Device and method for measuring rupture state of electrohydrodynamic spraying liquid belt
By triggering industrial camera photography by induced falling edges of current signals by using the collection plate, the problem of inconsistent injection state in the electrohydrodynamic droplet injection device is solved, and reliable detection of undesirable injection state is achieved, which improves application accuracy and reduces costs.
Patent Information
- Application Number
- CN202510201563.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-02-24
- Publication Date
- 2025-05-30
AI Technical Summary
In the electrodynamic droplet ejection device, the inconsistent ejection state under the steady and constant liquid supply flow rate and steady and constant voltage, especially the high-frequency oscillation of the liquid surface causes the liquid belt to not rupture, resulting in an undesirable ejection state, which affects the application accuracy of the liquid sample.
By using the collecting plate to sense the falling edge of the current signal as a reference time in the electrohydrodynamic droplet ejection device, ordinary industrial cameras are controlled to perform single-frame trigger shooting, recording the critical state of liquid belt rupture, and detecting an unsatisfactory ejection state in the droplet infusion mode.
The critical state of reliable recording of droplet fracture is achieved, and the unsatisfactory ejection state in droplet infusion mode is accurately detected, which improves the application accuracy of liquid samples and reduces the application cost of the device.
Smart Images
Figure CN120075366A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to a method for monitoring the spraying state of a micron diameter droplet spraying device based on the electrohydrodynamic principle, and belongs to the technical field of droplet generation and inkjet printing. Background Art
[0002] Normally, droplet ejection is done by applying a drive to squeeze the liquid out of the nozzle to form droplets, and the diameter of the droplets produced is larger than the nozzle diameter. In order to achieve high-resolution printing (smaller droplet diameter), the most direct way is to reduce the nozzle. However, as the nozzle diameter decreases, the pressure required for the liquid to be squeezed out of the nozzle will increase dramatically; especially when the liquid is highly viscous or contains fine particles, the nozzle is extremely prone to clogging. Therefore, the most critical issue of droplet ejection technology is how to improve the actual resolution that can be achieved while alleviating the risk of nozzle clogging. A new droplet ejection technology - electrohydrodynamic ejection, can effectively avoid the above difficulties.
[0003] In electrohydrodynamic jetting, the liquid is made to "protrude" slightly from the nozzle by pressure or other means. A strong electric field is generated near the nozzle by applying a high voltage between the nozzle and the receiving electrode. The surface of the liquid at the nozzle is charged and is subjected to force in the electric field. Under actual working conditions, the electric field force pulls the liquid at the nozzle into a cone (called a Taylor cone). The electric field force overcomes its surface tension, causing the liquid band to break and form droplets to be ejected. Compared with the traditional droplet jetting method, the electrohydrodynamic method has the following characteristics: (1) The droplet diameter can be significantly smaller than the nozzle diameter, even up to the submicron level, and it helps to improve the printing resolution while reducing the risk of nozzle clogging; (2) The droplets carry an electric charge, and the motion of the droplets can be controlled during the droplet falling process. In addition, this method is suitable for a wide range of fluids with wide viscosity and conductivity, so the jetting materials are wide. These characteristics have well solved the defects of the traditional jetting method. This technology can be applied to various fields such as printed electronics, microelectronics packaging, biomedical engineering, and drug development, and has broad development prospects.
[0004] The electrohydrodynamic droplet generation system is mainly composed of four parts: a liquid supply pump, a conductive nozzle, a collecting plate, and a constant high-voltage power supply. The liquid supply pump provides a stable liquid supply. A high voltage is applied between the conductive nozzle and the collecting plate, and the liquid at the nozzle is subjected to the electric field force, gravity, and the surface tension to produce droplets. When the voltage between the nozzle and the extraction electrode is very small, gravity and surface tension are dominant, and the droplets formed are usually much larger than the nozzle diameter, which is usually called the dripping mode. The most potential application mode of electrohydrodynamic droplet injection technology is often called the micro-dripping mode, in which the electric field force is greater than gravity and dominates the generation of droplets. The Taylor cone breaks at its end to form droplets. The droplets carry away most of the charge on the liquid surface, so that the electrical stress at the liquid surface is released and the liquid surface retracts. It should be pointed out that in this working mode, droplets are not generated on demand, but are a continuous droplet generation process under the action of an electric field under a constant liquid supply condition. The time interval and geometric size of droplet generation depend on the intrinsic oscillation behavior of the liquid band at the nozzle. Although a pulsed high voltage power supply can be used, which is also often referred to as on-demand (DOD) electrohydrodynamic droplet injection, the expensive pulsed high voltage power supply greatly increases the application cost of the technical solution. The present invention is mainly directed to an electrohydrodynamic droplet injection device under constant high voltage and stable liquid supply flow rate.
[0005] In the above-mentioned microdroplet injection mode, there are many factors that affect the electrohydrodynamic microdroplet injection, such as applied voltage, liquid supply flow rate, geometric structure and positional relationship including the distance between the conductive nozzle and the lead-out electrode, geometric structure and positional relationship of other nearby objects, etc., so that the generated microdroplets will be unstable or inconsistent in time interval and droplet size due to the disturbance of working conditions. In the microdroplet injection mode, an undesirable microdroplet injection state is common: the liquid surface oscillates at high frequency (the frequency can reach hundreds to thousands of hertz) under the combined action of electric field force and surface tension, the liquid surface stretches and retracts, but the end of the liquid band does not break; after the liquid surface oscillates many times, one or more larger droplets are generated. This undesirable injection state will deteriorate the application accuracy of the liquid sample. In practical applications, the device is usually required to produce droplets of consistent size. Therefore, reliably obtaining and accurately judging whether the droplet state is stable and consistent is the key to the application of this technology.
[0006] Machine vision is a common means of obtaining the state of droplet ejection. There are two methods for obtaining the state of electrohydrodynamic (EHD) droplet ejection: (1) By shooting with an ultra-high frame rate camera; the state when the droplet is about to break is of great significance for evaluating the stability and consistency of the droplet ejection state. However, in the droplet injection mode, the characteristic time of EHD droplet generation is in the order of microseconds. Even an ultra-high frame rate camera cannot guarantee to capture the state at this moment. Moreover, ultra-high frame rate cameras are expensive, greatly increasing the application cost of the droplet generation device. A large number of images usually have to be stored in memory first and then uploaded to a computer for image processing, which is a complex and time-consuming process. (2) The induced current of the collection plate is applied to trigger a normal single-trigger camera to shoot the EHD droplet ejection process. Usually, the rising edge of the current signal is used as the reference time, and by adjusting the delay time, the droplet ejection process is recorded (in fact, this technology records not a single ejection process, but different moments of multiple ejection processes). The applicability of this shooting technology highly depends on the periodic characteristics of droplet ejection. For the common non-ideal ejection states in the droplet injection mode, this shooting technology cannot be adapted.
[0007] Based on the acquisition of the induced current signal of the collection plate in the EHD droplet ejection device, our invention proposes a method of using the moment of the falling edge of the current signal as the reference time and controlling the single-frame trigger mode of an ordinary industrial camera to take pictures. This method can reliably record the critical state of liquid ribbon rupture and detect non-ideal ejection states in the droplet injection mode. Summary of the Invention
[0008] In the background introduction, aiming at the problem of inconsistent ejection states that may occur when the electrohydrodynamic micro-droplet ejection device with a steady liquid supply flow rate and a steady voltage operates in the droplet injection mode, the present invention proposes a method based on the induced current of the extraction electrode and the triggering of an ordinary industrial camera to take pictures, which can reliably record the critical state of droplet rupture and detect non-ideal ejection states in the droplet injection mode.
[0009] The electrohydrodynamic micro-droplet ejection device based on a steady liquid supply flow rate and a steady voltage includes a metal nozzle 1, a liquid supply module 2, a steady high-voltage power supply 3, a collection plate 4, a current amplifier 5, an industrial camera 6, an illumination module 7, a control system, and a signal acquisition system 8. The liquid supply module 2 conveys the "ink" to be ejected to the metal nozzle 1 at a steady flow rate; the positive pole of the steady high-voltage power supply 3 is connected to the metal nozzle 1, and the negative pole is grounded; the collection plate 4 is grounded through the current amplifier 5 and faces the metal nozzle 1; the industrial camera 6 is arranged on the side of the metal nozzle 1 and is connected to the control system; the control system and the signal acquisition system 8 are used to collect the signal of the current amplifier 5 and generate a trigger shooting signal for the industrial camera 6; the illumination module 7 provides background illumination for the industrial camera 6.
[0010] According to the electrohydrodynamic droplet generation principle, during the droplet generation process, the liquid ribbon stretches to form a Taylor cone, the liquid ribbon breaks, and after the droplet is generated, the remaining liquid shrinks. If the liquid ribbon does not break, that is, the droplet injection state is not ideal, the liquid ribbon will shrink under the action of surface tension. Along with the deformation of the liquid surface, the amount of charge on the liquid surface at the nozzle and the collecting plate changes with time. When the metal nozzle is connected to a positive voltage, the collecting plate induces a negative charge, and the current flows from the collecting plate to the ground through the ammeter.
[0011] The process of measuring the induced current of the collecting plate 4 is as follows: the collecting plate 4 is grounded through a series resistor, and a high-speed instrumentation amplifier is used to measure the voltage v across the resistor R. 0 , record the output voltage data through the data acquisition card, and calculate the induced current on the collection board according to formula (1).
[0012] i id =v o / R (1)
[0013] In the micro-droplet injection mode, the normal micro-droplet injection process is faithful, and the image acquisition time is marked on the induced current signal. The falling edge moment corresponds to the liquid band rupture, that is, state F. If the injection is not ideal, the liquid band will not rupture. Regardless of whether the injection process is normal or not, there are two obvious falling edges in the current signal, and the liquid band rupture corresponds to the second falling edge. Real-time detection of the induced current i id For each spraying cycle, i.e., each time the liquid ribbon stretches and retracts, an industrial camera is used to record the instantaneous state of the liquid ribbon when it breaks by a single-frame triggering method. Moreover, with only one photo, it can be determined whether the micro-droplet spraying device is spraying normally in the micro-droplet injection mode.
[0014] The present invention proposes two operable methods, which can trigger a common industrial camera to capture the state of droplet generation in the electrohydrodynamic micro-droplet injection mode based on the current signal.
[0015] Method 1: Detect the falling edge by differentiation. The current signal usually needs to be smoothed first, and then the falling edge is obtained by differentiation. At the falling edge, the differentiated current signal presents an extreme value. Specifically, after the induced current is inverted, it passes through an analog differential circuit. The design of the analog differential circuit is very mature and is not included in the claims of this application. The output signal d(-i id The two maximum values of d(-i ) / dt correspond to the two significant falling edges of the induced current signal. id) / dt can perform peak seeking through a high-speed digital circuit. The second one exactly corresponds to the moment when the microdroplet is about to break, which is set as the zero point moment of triggering. A trigger signal is sent to the industrial camera at the zero point moment to record the state at the moment of microdroplet rupture. If the camera is set to delay shooting, the camera will delay for a certain time based on the zero point for shooting. In this way, the state of the droplet at each moment during the generation process can be observed in real time.
[0016] Method 2: It is a more convenient triggering method: Set a level, take 20% of the current peak value, and the current is in the descending range. Under this triggering condition, for the microdroplet injection mode, if the spraying device sprays normally and the microdroplets have been generated, the microdroplets and the fully retracted liquid band can be seen in the photo. If the liquid band does not break, no microdroplets will be photographed. Description of the Drawings
[0017] Figure 1 Schematic diagram of an electrohydrodynamic microdroplet spraying device based on a steady liquid supply flow rate and a steady voltage
[0018] Figure 2 Schematic diagram for triggering an ordinary industrial camera to photograph the state of microdroplet generation in the electrohydrodynamic microdroplet injection mode based on a current signal.
[0019] Figure 3 Measurement diagram of the induced current of the collection plate.
[0020] Figure 4 The normal microdroplet spraying process is as shown in the figure.
[0021] Figure 5 Schematic diagram of the droplet state. Detailed Implementation Modes
[0022] The present invention will be described in detail below in conjunction with the drawings and embodiments.
[0023] The schematic diagram of the electrohydrodynamic microdroplet spraying device based on a steady liquid supply flow rate and a steady voltage is as Figure 1 shown. In the figure, 1 is a metal nozzle, 2 is a liquid supply module, 3 is a steady high-voltage power supply, 4 is a collection plate, 5 is a current amplifier, 6 is an industrial camera, 7 is an illumination module, and 8 is a signal acquisition system. The liquid supply module 2 supplies the "ink" to be sprayed to the metal nozzle 1 at a steady flow rate; the positive electrode of the steady high-voltage power supply 3 is metallically connected to the metal nozzle 1, and the negative electrode is grounded; the collection plate 4 is grounded through an ammeter; the control system and the signal acquisition system 8 are used to collect the ammeter signal and generate a trigger shooting signal for the industrial camera 6; the illumination module 7 provides background illumination for the industrial camera.
[0024] According to the principle of electrohydrodynamic microdroplet generation, the microdroplet generation process is qualitatively as Figure 2As shown. When the liquid band elongates, a Taylor cone is formed. After the liquid band breaks and micro-droplets are generated, the remaining liquid retracts. If the liquid band does not break (i.e., the non-ideal micro-droplet injection state introduced above), the liquid band will retract under the action of surface tension. Along with the deformation of the liquid surface, the electric charges on the liquid surface at the nozzle and on the collection plate both change with time (as Figure 2 shown, when the metal nozzle is connected to a positive voltage, the collection plate induces negative charges), and the current flows from the collection plate to the ground terminal through the ammeter.
[0025] The measurement of the induced current on the collection plate is as Figure 3 shown. The collection plate is grounded through a series resistor, and a high-speed instrumentation amplifier is used to measure the voltage v 0 across the resistor R. The output voltage data is recorded through a data acquisition card, and the induced current on the collection plate is calculated according to formula (1).
[0026] i id = v o / R (1)
[0027] In the micro-droplet injection mode, the normal micro-droplet injection process is as Figure 4 shown, and the image acquisition time is marked on the induced current signal. The falling edge moment exactly corresponds to the liquid band breakage, i.e., state F. If the injection is not ideal, the injection process is as Figure 5 shown, and the liquid band does not break at the falling edge moment, i.e., state F. It can be easily found from Figure 4 and Figure 5 that regardless of whether the injection process is normal or not, two obvious falling edges appear in the current signal, and the liquid band breakage corresponds to the second falling edge. Based on this experimental phenomenon, it is proposed that the induced current i id can be detected in real time. For each injection cycle (i.e., the cycle of each elongation and retraction of the liquid band), a common industrial camera is used and a single-frame trigger photography method is adopted to record the instantaneous state when the liquid band breaks. Moreover, just by one photo, it can be judged whether the micro-droplet injection device injects normally in the micro-droplet injection mode.
[0028] The present invention will propose two operable methods, which can trigger a common industrial camera to photograph the micro-droplet generation state in the electrohydrodynamic micro-droplet injection mode based on the current signal (such as Figure 2 ).
[0029] Method 1: Detect the falling edge through the differential method. The current signal usually needs to be smoothed first and then the falling edge is obtained through differentiation. At the falling edge moment, the differentiated current signal presents an extreme value. Specifically, the induced current can be inverted and then passed through an analog differential circuit. The design of the analog differential circuit is very mature and is not in the claims of this application. The two maximum values of the output signal d(-i id ) / dt of the differential circuit correspond to the two significant falling edges of the induced current signal. d(-iid ) / dt can perform peak seeking through a high-speed digital circuit. The second one exactly corresponds to the moment when the droplet is about to break, which is set as the zero point moment of triggering. Sending a trigger signal to the industrial camera at the zero point moment can record the state at the moment when the droplet breaks. If the camera is set to delay shooting, the camera will delay for a certain time based on this zero point for shooting. In this way, the state of the droplet at each moment during the generation process can be observed in real time.
[0030] Method 2: It is a more convenient triggering method: Set a level, usually 20% of the current peak value can be taken, and the current is in the descending range. Under this triggering condition, for the micro-droplet injection mode, if the spraying device sprays normally and the micro-droplet has been generated (such as Figure 4 State G), the micro-droplet and the fully retracted liquid band can be seen in the photo. If the liquid band does not break, no micro-droplet will be photographed (such as Figure 5 State G).
Claims
1. A device for measuring the rupture state of an electrohydrodynamic jet, characterized in that: It includes a metal nozzle, a liquid supply module, a constant high-voltage power supply, a collecting plate, a current amplifier, an industrial camera, a lighting module, a control system and a signal acquisition system; the liquid supply module delivers the "ink" to be sprayed to the metal nozzle at a constant flow rate; the positive metal of the constant high-voltage power supply is connected to the metal nozzle, and the negative electrode is grounded; the collecting plate is grounded through the current amplifier and faces the metal nozzle; the industrial camera is arranged on the side of the metal nozzle and connected to the control system; the control system and the signal acquisition system are used to collect the signal of the current amplifier and generate a trigger shooting signal for the industrial camera; the lighting module provides background lighting for the industrial camera.
2. The device for measuring the breakup state of electrohydrodynamic jetting liquid according to claim 1, characterized in that: According to the electrohydrodynamic droplet generation principle, during the droplet generation process, the liquid ribbon stretches to form a Taylor cone, and the liquid ribbon breaks. After the droplet is generated, the remaining liquid shrinks. If the liquid ribbon does not break, that is, the droplet injection state is not ideal, the liquid ribbon will shrink under the action of surface tension. Along with the deformation of the liquid surface, the amount of charge on the liquid surface at the nozzle and the collecting plate changes with time. When the metal nozzle is connected to a positive voltage, the collecting plate induces a negative charge, and the current flows from the collecting plate to the ground through the ammeter.
3. The device for measuring the breakup state of electrohydrodynamic jetting liquid according to claim 1, characterized in that: The measurement process of the induced current of the collection plate is as follows: the collection plate is grounded through a series resistor, a high-speed instrument amplifier is used to measure the voltage v0 across the resistor R, the output voltage data is recorded through a data acquisition card, and the induced current on the collection plate is calculated according to formula (1); i id =v o / R (1) In the micro-droplet injection mode, the normal micro-droplet ejection process is faithful, and the image acquisition moment is marked on the induced current signal; the falling edge moment corresponds to the liquid band rupture; If the injection is not ideal, the liquid belt will not break. Regardless of whether the injection process is normal or not, there are two obvious falling edges in the current signal, and the liquid belt rupture corresponds to the second falling edge. Real-time detection of the induced current i id For each injection cycle, that is, each extension and retraction cycle of the liquid ribbon, an industrial camera is used and a single-frame trigger photo is taken to record the instantaneous state when the liquid ribbon breaks; and only one photo can be used to determine whether the micro-droplet injection device is spraying normally in the micro-droplet injection mode.
4. The device for measuring the breakup state of electrohydrodynamic jetting liquid according to claim 1, characterized in that: The measurement method is as follows: based on the current signal, a common industrial camera is triggered to capture the state of droplet generation in the electrohydrodynamic micro-droplet injection mode; The falling edge is detected by differentiation. The current signal needs to be smoothed first, and then the falling edge is obtained by differentiation. At the falling edge, the differentiated current signal presents an extreme value. After the induced current is inverted, it passes through an analog differential circuit; the output signal d(-i id The two maximum values of ) / dt correspond to the two significant falling edges of the induced current signal; id ) / dt is peak-searched by a high-speed digital circuit; the second one corresponds to the moment when the droplet is about to break, which is set as the trigger zero time; a trigger signal is sent to the industrial camera at zero time to record the state of the droplet at the moment of breakage; if the camera is set to take delayed photos, the industrial camera will take photos after a certain delay based on the zero time; in this way, the state of the droplet at each moment in the production process can be observed in real time.
5. The device for measuring the breakup state of electrohydrodynamic jetting liquid according to claim 1, characterized in that: The measurement method is as follows: based on the current signal, a common industrial camera is triggered to capture the state of droplet generation in the electrohydrodynamic micro-droplet injection mode; Set a level to take 20% of the current peak value, and the current is in the decreasing range; Under this triggering condition, for the micro-droplet injection mode, if the ejection device ejects normally, micro-droplets have been produced, and the micro-droplets and the completely retracted liquid belt can be seen in the photo; if the liquid belt does not break, no droplets will be photographed.