A method for measuring the freezing time of microdroplets

Abstract

This invention relates to a method for measuring the freezing time of microdroplets. For microdroplets on a smooth solid surface, the formation process is captured by filming at a rate of 0.01 s / frame. MATLAB is used for image processing. Within the resolution range of the filming, based on the study of the brightness change of the central spot and the formation of a condensation ring during the microdroplet freezing process, the direction of grayscale change is determined. The grayscale intensity changes of the central bright spot and the grayscale intensity changes of the condensation ring region during the freezing process of droplets on a smooth surface under different experimental conditions are obtained. This allows for the determination of the freezing time of the droplets on the smooth surface, as well as the start and end times of freezing, and the establishment of a physical model of the central bright spot. The experimental results are compared with theoretical calculations. The advantages are: this invention provides a simple and intuitive method for measuring the freezing time of microdroplets, is pollution-free throughout the process, has low measurement costs, uses simple equipment, and has a simple procedure.

Description

Technical Field

[0001] This invention relates to the field of microdroplet freezing calculation, and more particularly to a method for measuring microdroplet freezing time. Background Technology

[0002] Icing is a common phenomenon in nature and industry. On hydrophobic, smooth surfaces, liquid water exists in the form of crown-shaped droplets. If the temperature of the cold surface is below the freezing point of water, the droplets will be in a supercooled state and have a probability of freezing. The phase change freezing of supercooled droplets is significantly different from the phase change freezing of water in a steady state. Macroscopic freezing / frost in production and daily life is actually a process in which countless droplets interact with the wall surface to freeze and gradually accumulate to form a macroscopic ice layer. However, for supercooled droplets in a metastable state, freezing occurs first and then manifests. The condensation and icing process begins with the formation of an ice shell, which then slowly completes the icing process from the side closest to the cold surface due to energy limitations. Icing is a common phenomenon in aviation. When an aircraft flies at high altitudes, droplets in the clouds collide with the wings and ic up. If a thick ice layer forms, it will change the laminar flow on the wing surface into turbulence and cause vortex separation at the tail of the wing. This will reduce the aircraft's lift, cause severe turbulence, reduce the stall angle of attack, and greatly increase the risk of air crashes. Therefore, studying the condensation and icing process has important practical application value.

[0003] The freezing process of droplets on a solid surface mainly begins with nucleation and ends with tip formation. The entire duration of this process is called the freezing time. Current methods for observing the freezing time of droplets mainly involve recording and determining the time difference between nucleation and tip formation using an optical camera. However, this measurement method is difficult to clearly observe the freezing process and time of droplets with even smaller micrometer-sized volumes. Therefore, if we want to continue to measure the freezing time directly, the camera performance requirements are extremely demanding, and the lighting methods and shooting procedures are very cumbersome. Summary of the Invention

[0004] The present invention provides a method for measuring the freezing time of microdroplets, primarily targeting the freezing process of microdroplets on smooth surfaces. This method is mainly based on experimental measurements to obtain the freezing time of microdroplets under different conditions. The method includes:

[0005] Step S1: Obtain the generated microdroplets;

[0006] Step S2: Use a speed of 0.01s / frame to film the above microdroplet formation process. Place the camera perpendicular to the sample surface and use coaxial lighting to illuminate the surface from inside the camera lens. The light is reflected from the surface to produce a clear image inside the camera.

[0007] Step S3: Use MATLAB for image processing. Within the resolution range of the image capture, based on the study of the brightness change of the central spot and the formation of condensation rings during the freezing process of microdroplets, determine the direction of grayscale change and obtain the grayscale intensity change of the central bright spot and the grayscale intensity change of the condensation ring region during the freezing process of droplets on a smooth surface under different experimental conditions. Thus, the freezing time of droplets on a smooth surface and the start and end times of freezing are obtained, and a physical model of the central bright spot is established.

[0008] Step S4: Obtain experimental results based on image processing, and determine the freezing time of the droplet by analyzing and judging the start and end measurement nodes of the droplet freezing according to the two gray intensity change laws. Calculate the freezing time of the supercooled droplet using the theoretical formula, and compare the experimental results with the theoretical formula calculation results.

[0009] Furthermore, the specific process of measuring the start and end points of droplet freezing time on a smooth surface is as follows:

[0010] First, a preliminary judgment is made on the image to determine the size of the central bright spot and the selection of the condensation ring region;

[0011] After determining the selected computational region, MATLAB was used to calculate the gray value change over time within the two regions and observe the change in average gray value within the two regions with the freezing process. This allowed us to obtain the start and end points of the droplet freezing time measurement on the smooth surface. Specifically, the average gray value change in the condensation ring region starts from the beginning of ice nucleation until the freezing is complete and reaches its minimum value, or the average gray value change in the droplet center region starts from the gray value intensity jump and reaches a stable stage.

[0012] Furthermore, the specific process of comparing the experimental results with the theoretical formula calculations is as follows:

[0013] S4l. Obtain the freezing time of the droplets in the experiment based on the measurement start and end points of the freezing time of the droplets in the experiment;

[0014] S42. Obtain the formula for calculating droplet volume: V = fR 3 Since a droplet maintains a stable clamping relationship with a smooth surface, the angle between the solid-liquid interface, the liquid interior, and the gas-liquid interface is called the contact angle θ. c ,in R is the lateral radius of the droplet;

[0015] S43. Theoretical formula for the freezing time of supercooled droplets Perform deformation to obtain Where ρ i It is the mass of ice, L m It is the latent heat of the ice-water mixture, H is the final height of the droplet, and k is the latent heat of the mixture.i It is the thermal conductivity, ΔT is the subcooling, and the droplet height H is related to the droplet radius R. d They are directly proportional;

[0016] S44. Compare the freezing time of the droplets in the experiment with the freezing time calculated by the theoretical formula.

[0017] Furthermore, the physical model for establishing the central bright spot specifically includes:

[0018] The light spot at the center of the droplet was observed due to refraction.

[0019] The relative intensity distribution of light in a droplet was quantified using Snell's law, and the intensity distribution on the cross-section of the droplet was calculated based on the symmetry of the droplet's top view.

[0020] The relative intensity distribution of light as a function of the distance from the droplet center under theoretical and practical conditions was obtained, and the image of the droplet under parallel light was simulated.

[0021] Compare the simulated images with the experimentally observed images.

[0022] Furthermore, the microdroplets were obtained using a condensation method. Specifically, a smooth sample surface was first prepared, and then the sample surface was placed on a cooler. The relative humidity of the condensation environment was controlled by adjusting the water vapor inlet rate using a mass flow meter and a wide-mouth bottle. Once the relative humidity stabilized, the temperature of the cooler was adjusted to the target temperature, and a large number of microdroplets formed on the sample surface, resulting in freezing.

[0023] Beneficial effects:

[0024] 1. A simple and intuitive method is provided to measure the freezing time of condensate droplets, with no pollution throughout the process and low measurement cost;

[0025] 2. The device is simple to use and the process is convenient. It provides a new measurement method that can effectively measure the freezing time of microdroplets on the surface. It avoids the various harsh conditions in existing methods for measuring the freezing time of micron-sized microdroplets, such as high requirements for camera performance and complex measurement process. Attached Figure Description

[0026] Figure 1 This is a process diagram of two freezing processes of microdroplets, which is a method for measuring the freezing time of microdroplets;

[0027] Figure 2 This is a graph showing the change in grayscale intensity of the condensation ring as a function of distance at different time intervals, which is a method for measuring the freezing time of microdroplets.

[0028] Figure 3This is a graph showing the change of average gray intensity over time in two regions of a microdroplet freezing time measurement method.

[0029] Figure 4 This is a physical model diagram of the central droplet in a microdroplet freezing time measurement method;

[0030] Figure 5 This is a comparison graph between the freezing time obtained by measuring grayscale changes and the theoretical freezing time in a microdroplet freezing time measurement method. Detailed Implementation

[0031] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0032] like Figure 1-5 The method for measuring the freezing time of microdroplets shown includes:

[0033] Step S1: Obtain the generated microdroplets. In this embodiment, the condensation method is used to obtain them, but other methods can also be used instead. First, a smooth sample surface is prepared, and then the sample surface is placed on a cooler. The relative humidity of the experimental environment is controlled by adjusting the water vapor inlet rate using a mass flow meter and a wide-mouth bottle. When the relative humidity is stable, the temperature of the cooler is adjusted to the target temperature. A large number of microdroplets are formed on the sample surface and freezing occurs.

[0034] Step S2: Use a speed of 0.01s / frame to film the above microdroplet formation process. Place the camera perpendicular to the sample surface and use coaxial lighting to illuminate the surface from inside the camera lens. The light is reflected from the surface to produce a clear image inside the camera.

[0035] Step S3: Use MATLAB for image processing. Within the resolution range of the image capture, based on the study of the brightness change of the central spot and the formation of condensation rings during the freezing process of microdroplets, determine the direction of grayscale change, obtain the grayscale intensity change of the central bright spot and the grayscale intensity change of the condensation ring region during the freezing process of droplets on a smooth surface under different experimental conditions, determine the start and end points of the measurement of the freezing time of droplets on a smooth surface, and establish a physical model of the central bright spot to facilitate the analysis and description of the cause of this phenomenon.

[0036] There are two main triggering mechanisms for icing on smooth surfaces; please refer to [link / reference] for details. Figure 1 These are spontaneous freezing and ice bridge-triggered freezing, respectively. Figure 1 a and Figure 1Image b shows images of spontaneous icing and ice-bridge-triggered icing processes. The images of the droplet icing process show that when a droplet ices, a ring of condensed droplets forms around it and then disappears, called a condensation ring. The bright spot at the center of the droplet changes due to the different refractive indices before and after icing. The changes in grayscale intensity of the condensation ring and the bright spot at the center of the droplet can be obtained through image processing using MATLAB. Specifically...

[0037] First, we will perform a preliminary assessment of the image, determining the size of the central bright spot and the selection of the condensation ring region. The central bright spot of the droplet can be determined based on the initial size of the light spot, but the condensation ring, due to the interference of surrounding droplets, is an irregular region, making it relatively more difficult to select a calculation area. Since the freezing process is very short, we can assume that the grayscale change in the condensation ring region during this process is only affected by the formation of the condensation ring. Therefore, we can analyze the grayscale change range and refer to... Figure 2 Quantitatively select regions;

[0038] After determining the computational region, MATLAB was used to calculate the grayscale changes over time within the two regions. (Refer to...) Figure 3 By observing the change in average grayscale value in two regions during the freezing process, the change in average grayscale value in the condensation ring region shows that from the beginning of ice nucleation, the formation of the condensation ring causes a sudden change in grayscale intensity until freezing is complete and the grayscale value reaches its minimum. The figure shows that the freezing time of this measurement method is 0.26 s. By calculating the time difference between the sudden change in grayscale value and the extreme value, the freezing process time can be indirectly reflected. The change in average grayscale value in the droplet center region shows that at the beginning of freezing, reglow causes a sudden change in grayscale intensity, followed by a slow change until freezing is complete and a stable state is reached. The freezing time is obtained by measuring the time difference between the initial grayscale intensity jump at the droplet center and the stable stage. The figure also shows that the freezing time of this measurement method is 0.26 s.

[0039] Step S4: Based on image processing, the experimental results are obtained. The freezing time of the droplet is determined by analyzing the two grayscale intensity variation patterns to identify the initial and final measurement nodes of the droplet freezing. The freezing time of the supercooled droplet is then calculated using the theoretical formula. The experimental results are compared with the theoretical calculation results. Specifically,

[0040] S41. The freezing time of the droplets in the experiment is obtained from the measurement start and end points of the freezing time of the droplets in the experiment. That is, the freezing time is obtained by measuring the average gray value change in the condensation ring area from the start of ice nucleation until the freezing is completed and reaches the minimum value, or by measuring the freezing time from the time difference between the average gray value change in the droplet center area from the start of the droplet center gray value intensity to the steady stage.

[0041] S42. Obtain the formula for calculating droplet volume: V = fR 3 Since a droplet maintains a stable clamping relationship with a smooth surface, the angle between the solid-liquid interface, the liquid interior, and the gas-liquid interface is called the contact angle θ. C ,in R is the lateral radius of the droplet;

[0042] S43. Theoretical formula for the freezing time of supercooled droplets Perform deformation to obtain Where ρ i It is the mass of ice, L m It is the latent heat of the ice-water mixture, H is the final height of the droplet, and k is the latent heat of the mixture. i It is the thermal conductivity, ΔT is the subcooling, and the droplet height H is related to the droplet radius R. d They are directly proportional;

[0043] S44. Comparing the freezing time of the droplets in the experiment with the freezing time calculated by the theoretical formula, it can be found that the freezing time obtained by measuring the average grayscale change on a smooth surface is quite close to the theoretical freezing time. Therefore, this demonstrates that the method can be well used to measure freezing time. (See reference for details.) Figure 5 .

[0044] In this embodiment, to illustrate the feasibility of the experiment, a physical model of the central bright spot was established in step S3, as detailed in the following reference. Figure 4 ,from Figure 4 As can be seen in a, the central light spot inside the droplet was observed due to the refraction of the droplet;

[0045] Figure 4 In b, Snell's law is used to quantify the relative intensity distribution of light. Based on the symmetry of the droplet top view, the intensity distribution on the cross-section of the droplet is calculated. The parallel light beam is refracted on the surface of the droplet and reflected on the solid surface. Then, the reflected light will be refracted a second time on the surface of the droplet. Here, only the first reflection and the second refraction on the surface of the droplet are considered.

[0046] Figure 4 Figure c shows the relative intensity distribution of light as a function of the distance from the droplet center under theoretical and practical conditions. Based on the calculation results, we can simulate the image of the droplet under parallel light, which is close to the experimental observation results. (The experimental observation results are referenced.) Figure 4 a and Figure 4 d.

[0047] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made using the present invention’s specification and content, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method of measuring the ice nucleation time of a microdroplet, the method comprising: providing a microdroplet; and measuring the ice nucleation time of the microdroplet. Specifically, it includes: Step S1: Obtain the generated microdroplets; Step S2: Use a speed of 0.01s / frame to film the above microdroplet formation process. Place the camera perpendicular to the sample surface and use coaxial lighting to illuminate the surface from inside the camera lens. The light is reflected from the surface to produce a clear image inside the camera. Step S3: Use MATLAB for image processing. Within the resolution range of the image capture, based on the study of the brightness change of the central spot and the formation of condensation rings during the freezing process of microdroplets, determine the direction of grayscale change and obtain the grayscale intensity change of the central bright spot and the grayscale intensity change of the condensation ring region during the freezing process of droplets on a smooth surface under different experimental conditions. Thus, the freezing time of droplets on a smooth surface and the start and end times of freezing are obtained, and a physical model of the central bright spot is established. Step S4: Obtain experimental results based on image processing, and determine the freezing start and end times of the droplet by analyzing the two gray intensity change patterns. Obtain the freezing time of the droplet using the theoretical formula for the freezing time of supercooled droplets, and compare the experimental results with the theoretical formula calculation results.

2. A method of measuring the ice nucleation time of a microdroplet as claimed in claim 1, wherein, The specific process for determining the start and end points of the freezing time of droplets on a smooth surface is as follows: First, a preliminary judgment is made on the image to determine the size of the central bright spot and the selection of the condensation ring region; After determining the selected computational region, MATLAB was used to calculate the gray value change over time within the two regions and observe the change in average gray value within the two regions with the freezing process. This allowed us to obtain the start and end points of the droplet freezing time measurement on the smooth surface. Specifically, the average gray value change in the condensation ring region starts from the beginning of ice nucleation until the freezing is complete and reaches its minimum value, or the average gray value change in the droplet center region starts from the gray value intensity jump and reaches a stable stage.

3. A method of measuring the ice nucleation time of a microdroplet as claimed in claim 1, wherein, Step S4 specifically includes: S41. Obtain the freezing time of the droplets in the experiment based on the measurement start and end points of the freezing time of the droplets in the experiment; S42, obtaining a droplet volume calculation formula V = fR 3 Since the droplet and the surface keep in a stable pinch on the smooth surface, the angle between the solid-liquid interface and the gas-liquid interface through the liquid interior is called the contact angle θ c wherein R is the lateral radius of the droplet; S43. Theoretical formula for the freezing time of supercooled droplets deformation is carried out to obtain where p i is the mass of ice, L m is the latent heat of the ice-water mixture, H is the final height of the droplet, k i is the thermal conductivity, and ΔT is the supercooling degree. The droplet height H is in a direct proportional relationship with the droplet radius R d . S44. Compare the freezing time of the droplets in the experiment with the freezing time calculated by the theoretical formula.

4. The method for measuring the freezing time of microdroplets as described in claim 1, characterized in that, The physical model for establishing the central bright spot specifically includes: The light spot at the center of the droplet was observed due to refraction. The relative intensity distribution of light in a droplet was quantified using Snell's law, and the intensity distribution on the cross-section of the droplet was calculated based on the symmetry of the droplet's top view. The relative intensity distribution of light as a function of the distance from the droplet center under theoretical and practical conditions was obtained, and the image of the droplet under parallel light was simulated. Compare the simulated images with the experimentally observed images.

5. The method for measuring the freezing time of microdroplets as described in claim 1, characterized in that, Specific methods for obtaining generated microdroplets include: First, a smooth sample surface is prepared; Then the sample surface was placed on the cooler, and the relative humidity of the condensation environment was controlled by adjusting the water vapor introduction rate using a mass flow meter and a wide-mouth bottle. Once the relative humidity stabilizes, the temperature of the cooler is adjusted to the target temperature, and a large number of microdroplets form on the sample surface, resulting in freezing.

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