80 results about "Droplet flow" patented technology

An electronic droplet delivery device having a mouthpiece of inhalation of droplets includes an aperture plate with a plurality of openings coupled to a piezoelectric actuator. A fluid reservoir supplies a solution with nicotine to the aperture plate for ejection out of the mouthpiece in a droplets stream.

The invention provides a uniform tin droplet target generating device for inhibiting axial shaking of tin droplets, which comprises a crucible, tin melt, a nozzle, a heating assembly and a disturbance assembly, the tin melt is located inside the crucible, and the heating assembly is located outside the crucible and is used for heating the tin melt in the crucible; the nozzle is located at the bottom of the crucible and used for spraying uniform tin liquid drops and reducing axial shaking of the tin liquid drops. The disturbance assembly comprises a signal generator, a piezoelectric transducer and a vibration transmission rod, the vibration transmission rod is located in the molten tin, the piezoelectric transducer is located above the crucible and connected with the vibration transmission rod through a connecting piece, and the signal generator is connected with the piezoelectric transducer; a sine disturbance signal generated by the signal generator is provided for the vibration transmission rod through the piezoelectric transducer, and the disturbance signal is transmitted into molten tin through the vibration transmission rod, so that the jetted tin jet flow is broken into uniform liquid drop flow; according to the device provided by the invention, the distance between the liquid drops can be regulated and controlled; the designed nozzle can reduce the axial jitter of the liquid drop flow and increase the axial stability of the liquid drop.

The utility model belongs to a spraying device, and provides a supersonic spraying device, a non-invasive spraying device and a spraying instrument.The problems that spraying is not uniform and operation is inconvenient still exist in an existing instrument, and the supersonic spraying device, the non-invasive spraying device and the spraying instrument.The supersonic spraying device comprises a liquid section, a gas inlet section, a contraction section, a throat part and an expansion section, and the gas inlet section, the contraction section, the throat part and the expansion section are coaxially and sequentially connected; liquid enters from the liquid section, gas is accelerated through the contraction section, reaches the throat part and is continuously accelerated at the expansion section, the liquid entering from the liquid section is atomized under the action of the gas at the throat part, is brought into the expansion section and is finally sprayed out from the spraying opening along with continuous acceleration, and the uniform spraying effect is achieved. According to the supersonic jet device, the ratio of the axial length L1 to the axial length L2 is designed, the structure of the supersonic jet device is further optimized, and actual verification shows that under the optimized ratio, the jet force of the supersonic jet device is obviously improved, the particle size and the jet range of jetted liquid drops are better, and the flow speed of the liquid drops is more stable.

The invention belongs to the technical field of microfluidics, and particularly relates to a microfluidic chip and a method for preparing liquid drops in a high-flux and large-scale manner. A step emulsification structure and a liquid drop splitting structure are used together, a liquid drop generation structure is arranged at the communication position of a branch splitting channel and a main channel unit correspondingly communicated with the branch splitting channel, a dispersion phase necks at the liquid drop generation structure under the action of interfacial tension to form liquid drops, and the liquid drops are generated under the leading of the interfacial tension, so that the dispersion phase is uniform in dispersion. Therefore, the size of the liquid drops is basically determined by the channel width of the liquid drop generation structure, the branch splitting channel is designed to be wide, the influence of manufacturing errors on the uniformity of the generated liquid drops can be reduced, the branch splitting channel is designed to be a plurality of serpentine channels, and the uniformity of the liquid drops is improved while high-flux liquid drop preparation is guaranteed; and then the liquid drops flow through the main channel unit and are converged into the main splitting channel, and the liquid drops are uniformly split into liquid drops with smaller sizes through the liquid drop splitting structure, so that the preparation of the liquid drops with high flux and uniform sizes is realized.

The invention provides a rod channel cooling liquid drop behavior analysis method based on machine learning, which comprises the following steps of: acquiring image data of a nuclear reactor fuel rod channel cooling liquid drop movement process, and temperature, pressure, liquid drop flow velocity distribution and liquid drop concentration in a channel; using a model trained based on machine learning to analyze coolant drop behaviors, including coolant drop behavior classification, short-term behavior prediction and trend analysis; the constructed model architecture is a hybrid model fusing a convolutional neural network, a recurrent neural network and a graph neural network. According to the method, the flowing state, the interaction and the phase change behavior of the cooling liquid drops can be classified in detail and accurately predicted, the probability value of each classification result is provided, and a more comprehensive and accurate decision basis is provided for operation and maintenance personnel.

The invention relates to a micro-fluidic chip based on a reflector light path and a preparation method of the micro-fluidic chip. The reflector light path comprises an incident optical fiber channel, an emergent optical fiber channel and a liquid drop flow channel, a first reflecting mirror runner and a second reflecting mirror runner are arranged on the two sides of the liquid drop runner, liquid metal is poured into the first reflecting mirror runner and the second reflecting mirror runner respectively, and then the liquid metal is cooled and solidified to form a first reflecting mirror and a second reflecting mirror. By utilizing a light path formed by multiple reflections, the effective absorption light path is increased, and the detection sensitivity and accuracy are improved; and the liquid drop flow channel is not bent, so that the liquid drops are not damaged, and the compatible liquid drop size range is wide.

The invention discloses a method for microfluidic high-throughput screening of AKK bacteria. The method comprises the following steps: 1) preparing a microfluidic chip; 2) collecting fresh excrement of healthy volunteers, diluting, and adding a fluorescence labeled antibody to obtain a dispersion phase; and (3) preparing liquid drops by taking paraffin oil and Span-80 as a mobile phase, enabling the liquid drops to enter a liquid drop culture tank through an S-shaped liquid drop circulation channel, culturing, introducing the mobile phase, introducing the liquid drops in the liquid drop culture tank into a liquid drop collection tank, taking out, detecting a fluorescence signal, culturing the liquid drops with the fluorescence signal on the improved brain heart infusion broth solid culture medium, and obtaining the brain heart infusion broth. Obtaining a to-be-identified strain; 4, the strain to be identified serves as a template, PCR amplification is conducted, sequencing is conducted on an amplification product, BLAST sequence comparison is conducted on a sequencing result on an NCBI website, and the target strain AKK bacterium is screened out. The method has the advantages of being high in throughput, simple in experimental process and high in specificity, and the screening efficiency of the AKK bacterium can be improved.

The utility model provides non-contact biological particle treatment equipment and a biological particle treatment device. The biological particle processing device comprises a liquid droplet generating chamber, a working chamber communicated with the liquid droplet generating chamber, and a sorting chamber communicated with the working chamber. The droplet generating chamber is configured to receive a first liquid, a biological particle located within the first liquid, and a second liquid insoluble in the first liquid. The droplet generating chamber is used for enabling the flow of the second liquid to be staggered with the flow of the first liquid, so that the biological particles and the first liquid part around the biological particles jointly generate a biological particle droplet flowing to the working chamber after penetrating through the second liquid; the biological particles are cultured or tested through the first liquid. Therefore, the biological particle liquid drop can protect at least one biological particle located in the biological particle liquid drop, and culture work or detection can be completed while the biological particle liquid drop moves.

The application discloses a microfluidic droplet generation control method, device, equipment and medium, relates to the field of microfluidic droplet generation, and comprises the following steps: first, dividing nodes and judging the flow state of each node according to a limited parameter by using a flow state classifier; predicting the droplet diameter by using a stacking integrated size regression model based on an ideal droplet flow state node; screening candidate nodes based on an accuracy tolerance constraint; determining local sensitivity based on a virtual flow rate micro-perturbation and a flow state boundary risk penalty; determining a robust candidate solution set based on a comprehensive risk score according to the local sensitivity and a confidence interval width; extracting a cluster and a representative solution for each candidate node in the robust candidate solution set; and calculating the two-phase flow control parameters for the representative solution of each cluster in the dimension of the continuous phase flow rate and the dispersed phase flow rate by using a univariate step-by-step double-criterion termination algorithm based on a cascade model, so that the time cost and the material cost are reduced.

The present invention discloses a controllable spray droplet efficient cooling and heat exchange experimental platform, including a droplet generating device, multiple groups of piezoelectric droplet generators that can realize droplet string arrays; a thin film heater located at the downstream end of the piezoelectric droplet generator process, the thin film heater is prepared using a magnetron sputtering process, the thin film heater uses a 500μm double-polished silicon wafer as a substrate, and a metal platinum film is magnetron sputtered on the substrate using a magnetron sputtering process; and an optical imaging and temperature measurement system. The experimental platform of the present invention is designed based on multiple groups of piezoelectric droplet generators, and can capture and accurately measure both the temperature field and the flow field. The advanced droplet generator achieves a droplet flow rate that the existing single group of droplets does not have, while also taking into account accuracy and controllability. The designed microchip thin film heater reduces resistance error and experimental error, and the heat loss is less than 1%. This experimental platform has an important impact on the exploration of droplet physics and the field of efficient heat exchange of continuous droplets.

This invention discloses a piezoelectric valve nozzle cleaning management method and system based on identifier traceability, specifically relating to the field of piezoelectric jet equipment operation and maintenance technology. The method involves reading the unique identifier of the piezoelectric valve nozzle to trace individualized cleaning records; dynamically generating customized cleaning strategies based on the records, determining the solvent type and piezoelectric excitation waveform parameters; first, driving a small atomizing valve to generate an atomized cloud for non-destructive pre-wetting of the nozzle surface, then driving the piezoelectric valve for active excitation cleaning; driving the small atomizing valve to generate standardized droplet flow as a probe, capturing the characteristic spectrum after passing through the nozzle area through a detection unit and comparing it with a standard spectrum to quantitatively evaluate the cleaning effect and accurately locate the blocked nozzle; updating the records based on the detection results, and if the cleaning is unqualified, optimizing the strategy based on the blockage location information and iteratively executing it until qualified.

The application discloses a non-contact flow field measuring device and method with nitrogen droplets as tracer particles, and belongs to the technical field of flow field visualization. A Laval nozzle is used as a nitrogen droplet generating device, and the Laval nozzle is arranged in a high-speed flow section. After the nitrogen droplets flowing out of the Laval nozzle are mixed with the main flow in the high-speed flow section, the nitrogen droplets pass through an expansion section and a flow regulation section and then enter a speed measuring section. A high-speed camera is used in the speed measuring section to capture the nitrogen droplets and obtain the air flow speed. By actively regulating and controlling the operating parameters of the wind tunnel, the total temperature, the Ma number and the expansion ratio of the Laval nozzle in the wind tunnel are reasonably controlled, so that the condensation nitrogen droplets are spontaneously generated at a proper position, the condensation degree is controlled, and the disturbance of the flow field near the test model is reduced as much as possible. The application aims to solve the key problem that suitable and available tracer particles are lacked in the application of the non-contact measurement technology in a low-temperature wind tunnel, and provides a new feasible way for the application of the non-contact flow field measurement technology in a transonic low-temperature wind tunnel.

The application relates to a device and a method for realizing particle aggregation in a droplet, which comprises: a microfluid channel for providing a channel for droplet flow; a first bulk acoustic resonator close to the microfluid channel, the working range of the first bulk acoustic resonator including the space in the microfluid channel; the overall shape of the first bulk acoustic resonator is arranged to be narrowed from the front end to the tail end, and the tail end is an outwardly convex arc-shaped end portion, the arc-shaped end portion being used for making particles in the droplet be aggregated at a first position in the droplet during the process that the droplet flows through the first bulk acoustic resonator; and the tail end is located on the downstream side in the microfluid channel relative to the front end. The scheme of the application can realize that particles in the droplet are aggregated at a first position in the droplet, so as to facilitate observation or the interaction of different particles in the droplet after aggregation.

To accurately detect fine particles without complicating a device configuration.SOLUTION: The condensation particle counter includes a supply unit configured to supply a working fluid to a periphery of an inlet of a condensation tube through which an aerosol containing sample particles flows, a saturation unit configured to saturate the aerosol with vapor of the working fluid, and a control unit configured to condense and grow the aerosol into droplets using the sample particles as nuclei and extract the aerosol containing the droplets from a center axis of a condensation-growth tube connected to the condensation tube. A condensation and growth section that and exhausts excess working fluid that has flowed down an inner wall of a condensation and growth tube, vapor of the working fluid that has condensed on the inner wall, and water vapor together with excess aerosol, a flow rate control section that controls a flow rate of the aerosol flowing through a capillary tube that the aerosol containing droplets after condensation and growth at a minute flow rate, a double cylindrical sheath flow that positions the aerosol that has passed through the capillary tube on an extension line of a central axis of the capillary tube, and a droplet counting section that counts the droplets contained in the aerosol based on the droplets flowing through the capillary tube, wherein the working fluid contains an alkane having a carbon number of 14 or less.SELECTED DRAWING: Figure 1

A miniaturized, automated method for controlled printing of large arrays of nano- to femtoliter droplets by actively transporting mother droplets over hydrophilic-in-hydrophobic (“HIH”) micropatches. The technology uses single or double-plate devices where mother droplets can be actuated and HIH micropatches on one or both plates of the device where the droplets are printed. Due to the selective wettability of the hydrophilic micropatches in a hydrophobic matrix, large nano- to femtoliter droplet arrays are created when mother droplets are transported over the arrays. The parent droplets are moved by various droplet actuation principles. Also, a method using two plates placed one top another while being separated by a spacer. One plate is dedicated to confirming and guiding parent droplets by using hydrophilic patches in a hydrophobic matrix, while the other plate contains HIH arrays for printing of the droplets. When the parent droplet guidance plate is rotated over the plate dedicated to printing of nano- to femtoliter droplets, the droplets are dispensed inside the HIH array utilizing their selective wettability. The methods allow the parent droplets to move over the HIH arrays many times, providing advantages for performing bio-assays or miniaturized materials synthesis in nano- to femtoliter sized droplets. With controlled evaporation of the dispensed droplets of solution, large arrays of printed material can be generated in seconds. The methods provide a nano- to femtoliter droplet printing technique for a wide variety of applications, e.g., protein- or cell-based bio-assays or printing of crystalline structures, suspensions of nanoparticles or microelectronic components.

Copper-boron-ferrite (Cu—B—Fe) composites may be prepared and immobilized on graphite electrodes in a silica-based sol-gel, e.g., from rice husks. Different bimetallic loading ratios can produce fast in-situ electrogeneration of reactive oxygen species, H2O2 and ·OH, e.g., via droplet flow-assisted heterogeneous electro-Fenton reactor system. Loading ratios of, e.g., 10 to 30 wt. % Fe3+ and 5 to 15% wt. Cu2+, can improve the catalytic activities towards pharmaceutical beta blockers (atenolol and propranolol) degradation in water. Degradation efficiencies of at least 99.9% for both propranolol and atenolol in hospital wastewater were demonstrated. Radicals of ·OH in degradation indicate a surface mechanism at inventive cathodes with correlated contributions of iron and copper. Copper and iron can be embedded in porous graphite electrode surface and catalyze the conversion of H2O2 to ·OH to enhance the degradation. Inventive cathodes can be stable catalytically after 20 or more cycles under neutral and acidic conditions.

Systems and methods for initializing an extreme ultraviolet (EUV) source are discussed. The EUV radiation source may include a droplet generator configured to generate a stream of droplets of a target material, a laser source communicatively coupled to a controller. The laser source is configured to generate a first pulsed beam of light, which upon interaction with a droplet, initiates a modification of a shape of the droplet, forming a modified droplet, and generate a second pulsed beam of light configured to substantially evaporate the modified droplet, thereby generating pulses of EUV light. The EUV radiation source comprises a first actuator communicatively coupled to the controller and configured to adjust a width of the second pulsed beam to minimize a variation of energy of the pulses of EUV light.

The application discloses a kind of multi-component monodisperse microdroplet digital electric control system and method, comprising: multi-component droplet generation module, for obtaining microdroplet by controlling the flow ratio between dispersed phase and continuous phase;Multi-component droplet digital electric control module is used to control the deformation, movement and fusion of the microdroplet by the combination electrode of several sharp electrodes and flat angle electrodes;The multi-component droplet generation module and multi-component droplet digital electric control module are connected by the flow channel of PVA solution injection, and are located on microdroplet chip.This application realizes the control of microdroplet component structure and the control of microdroplet size flow rate by adjusting flow ratio and adjusting overall flow rate through discrete droplet generation unit and multi-component droplet flow focusing structure, and finally forms a complex multi-component droplet independent generation system.

In one embodiment, a system includes a flow cell with a conductive electrode and a base having an opening allowing variable charged sheathed sample fluid to flow out along a fluid axis, a droplet deflection unit with a pair of charge plates and a deflection chamber with a pivot door closing over the chamber a hardware triggered camera mounted to the back of the deflection chamber, and an LED array strobe light mounted to the pivotal door opposite the camera. The LED array strobe light generates a backlighting for various droplet streams. Triggering of the camera is synchronized with the generation of the backlighting to periodically capture a brightfield still image of deflected droplets and center droplets in the deflection chamber. The captured images can be analyzed for droplet deflection angles and center line for visual feedback control of the deflected droplets and the center droplets in respective droplet streams.

A microfluidic chip system for generating droplets is provided by the present disclosure. The microfluidic chip system includes a droplet generation device for generating the droplets, a power generation device for supplying power to the droplet generation device, a collection container for collecting the droplets flowing out of the droplet generation device, a connection device connecting the droplet generation device, the power generation device, and the collection container to each other, and a preparation platform holds together the droplet generation device, the power generation device, and the collection container. A method for preparing the droplets is also provided by the present disclosure.

The invention discloses a droplet microfluidic in-situ test chip and method for microbial mineralization crystallization kinetic research, and relates to the technical field of rock and soil tests. In order to solve the problems that in the prior art, a single crystal crystallization process is difficult to visualize and nucleation sites of crystals are difficult to observe in situ and in real time, the following technical scheme is provided: the device comprises an upper layer channel pattern layer, solid polydimethylsiloxane and cover glass, an oil phase liquid injection port is formed in the top of the upper layer channel pattern layer, and is connected with an annular oil phase pipeline; a first water-phase liquid injection port and a second water-phase liquid injection port are formed in the middle of the area where the annular oil-phase liquid injection pipeline is located, a pipeline connecting the first water-phase liquid injection port and the second water-phase liquid injection port forms a reaction liquid mixing area at the communication position of the water-phase liquid injection pipeline, a liquid drop generation area is formed at the communication position of the oil-phase liquid injection pipeline and the water-phase liquid injection pipeline, and liquid drops form an in-liquid-drop solution mixing area in the snakelike circulation pipeline. And the liquid drops flow out through the liquid outlet. The method has the advantages of high flux, accurate reaction conditions, accurate observation and the like.

To provide a droplet deflector having an optimized deflection plate.SOLUTION: Droplet deflectors of interest include droplet deflectors comprising deflection plates configured for high angle deflection of a droplet flow stream. Droplet deflectors of interest further include those comprising deflection plates having a shape corresponding to the path of the deflected droplet flow stream, or deflection plates configured to apply a deflection force to the flow stream at a plurality of different angles. Droplet deflectors of interest further include droplet deflectors that are configured to apply a constant deflection force to the deflected droplet flow stream from a plurality of different lateral positions, e.g., wherein the deflector plate is a segmented deflector plate. Further provided are flow cytometers comprising the subject droplet deflectors, as well as methods of using and assembling or configuring the same.SELECTED DRAWING: None

The invention provides a double-laser liquid drop control measurement method and device.The double-laser liquid drop control measurement method comprises the steps that nano-particle liquid drops are prepared, and the nano-particle liquid drops are placed on a glass substrate of a pre-configured liquid drop operation table; laser is emitted through the liquid drop operation table, laser parameters are adjusted, the laser is made to be focused in the nano-particle liquid drops, and the nano-particle liquid drops are driven to move; raman laser penetrating through the nano-particle liquid drops is emitted through the liquid drop operation table, and the internal characteristics and the movement condition of the nano-particle liquid drops are determined according to the imaging result of the Raman laser; the internal characteristics include real-time temperature and structural characteristics. By means of the method and device, accurate control over liquid drop fluid flowing is achieved, the movement condition of nano-particle liquid drops can be fed back in real time in the control process, and the problem that in the prior art, the liquid drop control accuracy degree is poor is solved.

An illumination source includes a droplet generator, a laser, a detector, and a controller. The droplet generator produces a stream of droplets of target material directed to an interaction region. The laser irradiates the droplets at the interaction region using a beam of pulses. The irradiated droplets generate output illumination of the illumination source. The detector measures an aspect of an interaction between a pulse from the beam and a droplet from the stream. Controller includes circuitry and determines a pulse timing adjustment that is nonlinearly dependent on the measured aspect. The controller also controls a subsequent laser pulse of the beam based on the pulse timing adjustment.

The present invention discloses an insulator electric field characteristic evaluation method based on an equivalent growth length model, which includes: constructing a fluid-electric field coupling simulation model based on the dynamic deformation of melting ice droplets; analyzing the influence of the water droplet flow velocity on the electric field strength at the ice ridge gap based on the fluid-electric field coupling simulation model, and obtaining the analysis results; fitting to obtain a water droplet remaining length fitting formula based on the analysis results, and constructing a static model of the equivalent growth length of the ice ridge tip; analyzing and evaluating the spatial electric field characteristics of the ice-covered insulator based on the equivalent growth length static model. The present invention uses the equivalent growth length static model to replace the conventional dynamic model, which can not only effectively improve the simulation speed but also ensure the accuracy of the analysis of the electric field characteristics of the insulator during the ice melting period, thereby greatly improving the efficiency of the analysis of the electric field characteristics of the insulator during the ice melting period, and then being widely applied to the discharge risk assessment of ice-covered insulators in different ice melting environments.

Aspects of the present disclosure include a particle sorter with a droplet deflector configured to apply a known offset deflection force to a droplet stream. Particle sorters according to certain embodiments include a flow cell, a light source, e.g., laser, for irradiating an interrogation point of the flow cell, a detector for detecting light from the interrogation point, a droplet generator for producing a droplet stream from fluid exiting the flow cell and a droplet deflector configured to apply a known offset deflection force to the droplet stream. In some cases, the droplet deflector comprises first and second plates configured to be offset from one another. Methods and particle sorting modules for applying a known offset deflection force are also provided.

In one embodiment, a system includes a flow cell with a conductive electrode and a base having an opening allowing variable charged sheathed sample fluid to flow out along a fluid axis, a droplet deflection unit with a pair of charge plates and a deflection chamber with a pivot door closing over the chamber a hardware triggered camera mounted to the back of the deflection chamber, and an LED array strobe light mounted to the pivotal door opposite the camera. The LED array strobe light generates a backlighting for various droplet streams. Triggering of the camera is synchronized with the generation of the backlighting to periodically capture a brightfield still image of deflected droplets and center droplets in the deflection chamber. The captured images can be analyzed for droplet deflection angles and center line for visual feedback control of the deflected droplets and the center droplets in respective droplet streams.

The embodiment of the present application discloses a kind of microfluidic chip.The microfluidic chip includes oppositely arranged first substrate and second substrate, microfluidic channel is formed between first substrate and second substrate, microfluidic channel is used to accommodate at least one droplet;Multiple drive electrodes, multiple first sensing electrodes and multiple second sensing electrodes are located on the side of first substrate, adjacent drive electrodes are loaded with different drive voltage signals to drive droplet to move;First sensing electrode and second sensing electrode are loaded with detection signal, and the position of droplet is determined according to the change of the capacitance formed by first sensing electrode and a certain electrode and the change of the capacitance formed by second sensing electrode and a certain electrode when droplet flows.The microfluidic chip provided by the present application can obtain the position of droplet while driving the movement of droplet, and solve the problem of low reliability of equipment caused by the inability to detect the position of droplet in the prior art.

This invention relates to the field of droplet hydrodynamics, specifically to a visualization experimental system and method for studying droplet thermocapillary phenomena. The visualization experimental system for studying droplet thermocapillary phenomena includes: a droplet control device, a heating device, and a data acquisition device. The droplet control device generates two suspended droplets and controls at least one of the suspended droplets to approach and collide with the other droplet in a horizontal direction. The two suspended droplets contain fluorescein and tracer particles. The heating device radiates ultraviolet light onto the two suspended droplets and controls their temperature. The data acquisition device acquires video and real-time images of at least one of the suspended droplets approaching and colliding with the other droplet in a horizontal direction.

A method for counter-current liquid-liquid extraction in a sub-millimeter conduit is implemented from an initial liquid drop stream. The stream has alternating drops of a first liquid, and a second liquid less wetting than the first liquid and immiscible with the first liquid. One of the liquids includes a component to extract towards the other liquid. A first pressure gradient applied along the conduit generates a visco-inertial flow displacing a first drop stream volume according to the first gradient and generating a film of first liquid displaced along the opposite orientation, the film being located between the drops of second liquid and the conduit. The application of the first pressure gradient is stopped. A second pressure gradient is applied along the conduit, in the opposite orientation to generate a viscous-capillary flow displacing a second volume of according to the second gradient and application of the second pressure gradient stops.