A method for calculating the mass and heat adsorption coefficients of water vapor molecules on the surface of a droplet

Abstract

The application discloses a kind of liquid drop surface water vapor molecule mass and heat adsorption coefficient calculation method, it is related to heat and mass transfer field, comprising: constructing molecular dynamics model, including single water molecule, single succinic acid molecule and multiple water molecules and multiple succinic acid droplet model of molecule;Based on the NVT ensemble balance simulation of liquid drop to Lammps, output result file;Based on result file, the isometric radius of liquid drop is calculated;According to droplet centroid, molecular average diameter, force field cutoff radius and isometric radius, the simulation system is partitioned along the radial direction with droplet centroid as origin;Water molecule trajectory analysis, and then the mass and heat adsorption coefficient is calculated.The application obtains all molecular motion trajectories after liquid drop equilibrium using molecular dynamics simulation method, divides liquid drop and surrounding space into different regions along radial direction, labels all molecules and carries out trajectory analysis, and quantitatively obtains the evolution mechanism of liquid drop surface mass and heat adsorption coefficient from molecular level, which is more reasonable and more accurate than empirical result.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of heat and mass transfer, and particularly relates to a method for calculating the mass and heat adsorption coefficients of water vapor molecules on the surface of a droplet. BACKGROUND

[0002] The condensation and evaporation of water in droplets widely exist in the growth and evolution of atmospheric aerosol particles, spray drying, spray cooling, and pulmonary drug delivery, and other environmental, industrial, and medical processes. The mass and heat transfer at the gas-particle interface is the core process of droplet condensation and evaporation. For micro-nano scale droplets, the mass adsorption coefficient and the heat adsorption coefficient are key parameters in the theory of heat and mass transfer. The physical meaning of the former is the probability of water vapor molecules that hit the surface of the droplet and stay in the liquid phase, and the physical meaning of the latter is the probability of water vapor molecules that hit the surface of the droplet and reach equilibrium with the liquid surface temperature.

[0003] However, the existing experimental methods cannot obtain the mass and heat adsorption coefficients of water vapor molecules on the surface of micro-nano droplets, which leads to poor prediction accuracy of the droplet evaporation model. With the development of computer technology, molecular dynamics simulation has become an effective method for studying nanoscale physical and chemical problems. It is based on different force field models to calculate the motion state of each atom in the system. It not only can break through the condition limitation of general experimental methods to obtain molecular level information, but also can quantitatively obtain micro parameters by combining with some statistical physics analysis methods. Therefore, molecular dynamics simulation is an ideal method for obtaining the mass and heat adsorption coefficients of the gas-particle interface.

[0004] Therefore, the person skilled in the art is committed to developing a method for calculating the mass and heat adsorption coefficients of water vapor molecules on the surface of a droplet. SUMMARY

[0005] In view of the above defects of the prior art, the technical problem to be solved by the present application is that the existing experimental instruments cannot measure the mass and heat adsorption coefficients of surface water molecules in the droplet condensation and evaporation process. In the existing model, the above two parameters are both empirical values, which affects the accuracy of the model.

[0006] To achieve the above-mentioned purpose, the present application provides a method for calculating the mass and heat adsorption coefficients of water vapor molecules on the surface of a droplet, characterized in that the method comprises the following steps:

[0007] S101: constructing a molecular dynamics model, the model comprising a single water molecule geometric model, a single succinic acid molecule geometric model, and a droplet initial model, the droplet initial model comprising a plurality of water molecules and a plurality of succinic acid molecules;

[0008] S103: performing NVT ensemble equilibrium simulation on the droplet based on Lammps, and outputting a result file;

[0009] S105: calculating the equal molar radius of the droplet based on the acquired molecular dynamics simulation result file;

[0010] S107: partitioning the simulation system along the radial direction with the droplet centroid as the origin according to the droplet centroid, the average diameter of the molecules, the force field cutoff radius, and the equal molar radius of the droplet;

[0011] S109: analyzing the water molecule trajectory to calculate the total number of events in which the water molecules hit the surface of the droplet;

[0012] S111: calculating the mass adsorption coefficient and the heat adsorption coefficient.

[0013] Further, in the step S101, the data file required for establishing the LAMMPS simulation by msi2lmp is also included, and the data file contains the side length of the simulation cubic box, the initial coordinates of the atoms, the atomic charge, the bond stretching, the bond angle bending, and the dihedral angle information.

[0014] Further, in the step S103, the simulation system of the NVT ensemble equilibrium simulation adopts a three-dimensional periodic boundary condition, the fixed force field cutoff radius in the molecular dynamics simulation is 1.4 nm, the time step is 1 fs, the water molecules adopt the SPC / E force field model, and the force field parameters of the succinic acid molecules are derived from the OPLS-aa force field.

[0015] Further, the NVT ensemble equilibrium simulation supports simulation under various temperature conditions, and the temperature conditions adopt the Nose-Hoover heat bath method to maintain the temperature at 280 K, 290 K, 300 K, 310 K, or 320 K, respectively.

[0016] Further, when the droplet reaches the equilibrium state during the simulation process, 100 ns is run, and the result file is output every 1500 steps, and each result file contains the type of atom and molecule, the atomic serial number, and the coordinate information of the atom, and the equilibrium state adopts the following judgment criterion: when the interaction potential energy between all water molecules in the particle and their own molecules remains unchanged, it is considered that the droplet reaches the equilibrium state.

[0017] Further, in the step S105, the radial number density distribution of all atoms along the radial direction is calculated with the droplet centroid as the center, and the equal molar radius of the droplet is calculated by data fitting:

[0018]

[0019]

[0020] where ρ(r) is the total number density of all atoms in the droplet at radius r, R e is the equal-molar radius of the droplet, ρ α is the liquid phase density, ρ β is the gas phase density, r0 is the interface radius, and ξ is the interface thickness.

[0021] Further, the method for determining the droplet centroid comprises the following steps:

[0022] calculating the centroid coordinates of each molecule;

[0023] tracking the number of molecules within 1 nm of the molecule;

[0024] determining whether the molecule is in the droplet phase: if the number of molecules is greater than 8, the molecule is in the droplet phase;

[0025] calculating the centroid of all droplet phase molecules, which is the centroid of the droplet.

[0026] Further, in the step S107, the simulation system is divided into four regions: the droplet bulk phase region, the droplet surface region, the incident region, and the gas phase region, and the centroid of a water molecule entering the four regions is marked: entering the droplet bulk phase region is marked as 2, entering the droplet surface region is marked as 1, entering the incident region is marked as 0, and entering the gas phase region is marked as -1.

[0027] Further, in the step S109, in one incident event, the water molecule hitting the droplet surface event includes adsorption event, dispersion event, desorption event, and absorption event, wherein,

[0028] the incident event: when the mark of a water molecule changes from -1 to non-1, it is recorded as an incident event, until the next -1 appears or the simulation ends;

[0029] the adsorption event: when the mark of the water molecule only appears 0 or 1 after -1, the event is defined as an adsorption event, and the total number of adsorption events is n ads ;

[0030] the dispersion event: when the mark of the water molecule appears 1 after -1, and then -1 again, and the time interval between the two -1s is less than 3 ps, the event is defined as a dispersion event, and the total number of dispersion events is n scatt ;

[0031] the desorption event: when the mark of the water molecule appears 1 after -1, and then -1 again, and the time interval between the two -1s is greater than 3 ps, the event is defined as a desorption event, and the total number of desorption events is n des ;

[0032] The absorption event: when the water molecule assignment value is -1 after 2, the event is defined as an absorption event, and the total absorption event occurrence number is n abs ;

[0033] In one incident of the incident, the total number of events in which the water molecule hits the droplet surface is n ads +n scatt +n des +n abs .

[0034] Further, the case where water molecules enter the droplet bulk phase includes absorption events and partial adsorption events (represented by a correction factor P k ), the mass adsorption coefficient is the ratio of the number of molecules entering the droplet bulk phase to the number of molecules hitting the liquid surface, when the trajectory of the molecule is adsorption, absorption or desorption, the molecule and the droplet reach temperature equilibrium, the heat adsorption coefficient is the ratio of the number of molecules hitting the liquid surface to the number of molecules hitting the liquid surface, wherein:

[0035] The calculation method of the mass adsorption coefficient is:

[0036]

[0037]

[0038] The calculation method of the heat adsorption coefficient is:

[0039]

[0040] Wherein, α m is the mass adsorption coefficient, α h is the heat adsorption coefficient, P k is the correction factor, which represents the probability of converting the adsorption event to the absorption event without time limitation.

[0041] In the preferred embodiment of the present application, compared with the prior art, the present application has the following beneficial effects:

[0042] 1. The present application uses molecular dynamics simulation method to obtain the motion trajectory of all molecules after the droplet is balanced, on this basis, the droplet and the surrounding space are divided into different regions along the radial direction, all water molecules are labeled at each time according to the different regions, and finally the accurate results are calculated according to the physical meaning of the mass adsorption coefficient and the heat adsorption coefficient, so that the evolution mechanism of the mass adsorption coefficient and the heat adsorption coefficient of the droplet surface is obtained quantitatively from the molecular level, which is more reasonable and accurate than the empirical results.

[0043] 2. The application proposes to determine whether the system is balanced based on tracking the intermolecular potential energy in the simulation system, to calculate and record the interaction potential energy between all water molecules and their own molecules in the particles during the simulation process, and to consider that the droplet reaches the balanced state when the potential energy remains unchanged, so as to accurately obtain the balance time of the simulation system and ensure that the molecular trajectory information used for analysis is in the balanced state.

[0044] 3. The application proposes to record the positions of the molecules at different times based on the method of radial partitioning of the droplet and marking the positions of the water molecules, to partition the droplet and the surrounding space along the radial direction of the droplet according to the droplet centroid, the average diameter of the molecules, the force field cutoff radius and the molar radius, to mark the position of each water molecule at different times with a number, and to track the position information of each water molecule at each time.

[0045] The concept, specific structure and technical effects of the application will be further described below with reference to the accompanying drawings, so as to fully understand the purpose, features and effects of the application. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 is a flow chart of the calculation method of a preferred embodiment of the application;

[0047] Figure 2 is a geometric model schematic diagram of a single water molecule and a succinic acid molecule of a preferred embodiment of the application;

[0048] Figure 3 is a schematic diagram of a mixed droplet of a plurality of water molecules and a plurality of succinic acid molecules of a preferred embodiment of the application;

[0049] Figure 4 is a schematic diagram of partitioning of a droplet and surrounding space of a preferred embodiment of the application;

[0050] Figure 5 is a variation of the heat adsorption coefficient and the mass adsorption coefficient with temperature of a preferred embodiment of the application. DETAILED DESCRIPTION

[0051] The application can be embodied in many different forms and the scope of protection of the application is not limited to the embodiments described herein.

[0052] In the drawings, components of the same structure are denoted by the same reference numerals, and components having similar structures or functions are denoted by similar reference numerals. The size and thickness of each component shown in the drawings are arbitrarily shown, and the size and thickness of each component are not limited in the application. In order to make the drawing clearer, the thickness of some components is appropriately exaggerated in some places in the drawing.

[0053] As Figure 1 shown, for the existing test instrument cannot measure the mass and heat absorption coefficient of surface water molecules in the process of droplet condensation and evaporation, both parameters are used to affect the model accuracy, and the two coefficients are the key parameters in the theoretical model of micro-nano scale droplet condensation and evaporation, therefore, the embodiment of the present application provides a droplet surface water vapor molecule mass and heat absorption coefficient calculation method, including the following steps:

[0054] S101: Construct a molecular dynamics model, the model includes a single water molecule geometric model, a single succinic acid molecule geometric model and a droplet initial model, the droplet initial model contains a plurality of water molecules and a plurality of succinic acid molecules.

[0055] In the process of constructing the molecular dynamics model, it also includes using msi2lmp to establish the data file required for LAMMPS simulation, the data file contains the edge length of the simulation cubic box, the initial coordinates of the atoms, the atomic charge, the bond stretching, the bond angle bending and the dihedral angle information, as Figure 2 , Figure 3 shown.

[0056] S103: Based on Lammps, the NVT ensemble balance simulation of the droplet is carried out, and the result file is output.

[0057] The simulation system of the above-mentioned NVT ensemble balance simulation adopts three-dimensional periodic boundary conditions, the fixed force field cutoff radius in the molecular dynamics simulation is 1.4nm, the time step is 1fs, the water molecule adopts the SPC / E force field model, and the succinic acid molecule force field parameters are derived from the OPLS-aa force field;

[0058] The NVT ensemble balance simulation supports simulation under a variety of temperature conditions, and the temperature conditions adopt the Nose-Hoover heat bath method to keep the temperature at 280K, 290K, 300K, 310K or 320K respectively.

[0059] When the droplet reaches the equilibrium state in the simulation process, run 100ns, and output a result file every 1500 steps. Each of the above-mentioned result files contains the type of atom and molecule, the atomic serial number and the coordinate information of the atom.

[0060] The equilibrium state adopts the following judgment criteria: when the interaction potential energy between all water molecules in the particle and their own molecules remains unchanged, it is considered that the droplet reaches the equilibrium state.

[0061] S105: Based on the obtained molecular dynamics simulation result file, the droplet isovolumic radius is calculated.

[0062] Taking the droplet centroid as the center of the circle, the radial number density distribution of all atoms along the radial direction is calculated, and the isovolumic radius of the droplet is calculated by data fitting:

[0063]

[0064]

[0065] where ρ(r) is the total number density of all atoms in the droplet at radius r, R e is the isometric radius of the droplet, ρ α is the density of the liquid phase, ρ β is the density of the gas phase, r0 is the interface radius, and ξ is the interface thickness.

[0066] Optionally, the method for determining the centroid of the droplet comprises the following steps:

[0067] calculating the centroid coordinates of each molecule;

[0068] tracking the number of molecules within 1 nm of the molecule;

[0069] determining whether the molecule is in the droplet phase: if the number is greater than 8, the molecule is in the droplet phase;

[0070] calculating the centroid of all droplet phase molecules, which is the centroid of the droplet.

[0071] S107: Partitioning the simulation system radially with the droplet centroid as the origin according to the droplet centroid, the average diameter of the molecules, the force field cutoff radius, and the isometric radius.

[0072] The simulation system is divided into four regions: the droplet bulk phase region, the droplet surface region, the incident region, and the gas phase region, and the centroid of the water molecule is marked when it enters the four regions: marked as 2 when it enters the droplet bulk phase region, marked as 1 when it enters the droplet surface region, marked as 0 when it enters the incident region, and marked as -1 when it enters the gas phase region, as shown in Figure 4 .

[0073] S109: Water molecule trajectory analysis to calculate the total number of water molecule impact events on the droplet surface.

[0074] In a single incident event, the water molecule impact event on the droplet surface includes adsorption event, dispersion event, desorption event, and absorption event, wherein,

[0075] incident event: when the marker of the water molecule changes from -1 to non-1, it is recorded as an incident event, until the next -1 appears or the simulation ends;

[0076] adsorption event: when the marker of the water molecule only appears 0 or 1 after -1, the event is defined as an adsorption event, and the total number of adsorption events is n ads ;

[0077] Dispersion event: when the label of water molecule is -1 after 1, then 1 again, and the time interval of two -1 is less than 3 ps, the event is defined as dispersion event, the total number of dispersion event is n scatt ;

[0078] Desorption event: when the label of water molecule is -1 after 1, then 1 again, and the time interval of two -1 is greater than 3 ps, the event is defined as desorption event, the total number of desorption event is n des ;

[0079] Absorption event: when the label of water molecule is 2 after -1, the event is defined as absorption event, the total number of absorption event is n abs ;

[0080] In the incident event statistics, the total number of events of water molecules hitting the surface of the droplet is n ads +n scatt +n des +n abs .

[0081] S111: calculate the mass adsorption coefficient and the heat adsorption coefficient.

[0082] The condition of water molecules entering the bulk phase of the droplet includes absorption event and partial adsorption event (represented by correction factor P k ), the mass adsorption coefficient is the ratio of the number of molecules entering the bulk phase of the droplet to the number of molecules hitting the surface of the liquid, when the trajectory of the molecule is adsorption, absorption or desorption, the molecule and the droplet reach temperature equilibrium, the heat adsorption coefficient is the ratio of the number of molecules hitting the liquid surface to the number of molecules hitting the liquid surface, wherein:

[0083] The calculation method of the mass adsorption coefficient is:

[0084]

[0085]

[0086] The calculation method of the heat adsorption coefficient is:

[0087]

[0088] Wherein, α m is the mass adsorption coefficient, α h is the heat adsorption coefficient, P k is the correction factor, which represents the probability of the adsorption event being converted into the absorption event without the limitation of simulation time.

[0089] Compared with the prior art, the method for calculating the mass and heat adsorption coefficients of water molecules on the surface of droplets has the following beneficial effects:

[0090] 1. The existing test instrument cannot measure the mass and heat adsorption coefficients of water molecules on the surface of droplets in the condensation and evaporation process, and the two coefficients are key parameters in the theoretical model of micro-nano scale droplet condensation and evaporation, and the two parameters are both empirical values that affect the accuracy of the model. In view of the above problems, the motion trajectories of all molecules after the droplet is balanced are obtained by using a molecular dynamics simulation method, the droplet and the surrounding space are divided into different regions along the radial direction on this basis, all molecules are labeled according to different regions at each moment, and it is judged which of the incident, dispersion, desorption, adsorption and absorption each molecule belongs to according to the change of the label of each molecule with time. Finally, the accurate results are calculated according to the physical meaning of the mass and heat adsorption coefficients, and the evolution mechanism of the mass and heat adsorption coefficients of the droplet surface is quantitatively obtained from the molecular level, which is more reasonable and accurate than the empirical results.

[0091] 2. In view of the fact that the starting time of the balanced state of the droplet cannot be accurately judged in the simulation process of the prior art, the present application proposes a method for judging whether the system is balanced based on the tracking of the potential energy between molecules in the simulation system. The interaction potential energy between all water molecules in the particle and their own molecules is calculated and recorded during the simulation process, and the droplet is considered to reach a balanced state when the potential energy remains unchanged. The balance time of the simulation system is accurately obtained, and the trajectory information of the molecules used for analysis is ensured to be in a balanced state.

[0092] 3. In the simulation process of the prior art, the position of the droplet in the system is not fixed, and there are water vapor molecules around the droplet, which makes it difficult to accurately calculate the droplet centroid at each moment. The present application proposes a method for judging the liquid and vapor phases of the droplet based on the number of other molecules around the molecule, and accurately calculates the droplet centroid at each moment on this basis. The number of other molecules appearing in the range of less than 1 nm around each molecule is calculated, and if the total number of molecules around a certain molecule is more than 8, the molecule is considered to be in the liquid phase of the droplet, otherwise, the molecule is considered to be in the gas phase. The centroid of all molecules in the liquid phase of the droplet is the centroid of the droplet, and the centroid of the droplet at different times is accurately obtained and used as the basis for the partition of the system.

[0093] 4. In the prior art, there is no reasonable basis for classifying the positions of molecules in the system. The present application proposes a method for recording the positions of molecules at different times based on radial partitioning of the droplet and labeling of the positions of molecules. The droplet and the surrounding space are partitioned along the radial direction of the droplet according to the isomolar radius, the average diameter of the molecule and the cutoff radius, and the positions of each molecule at different times are labeled with numbers, so that the position information of each molecule at each moment can be quickly tracked.

[0094] 5、The application proposes the calculation formula according to the physical meaning of the mass adsorption coefficient and the heat adsorption coefficient, considers the factor that whether the water molecules adsorbed to the liquid drop surface area can reach the liquid drop bulk phase area due to the limited simulation time, and quantitatively obtains the evolution mechanism of the liquid drop surface mass and heat adsorption coefficient from the molecular level, which is more reasonable and more accurate than the empirical result.

[0095] The application will be described in detail below in combination with the preferred embodiments of the application.

[0096] The preferred embodiment of the application provides a liquid drop surface water vapor molecule mass and heat adsorption coefficient calculation method, relating to the calculation of the heat and mass adsorption coefficient of the nanoparticle gas-particle interface, and a flow chart of the method is shown in Figure 1 The detailed steps include:

[0097] Step 1: Construct a molecular dynamics model.

[0098] A single water molecule and a single succinic acid molecule are constructed by using Materials Studio, as shown in Figure 2 A liquid drop model containing 500 water molecules and 90 succinic acid molecules is established by using Packmol, as shown in Figure 3 The data file required for LAMMPS simulation is established by using msi2lmp, and the file contains the side length of the simulation cubic box (12 nm), the initial coordinates of the atoms, the atomic charge, the bond stretching, the bond angle bending and the dihedral angle information.

[0099] The NVT ensemble simulation of the simulation system under different temperature conditions (the total number of atoms, the volume of the simulation system and the temperature remain unchanged) is performed by using LAMMPS, and the temperature is maintained at 280K, 290K, 300K, 310K and 320K respectively by using the Nose-Hoover heat bath method, and the simulation system adopts three-dimensional periodic boundary conditions. The force field cutoff radius (r c = 1.4 nm) and the time step (1 fs) are specified in the molecular dynamics simulation. In the molecular dynamics simulation, the SPC / E force field model is used for water molecules, and the force field parameters of succinic acid molecules are derived from the OPLS-aa force field.

[0100] Step 2: Liquid drop equilibrium and trajectory information output.

[0101] The potential energy between all water molecules in the particle and their own molecules is calculated and recorded during the simulation, and the liquid drop is considered to reach the equilibrium state when the potential energy remains unchanged. Thereafter, 100 ns is run, and a result file is output every 1500 steps, and each result file contains the type of atom and molecule, the atomic serial number and the coordinate information of the atom.

[0102] Step 3: Calculate the liquid drop isometric radius.

[0103] Based on the molecular dynamics simulation result file obtained in step 2, the radial number density distribution ρ(r) of all atoms along the radial direction with the center of mass of the droplet as the center of the circle is calculated, and the liquid phase density ρ is obtained by fitting based on formula (1). α , gas phase density ρ β , interface radius r0, interface thickness ξ, and then the droplet equimolar radius R is obtained based on formula (2): e .

[0104]

[0105]

[0106] Where ρ(r) is the total density of all atoms in the droplet at radius r, R e is the equimolar radius of the droplet, ρ α is the liquid density, ρ β is the gas phase density, r0 is the interface radius, and ξ is the interface thickness.

[0107] In step three, the method for determining the droplet center of mass is as follows: after calculating the coordinates of the center of mass of each molecule, track the number of molecules smaller than 1 nm around each molecule. If the total number of molecules around a molecule is greater than 8, the molecule is considered to be in the droplet phase; otherwise, the molecule is in the gas phase. The center of mass of all droplet phase molecules is then calculated as the droplet center of mass.

[0108] In this simulation example, the equimolar radii of the droplets under different temperature conditions are shown in Table 1.

[0109] Table 1 Equimolar radius of droplets (R e )

[0110] T / K 280 290 300 310 320 [R e / nm]]> 1.7846 1.8169 1.8371 1.8180 1.8577

[0111] Step 4: Simulate system partitioning.

[0112] The simulation system is divided into four regions along the radial direction with the center of mass of the droplet as the origin, such as Figure 4 As shown. Corresponding to three radius limit values ​​(the average molecular diameter δ in the simulation system is 0.4nm): R b =R e -0.4nm, R s =R e +0.4nm, R ss =R e +1.4nm. r≤R b is the droplet phase region, R b <r≤R s is the droplet surface area, R s <r≤Rss r > R is the incident region ss r < R is the gas phase region. The water molecule is labeled -1 when its center of mass is in the gas phase region, 0 when its center of mass enters the incident region, 1 when its center of mass is in the droplet surface region, and 2 when its center of mass is in the droplet bulk region.

[0113] Step five: Water molecule trajectory analysis.

[0114] The trajectory of a water molecule is analyzed based on the Matlab program. When the label of a water molecule changes from -1 to non-1, it is recorded as an incident event, until the next -1 appears or the simulation ends. In an incident event, when the label of a water molecule only appears 0 and 1 after -1, the event is defined as adsorption (the total number of adsorption events is n ads ); when the label of a water molecule appears 1 after -1, and then -1 appears again, and the time interval between the two -1s is less than 3 ps, the event is defined as dispersion (the total number of dispersion events is n scatt ); when 1 appears between the two -1s and the time interval between the two -1s is greater than 3 ps, the event is defined as desorption (the total number of desorption events is n des ); and when the label of a water molecule appears 2 after -1, the event is defined as absorption (the total number of absorption events is n abs ). Therefore, the total number of events of water molecules hitting the droplet surface is n ads + n scatt + n des + n abs .

[0115] Step six: Calculation of the mass adsorption coefficient.

[0116] The ratio of the number of molecules entering the droplet bulk region to the number of molecules hitting the droplet surface is defined as the mass adsorption coefficient a. However, due to the limited simulation time, it cannot be determined whether the water molecules adsorbed to the droplet surface region can reach the droplet bulk region. Therefore, the upper limit of a is a upper , which can be expressed as:

[0117]

[0118] Considering the effect of simulation time on the value of a, a correction factor P k is added to the above equation, and the final mass adsorption coefficient a m is:

[0119]

[0120]

[0121] Step seven: Calculation of the thermal adsorption coefficient.

[0122] Based on the analysis of the molecular trajectory in step 5, when the molecular trajectory is adsorption (n ads ), absorption (n abs ), desorption (n des ), the molecules and the droplet are considered to have reached temperature equilibrium. The ratio of the number of molecules that have reached equilibrium at the liquid temperature to the number of molecules that have reached the liquid surface is defined as the thermal adsorption coefficient α h , which is calculated as follows:

[0123]

[0124] The final obtained mass adsorption coefficient and heat adsorption coefficient change with temperature are shown in the following figure: Figure 5 As shown, from Figure 5 It can be seen that the heat adsorption coefficient is negatively correlated with temperature, and the mass adsorption coefficient is positively correlated with temperature.

[0125] The preferred embodiments of the present invention have been described in detail above. It should be understood that numerous modifications and variations based on the concepts of the present invention are possible without inventive effort by those skilled in the art. Therefore, any technical solution that can be derived by one skilled in the art through logical analysis, reasoning, or limited experimentation based on the concepts of the present invention and the prior art should be within the scope of protection defined by the claims.

Claims

1. A method for calculating the molecular mass and heat adsorption coefficient of water vapor on a droplet surface, characterized in that: The method comprises the following steps: S101: constructing a molecular dynamics model, wherein the model includes a single water molecule geometric model, a single succinic acid molecule geometric model, and a droplet initial model, wherein the droplet initial model includes multiple water molecules and multiple succinic acid molecules; S103: Perform NVT ensemble equilibrium simulation on the droplet based on Lammps and output the result file; S105: Calculating the droplet equimolar radius based on the acquired molecular dynamics simulation result file; S107: partitioning the simulation system along the radial direction with the droplet mass center as the origin according to the droplet mass center, the average molecular diameter, the force field cutoff radius, and the equimolar radius; S109: analyzing water molecule trajectories, calculating the total number of events in which the water molecules collide with the droplet surface; S111: Calculate mass adsorption coefficient and heat adsorption coefficient; in, In step S105, the radial number density distribution of all atoms along the radial direction is calculated with the center of mass of the droplet as the center of the circle, and the equimolar radius of the droplet is calculated by data fitting: in, is the radius of all atoms in the droplet r The total density at is the equimolar radius of the droplet, is the liquid density, is the gas phase density, is the interface radius, ξ is the interface thickness; In step S109, in one incident event, the water molecules impacting the droplet surface include an adsorption event, a dispersion event, a desorption event, and an absorption event, wherein: The incident event: When the mark of the water molecule changes from -1 to non--1, it is recorded as an incident event until the next -1 appears or the simulation ends; The adsorption event: When the water molecule mark only appears 0 or 1 after -1, the event is defined as an adsorption event. The total number of adsorption events is ; The dispersion event: When the water molecule label appears 1 after -1, and then appears -1 again, and the time interval between the two -1s is less than 3 ps, the event is defined as a dispersion event, and the total number of dispersion events is ; The desorption event: When the water molecule label appears 1 after -1, and then appears -1 again, and the time interval between the two -1 is greater than 3 ps, the event is defined as a desorption event, and the total number of desorption events is ; The absorption event: When the value of the water molecule is 2 after -1, the event is defined as an absorption event. The total number of absorption events is ; In the incident event statistics, the total number of events in which the water molecules hit the droplet surface is .

2. The method according to claim 1, wherein In the step S101, msi2lmp is also used to create a data file required for LAMMPS simulation, wherein the data file contains the side length of the simulation cube box, the initial coordinates of atoms, the atomic charges, bond stretching, bond angle bending and dihedral angle information.

3. The method according to claim 1, wherein In step S103, the simulation system of the NVT ensemble equilibrium simulation adopts three-dimensional periodic boundary conditions, the force field cutoff radius in the molecular dynamics simulation is 1.4 nm, the time step is 1 fs, the water molecule adopts the SPC / E force field model, and the succinic acid molecule force field parameters are derived from the OPLS-aa force field.

4. The method according to claim 3, wherein The NVT ensemble equilibrium simulation supports simulation under various temperature conditions, where the temperature is maintained at 280 K, 290 K, 300 K, 310 K, or 320 K using the Nose-Hoover heat bath method.

5. The method according to claim 4, wherein When the droplet reaches equilibrium during the simulation, the simulation runs for 100 ns and outputs the result file every 1500 steps. Each result file contains the type of atom, the sequence number of the atom and molecule, and the coordinate information of the atom. The equilibrium state is determined by the following criterion: when the potential energy of the interaction between all water molecules and all succinic acid molecules in the particle remains unchanged, the droplet is considered to have reached equilibrium.

6. The method according to claim 5, wherein The method for determining the centroid of the droplet comprises the following steps: Calculate the center of mass coordinates of each molecule; Tracking the number of molecules smaller than 1 nm around the molecule; Determining whether the molecules are in the droplet phase: if the number of the molecules is greater than 8, the molecules are in the droplet phase; Calculate the center of mass of all droplet phase molecules, which is the center of mass of the droplet.

7. The method according to claim 1, wherein In step S107, the simulation system is divided into four regions: a droplet bulk phase region, a droplet surface region, an incident region, and a gas phase region. The center of mass of the water molecule is marked when it enters the four regions: entering the droplet bulk phase region is marked as 2, entering the droplet surface region is marked as 1, entering the incident region is marked as 0, and entering the gas phase region is marked as -1.

8. The method according to claim 7, wherein The entry of water molecules into the bulk phase of the droplet includes absorption events and partial adsorption events. The mass adsorption coefficient is the ratio of the number of molecules entering the bulk phase of the droplet to the number of molecules impacting the liquid surface. When the trajectory of the molecule is adsorption, absorption, or desorption, the molecule and the droplet reach temperature equilibrium. The thermal adsorption coefficient is the ratio of the number of molecules impacting the liquid surface that have reached equilibrium with the liquid temperature, where: The calculation method of the mass adsorption coefficient is: The calculation method of the heat adsorption coefficient is: in, is the mass adsorption coefficient, is the heat adsorption coefficient, is a correction factor that represents the probability of an adsorption event being converted into an absorption event without being limited by the simulation time.

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