A method for measuring three-dimensional motion trajectory of single-phase camera droplet bounce

Abstract

The present application relates to the technical field of interface, and in particular discloses a kind of top-down single camera droplet bounce three-dimensional motion trajectory measurement method, comprising the following steps: step 1, utilize high-speed camera, coaxial optical lens, three-axis displacement platform, super-hydrophobic substrate builds experimental platform, utilize the experimental platform, combined with through top-down perspective to the droplet coalescence bounce process is observed;Step 2, for the sequence image collected is handled as follows: filtering denoising, edge detection, edge gradient calculation;Utilize the characteristics that the edge of droplet is most clear and the edge gray scale gradient is maximum when bounce droplet passes through focal plane, realize droplet bounce feature calculation by key time point determination.This measurement method is simple in operation, good in reliability, wide in scope of application, need not to be calibrated to imaging system before experiment, and the influence of surrounding environment and camera parameter on this method can be ignored, compared with traditional side view observation method, bounce feature measurement precision and reliability can satisfy the experimental demand.

Description

Technical Field

[0001] This invention relates to the field of interfaces, and in particular to a method for measuring the three-dimensional motion trajectory of a droplet bouncing using a single camera viewed from above. Background Technology

[0002] The trajectory of simple droplet merging and bouncing is relatively straightforward. Traditional high-speed camera side-view methods can record the merging and bouncing process and calculate bouncing features, but the experimental procedure is relatively cumbersome, and the imaging system needs to be recalibrated after changing the imaging system parameters. In addition, when droplets are mutually occluded at the initial contact, the side-view perspective cannot achieve complete acquisition of images of two or more initial droplets.

[0003] Therefore, for the recording of droplet merging and bouncing processes and the measurement of bouncing characteristics under complex conditions, a simple, reliable, and widely applicable top-view single-camera method for measuring the three-dimensional motion trajectory of droplet bouncing is designed, which is of great significance to the development of the interface field.

[0004] No patent documents related to this application were found through a search. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies and better realize the calculation of droplet bouncing characteristics, this invention proposes a top-view single-camera method for measuring the three-dimensional motion trajectory of bouncing droplets. An experimental setup for measuring the three-dimensional trajectory of bouncing droplets is constructed using a high-speed camera, a coaxial optical lens, a three-axis displacement platform, and a superhydrophobic substrate. Based on the characteristic that the droplet edge is clearest and has the largest edge grayscale gradient when it crosses the focal plane, the droplet bouncing characteristics are calculated by determining key time points. This measurement method does not require pre-experimental calibration of the imaging system, and the influence of the surrounding environment and camera parameters on this method is negligible. Compared with traditional side-view observation methods, the accuracy and reliability of bouncing characteristic measurement meet experimental requirements.

[0006] The technical problem solved by this invention is achieved through the following technical solution:

[0007] A method for measuring the three-dimensional motion trajectory of a droplet bouncing using a single camera from a top-down perspective includes the following steps:

[0008] Step 1: An experimental platform was built using a high-speed camera, a coaxial optical lens, a three-axis displacement platform, and a superhydrophobic substrate. Using this experimental platform, the droplet merging and bouncing process was observed from a top-down perspective.

[0009] Step 2: The acquired sequence images are processed as follows: filtering and denoising, edge detection, and edge gradient calculation; taking advantage of the fact that the droplet edge is clearest and the edge gray-scale gradient is largest when the bouncing droplet passes through the focal plane, the droplet bouncing feature is calculated by determining key time points.

[0010] Furthermore, in step 1, the observation of the droplet merging and bouncing process specifically includes:

[0011] Step 1.1: Focus the high-speed camera's focal plane on the outline of the initial droplet to obtain the initial droplet's radius and position; based on the size and position of the stationary droplet, drive the X-axis and Y-axis linear displacement stages to adjust the initial droplet's position in the center of the high-speed camera's image window. Observe the clearest static droplet image in the high-speed camera's imaging window, at which point the droplet's edges are clearly visible;

[0012] Step 1.2: Raise the focal plane of the high-speed camera by a known distance based on the droplet size; determine the focal plane raising distance h1 based on the droplet radius measurement results in Step 1.1. The raising distance h1 is approximately 1-4 times the measured initial droplet radius. The focal plane of the high-speed camera is raised by changing the relative distance between the substrate and the high-speed camera through the Z-axis linear displacement stage. A blurred image is observed in the imaging window of the high-speed camera after the focal plane is raised.

[0013] Step 1.3: Begin the droplet merging and bouncing experiment and observe the high-speed camera imaging window. The bouncing droplet will generate an upward bouncing velocity perpendicular to the substrate surface, and will show the clearest droplet edge outline when it passes through the raised focal plane.

[0014] 4. The method for measuring the three-dimensional motion trajectory of a droplet bouncing using a single camera from above, as described in claim 1, is characterized in that, in step 2, the droplet bouncing feature is calculated, and the specific calculation process is as follows:

[0015] Step 2.1: The edge of the bouncing droplet is detected using the Canny operator. The key time point is determined by the gray-scale gradient curve of the edge of the bouncing droplet. The maximum edge gradient corresponds to the droplet being at the focal plane position and time point t1, and the minimum edge gradient corresponds to the highest bouncing height of the droplet and time point t2.

[0016] Step 2.2: Determine the bouncing velocity in the Z direction based on key time points t1, t2 and key locations; analyze the centroid position of the jumping droplets at times t1 and t2; determine the bouncing velocity and direction of the droplets in the X and Y directions; calculate the centroid height of the merged droplets when they detach from the substrate; and realize the observation and measurement of the bouncing characteristics of droplet merging on a substrate with a superhydrophobic structure.

[0017] Furthermore, the triaxial displacement platform includes an X-axis linear displacement stage, a Y-axis linear displacement stage, and a Z-axis linear displacement stage, wherein the X-axis linear displacement stage and the Y-axis linear displacement stage are used to adjust the position of the base in the plane, and the Z-axis linear displacement stage is used to adjust the height of the base.

[0018] The advantages and positive effects of this invention are:

[0019] This invention proposes a method for measuring the three-dimensional motion trajectory of a bouncing droplet using a top-down single-camera perspective. The top-down view allows for direct acquisition of initial droplet characteristics, including initial droplet velocity, direction of motion, and relative position during merging. Utilizing the characteristic that the droplet edge is clearest and has the largest edge grayscale gradient when it crosses the focal plane, the bouncing characteristics are calculated by determining key time points. This method eliminates the need to determine the merging time from initial contact to the detachment of the merged droplets from the superhydrophobic surface, avoiding the influence of the initial droplet merging condition on the merging time. Furthermore, the method for determining the bouncing droplet position and time point is applicable to any camera parameters and lighting conditions. Compared to traditional side-view observation methods, the accuracy and reliability of the bouncing characteristic measurement meet experimental requirements. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the three-dimensional trajectory measurement experimental device of the present invention.

[0021] Figure 2-a This is a schematic diagram of three-dimensional measurement of droplet bouncing - Y-view.

[0022] Figure 2-b This is a schematic diagram of three-dimensional measurement of droplet bouncing - Z+ perspective.

[0023] Figure 3 This is a diagram showing the experimental results of the dimensionless height of the centroid of the merged droplets under different merging conditions.

[0024] Figure 4 It is the process of two stationary droplets with a radius of about 0.25 mm merging and bouncing.

[0025] Figure 5 This is a schematic diagram of the evolution of the droplet edge gradient during the merging and bouncing process of two droplets with a radius of approximately 0.25 mm.

[0026] Figure 6 The results are from droplet merging and bouncing experiments using a top-down single-camera 3D measurement method and a traditional side-view measurement method. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0028] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It should be noted that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0029] The experimental device for measuring the three-dimensional trajectory of bouncing droplets using a single camera from above in this invention is as follows: Figure 1As shown, the system includes a high-speed camera 1, a coaxial optical lens 2, a 3-axis displacement platform 4, and a superhydrophobic substrate 3. The X-axis and Y-axis linear displacement platforms are used to adjust the position of the substrate in the plane, and the Z-axis linear displacement platform is used to adjust the height of the substrate. The coaxial optical lens is connected to the high-speed camera, and the droplet merging and bouncing process is observed from a top-down perspective.

[0030] The measurement principle used in this invention is as follows:

[0031] According to the defocus imaging principle of cameras, the edge of a target object located at the camera's focal plane corresponds to the sharpest image. When the target deviates from the camera's focal plane, the target image will produce edge blurring, and the degree of blurring increases with the defocus distance. The droplet merging and bouncing process is extremely brief, and the bouncing droplet is in real-time dynamic flight. The movement from the over-focus side through the camera's focal plane to the under-focus side is a continuous process. The droplet edge state is defocus-focus-defocus, corresponding to the edge of the acquired droplet image being blurred-sharp-blurred. By judging the sharpness of the bouncing droplet edge, the moment when the bouncing droplet reaches the focal plane and the moment when it reaches the maximum bouncing height can be determined, as shown in Figure 2.

[0032] Because bouncing droplets undergo oscillating motion and are spherical or ellipsoidal in shape, the edge sharpness is determined by the gray-scale gradient of the droplet's edge in assessing whether it has reached the focal plane. The criterion is that the greater the defocus of the droplet, the smaller the corresponding gray-scale gradient at the droplet's edge. Therefore, during the bouncing process, the gray-scale gradient at the edge of the bouncing droplet increases from small to large, then decreases again until the droplet reaches its maximum bouncing height. The maximum edge gradient corresponds to the droplet being at the focal plane position and time point t1, while the minimum edge gradient corresponds to the droplet's maximum bouncing height and time point t2. The bouncing velocity in the Z direction is determined based on the above key position and time point information. Furthermore, analyzing the centroid position of the bouncing droplet at times t1 and t2 allows us to determine the bouncing velocity and direction in the X and Y directions. The acquired image sequence is processed as follows: filtering and denoising, edge detection, and edge gradient calculation. Key time points are determined using the gray-scale gradient curve of the bouncing droplet's edge.

[0033] The specific detection process involved in this invention is described below.

[0034] The gradient calculation for bouncing droplet edges first involves detecting the edges of the bouncing droplets. Because the Canny operator has superior resistance to noise interference, it can detect weak edges where the target edge is not clearly distinguishable from the background. Therefore, the Canny operator is used for bouncing droplet edge detection.

[0035] The bouncing velocity of the droplet in the Z direction was calculated using key time points t1 and t2. When the droplet reached a high jump height, the upward velocity was 0 m / s. Since only gravity exerted a force on the droplet after it detached from the superhydrophobic surface, the upward velocity V2 of the droplet as it lifted the focal plane was calculated as follows:

[0036] V2=g×(t2-t1) (1)

[0037] The bouncing velocity of the droplet in the Z direction is calculated as follows:

[0038] V z =[V2 2 +2g×(h1-h m )] 1 / 2 (2)

[0039] Where h m This is the height of the center of mass of the merging droplets when they separate from the substrate surface. The dimensionless height of the center of mass of the merging droplets is h. m * Defined as:

[0040]

[0041] Where, r c This is the radius of the merged droplets after the jump, assuming the merged droplet shape is a perfect sphere, calculated based on the initial droplet radius:

[0042] r c =[(r1) 3 +r2 3 )×(2-3cosθ+cos 3 θ) / 4] 1 / 3 (4)

[0043] Where r1 and r2 are the radii of the two initial droplets, and θ is the contact angle of the droplets on the superhydrophobic surface.

[0044] The bouncing velocity in the X and Y directions is calculated using the displacement of the droplet's center of mass in these directions during the bouncing droplet motion. The bouncing velocity in the X direction is calculated as follows:

[0045] V x =l x / (t2-t1) (5)

[0046] The Y-direction bounce speed is calculated as follows:

[0047] V y =l y / (t2-t1) (6)

[0048] To measure the dimensionless centroid height h of the merged droplets m* Under different initial droplet merging conditions, including different initial droplet radii, initial droplet radius ratios, initial velocities, and deviations between the initial velocity direction and the droplet centroid, the dimensionless centroid height of the merged droplets was measured using a side-view perspective. Figure 3 As shown.

[0049] Experimental results were used to estimate the centroid height of merging droplets upon detachment from the substrate surface, providing a simple and effective model for calculation. The dimensionless centroid height of the merging droplets is concentrated between 0.9 and 1.0, with the highest concentration at 0.95. Therefore, the dimensionless centroid height of 0.95 can be used as a reference value in the calculation of merging bounce velocity. During static droplet merging, the merging droplets have similar morphologies, and their centroid height is relatively stable upon detachment from the superhydrophobic surface. However, dynamic droplet merging involves more complex factors; initial velocity, the deviation between velocity and centroid, etc., all affect the morphology of the merging droplets upon detachment, directly impacting the estimation of the droplet centroid height. In practical applications, the droplet centroid height is calculated based on the initial droplet merging conditions to reduce estimation errors. For droplet merging on a superhydrophobic substrate, the droplet centroid height is the height of the merged droplet centroid from the bottom surface of the superhydrophobic substrate at the moment of detachment. Therefore, by simply changing the calculation of the merged droplet centroid height upon detachment from the substrate, observation and measurement of the bounce characteristics of merging droplets on a superhydrophobic substrate can be achieved.

[0050] A method for measuring the three-dimensional motion trajectory of a droplet bouncing using a single camera from a top-down perspective includes the following steps:

[0051] Step 1: An experimental platform was built using a high-speed camera, a coaxial optical lens, a 3-axis displacement platform, and a superhydrophobic substrate to observe the droplet merging and bouncing process from a top-down perspective.

[0052] Step 2 involves processing the acquired image sequence as follows: filtering and denoising, edge detection, and edge gradient calculation. Utilizing the characteristic that the droplet edge is clearest and has the largest grayscale gradient at the moment the bouncing droplet crosses the focal plane, the droplet bouncing feature is calculated by determining key time points.

[0053] Furthermore, in step 1, the observation of the droplet merging and bouncing process is specifically as follows:

[0054] Step 1.1: Focus the high-speed camera's focal plane on the outline of the initial droplet to obtain its radius and position. Based on the size and position of the stationary droplet, drive the X-axis and Y-axis linear displacement stages to adjust the initial droplet's position in the center of the high-speed camera's image window. Observe the clearest static droplet image in the high-speed camera's imaging window, where the droplet edges are clearly visible.

[0055] Step 1.2: Raise the focal plane of the high-speed camera by a known distance based on the droplet size. The focal plane raising distance h1 is determined based on the droplet radius measurement results from Step 1.1. The raising distance h1 is approximately 1-4 times the measured initial droplet radius. The focal plane of the high-speed camera is raised by changing the relative distance between the substrate and the high-speed camera using a Z-axis linear displacement stage. A blurred image is observed in the imaging window of the high-speed camera after the focal plane has been raised.

[0056] Step 1.3: Begin the droplet merging and bouncing experiment and observe the high-speed camera imaging window. The bouncing droplet will generate an upward bouncing velocity perpendicular to the substrate surface, and will show the clearest droplet edge outline when it passes through the raised focal plane.

[0057] Furthermore, in step 2, the droplet bouncing characteristics are calculated, and the specific calculation process is as follows:

[0058] Step 2.1: The Canny operator is used to detect the edge of the bouncing droplet. The key time points are determined by the gray-scale gradient curve of the edge of the bouncing droplet. The maximum edge gradient corresponds to the droplet being at the focal plane position and time point t1, and the minimum edge gradient corresponds to the highest bouncing height of the droplet and time point t2.

[0059] Step 2.2: Determine the bouncing velocity in the Z direction based on key time points t1, t2, and key locations. Analyze the centroid positions of the jumping droplets at times t1 and t2 to determine the bouncing velocities and directions of the droplets in the X and Y directions. Calculate the centroid height of the merged droplets upon detachment from the substrate, thus realizing the observation and measurement of droplet merging and bouncing characteristics on a substrate with a superhydrophobic structure.

[0060] Verification is as follows: Example 1

[0061] Droplet Coupling and Bouncing Measurement

[0062] A top-view image of the merging and bouncing process of two equally sized droplets with a radius of approximately 0.25 mm. Figure 4 As shown, the gray-scale gradient at the edge of the merged droplet during the bouncing process is as follows: Figure 5 As shown. Due to the small initial droplet defocus distance, although the initial droplet image is relatively blurry, the droplet merging process can still be clearly identified. At 0 ms, the two droplets make initial contact, merge, oscillate, and detach from the superhydrophobic surface, continuing to rise under gravity. At 2.8 ms, the merged droplets reach the raised focal plane, exhibiting the clearest edges. Subsequently, the merged droplets continue to rise, reaching their maximum bounce height at 15.1 ms, at which point the bounce velocity in the Z direction decreases to 0 m / s.

[0063] Example 2

[0064] Comparative verification with traditional side-view measurement methods

[0065] To verify the accuracy of the top-view single-camera three-dimensional measurement method for droplet bouncing, five sets of experiments were conducted on the merging and bouncing of droplets with the same radius. The traditional side-view recording and bouncing velocity calculation methods were then compared and verified. The measurement results of the Z-direction bouncing velocity of merging droplets of different radii and equal sizes are shown below. Figure 6 As shown, the theoretical capillary inertial velocity (0.2u) of two equally sized droplets bouncing together was added to the image. ic ~0.25u ic The experimental results are consistent with the theoretical capillary inertial velocity range, and the results of the top-view single-camera droplet bouncing three-dimensional measurement method are in good agreement with the measurement results of the side-view method, verifying the accuracy and reliability of the method.

[0066] Although preferred embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other modifications under the guidance of the present invention without departing from the spirit and scope of the claims, and all of these modifications are within the scope of protection of the present invention.

[0067] Although embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A method for measuring the three-dimensional motion trajectory of a droplet bouncing using a single-camera top-down view, characterized in that, Includes the following steps: Step 1: An experimental platform was built using a high-speed camera, a coaxial optical lens, a three-axis displacement platform, and a superhydrophobic substrate. Using this experimental platform, the droplet merging and bouncing process was observed from a top-down perspective. Step 2: The acquired sequence images are processed as follows: filtering and denoising, edge detection, and edge gradient calculation; taking advantage of the fact that the droplet edge is clearest and the edge gray-scale gradient is largest when the bouncing droplet passes through the focal plane, the droplet bouncing feature is calculated by determining key time points. In step 1, the observation of the droplet merging and bouncing process is specifically as follows: Step 1.1: Focus the high-speed camera's focal plane on the outline of the initial droplet to obtain the initial droplet's radius and position; based on the size and position of the stationary droplet, drive the X-axis and Y-axis linear displacement stages to adjust the initial droplet's position in the center of the high-speed camera's image window, and observe the clearest static droplet image in the high-speed camera's imaging window, at which point the droplet's edge is clearly visible; Step 1.2: Raise the focal plane of the high-speed camera by a known distance based on the droplet size; determine the focal plane raising distance based on the droplet radius measurement results in Step 1.

1. h 1. Increase distance h 1 is 1-4 times the measured initial droplet radius. The relative distance between the substrate and the high-speed camera is changed by the Z-axis linear displacement stage to complete the focal plane elevation of the high-speed camera. A blurred image is observed in the imaging window of the high-speed camera after the focal plane is raised. Step 1.3: Begin the droplet merging and bouncing experiment. Observe the imaging window of the high-speed camera. The bouncing droplet will generate an upward bouncing speed perpendicular to the substrate surface. When it passes through the raised focal plane, it will show the clearest droplet edge outline. In step 2, the droplet bouncing characteristics are calculated, and the specific calculation process is as follows: Step 2.1: The Canny operator is used to detect the edges of the bouncing droplets. Key time points are determined by the gray-scale gradient curve of the droplet edges. The maximum edge gradient corresponds to the droplet's position and time point at the focal plane. t 1. The minimum edge gradient corresponds to the droplet's maximum bounce height and time point. t 2; Step 2.2, based on key time points t 1. t 2. Determine the jump speed in the Z direction at key locations and analyze. t 1 and t The centroid position of the jumping droplet at time 2 is determined, and the jumping velocity and direction of the droplet in the X and Y directions are determined. The centroid height of the merged droplet when it detaches from the substrate is calculated, so as to realize the observation of droplet merging and bouncing on a substrate with a superhydrophobic structure and the measurement of bouncing characteristics.

2. The method for measuring the three-dimensional motion trajectory of a droplet bouncing using a single camera from above, as described in claim 1, is characterized in that... The triaxial displacement platform includes an X-axis linear displacement stage, a Y-axis linear displacement stage, and a Z-axis linear displacement stage. The X-axis and Y-axis linear displacement stages are used to adjust the position of the base in the plane, and the Z-axis linear displacement stage is used to adjust the height of the base.

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