can accommodate and highlight more SES resonance peaks, and the spontaneous Raman signal is 6-8 orders of magnitude lower than the SES signal. Therefore, the SES signal measured in this invention may have better potential in detecting physicochemical parameters such as the radius and refractive index of atmospheric aerosol droplets.

[0126] Comparative Example 2

[0127] For droplets containing volatile components, such as NH4Cl, the change in droplet radius before and after evaporation was screened by refractive index range. Comparative examples show the optical signals detectable by a 501CCD502 spectrometer after NH4Cl evaporation, measured using both stimulated elastic scattering and stimulated Raman scattering methods. The smallest detectable droplet radius after evaporation was obtained by Mie scattering fitting and inversion. Figure 8 and Figure 9 As shown, stimulated elastic scattering can detect a minimum droplet radius of 1582.9 nm, while stimulated Raman scattering can detect a minimum droplet radius of 2523.0 nm. This result indicates that the droplet radius detected by stimulated elastic scattering can break through the detection limit of the original method.

[0128] Comparative Example 3

[0129] The stimulated signals of NH4Cl volatile component droplets with a radius of 2.9-2.5 μm are detected by stimulated elastic scattering and stimulated Raman scattering respectively, and the peak shift trajectory and peak intensity are compared. Figure 10 and Figure 11 As shown in the figures, the peak shift trajectory of stimulated elastic scattering is obviously distinguished from the background information, and both the peak intensity and the peak shift have obvious advantages compared with stimulated Raman scattering. The results show that the stimulated elastic scattering has the advantages of high intensity and good optical signal, and the measurement results obtained by Mie scattering inversion are obviously improved compared with the previous measurement methods.

[0130] Comparative Example 4

[0131] The stimulated signals of NH4Cl volatile component droplets with a smaller radius are detected by stimulated elastic scattering and stimulated Raman scattering respectively, and the peak shift trajectory and peak intensity are compared. Figure 12 and Figure 13 As shown in the figures, the shift trajectory and peak intensity of the resonance peak can be better distinguished, fitted and inverted for the radius and refractive index. Based on this, the minimum droplet radius range of NH4Cl that can be detected by stimulated elastic scattering fitting inversion is 2.5-1.5 μm, which is better than the detection lower limit 2.5 μm of stimulated Raman scattering. Therefore, the method of directly measuring the droplet radius and refractive index by stimulated elastic scattering is better. Therefore, the droplet radius and refractive index directly measured by the stimulated elastic scattering method of the present application can break through the detection limit of the original technology, that is, the detection range of the droplet radius and refractive index measured by the present application is widened, and the measurement result error is smaller.

[0132] The above is only a preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A device for measuring the radius and refractive index of a suspended single droplet, characterized by, The LED beam shaping system is in communication with the optical trap system, and the optical trap system is in communication with the single droplet suspension sample pool, the optical signal detection system and the optical imaging system respectively; meanwhile, the single droplet suspension sample pool is in communication with the microdroplet pumping system and the atmosphere control system respectively; The optical trap system includes a laser, a first half-transmission half-reflection mirror, a beam expander, a second half-transmission half-reflection mirror, a microscope objective, a third half-transmission half-reflection mirror and a mirror; The optical trap system is used for capturing and suspending single droplets; The laser is used for emitting a laser beam, and the laser beam is split after passing through the first half-transmission half-reflection mirror, the beam expander and the second half-transmission half-reflection mirror, and the upward laser beam forms an optical trap after being focused by the microscope objective to capture single droplets; The LED beam shaping system includes an LED light source; The LED beam shaping system can shape the divergent light of the LED light source into parallel light, and the parallel light enters the optical trap system and can be coupled with the laser emitted laser beam, and after coupling, the laser beam is focused in the sample pool by the microscope objective to suspend single droplets, the LED is lit to suspend single droplets, and the excited elastic scattering signal of the single droplets is collected. The microdroplet pumping system is used for pumping aerosol into aerosol microdroplets and pumping them into the single droplet suspension sample pool.

2. The device for measuring the radius and the refractive index of a suspended single droplet according to claim 1, wherein The optical signal detection system includes a spectrometer, a spectral CCD detector and a data processing unit, and the spectrometer is connected with the spectral CCD detector and the data processing unit respectively.

3. The apparatus according to claim 1, wherein The optical imaging system includes a lens and an imaging CCD; 4. The apparatus according to claim 1, wherein The lens and the imaging CCD are connected; after the light beam is focused by the lens, the droplet signal is focused on the imaging CCD, and the macroscopic morphology of the suspended droplets is observed through the imaging CCD. The atmosphere control system includes a gas generating device, a pressure reducing valve and a mass flow meter; the gas generating device, the pressure reducing valve and the mass flow meter are connected in sequence.

5. The apparatus according to claim 1, wherein The measurement method of the radius and refractive index of the suspended single droplet includes the following steps:

6. A method of measuring the radius and refractive index of a suspended single droplet, characterized by, Step 1, pumping aerosol into the single droplet suspension sample pool by using the microdroplet pumping system; Step 2, at the same time, turn on the optical trap system and the atmosphere control system, use the laser emitted by the optical trap system as the trapping beam, capture and suspend single microdroplets of aerosol in the single droplet suspension sample pool; at the same time, introduce the required gas into the single droplet suspension sample pool through the atmosphere control system; Step 3, turn on the LED beam shaping system and obtain the excited elastic scattering signal of the microdroplets by using the LED beam shaping system. In the step 1, the diameter of the single droplet is ≤2.5 μm.

7. The method of measuring the radius and refractive index of a suspended single droplet according to claim 6, wherein ​

Images