Method for calculating wettability of rough surface of mineral

Abstract

The invention provides a method for calculating the wettability of a rough surface of a mineral. The method comprises the following steps: obtaining a calcined mineral crystal model of various soil minerals after action at different temperatures through reaction molecular dynamics simulation; constructing a water drop model to obtain a calcined mineral-water drop model, and performing wettability simulation on the surface of a calcined mineral crystal; obtaining a water drop surface tension coefficient and water drop / mineral interface adhesion work through the water drop model and the calcined mineral-water drop model; and the contact angle of the rough surface is calculated through a Young-Drupe equation. According to the method, an accurate mineral crystal model after high-temperature action can be obtained through reaction molecular dynamics, and the influence of high temperature on the surface wettability can be better reflected; according to the method, the mineral surface wettability can be rapidly represented, errors caused by a profile curve fitting method are reduced, system errors caused by the rough surface are avoided, the water contact angle of the rough surface is accurately calculated, and then the mineral rough surface wettability after the fire disaster is accurately represented.

Description

Technical Field

[0001] The invention relates to the technical field of soil wettability, in particular to a method for calculating the wettability of a rough mineral surface. Background Art

[0002] Soil is hydrophilic and mainly composed of minerals (montmorillonite, kaolinite, illite, quartz, etc.). In recent years, it has been found that soil after drought and forest fire has become hydrophobic, that is, water droplets cannot spontaneously infiltrate the soil, which seriously threatens the safe production of agriculture and animal husbandry, and reduces the water retention and permeability of the soil, leading to soil erosion and floods.

[0003] Under high temperature, the crystal structure of the soil surface has changed. However, it is impossible to determine where exactly the minerals in the soil have changed. Molecular dynamics simulation can analyze the interaction between molecules from a molecular perspective. However, molecular dynamics under the empirical force field infers the chemical mechanism between molecules through physical reactions, but it cannot accurately explore the physical and chemical transformations of minerals under high temperature. Reaction molecular dynamics simulation can monitor the physical and chemical transformations of mineral crystal structures under high temperature. Through reaction molecular dynamics simulation, an accurate crystal model of minerals after high temperature can be obtained, and changes in the crystal structure will lead to changes in the wettability of the soil surface. Therefore, the premise for accurate analysis of wettability is to obtain accurate mineral molecular structures.

[0004] The most important indicator of mineral surface wettability is the contact angle. In unsaturated soil, the contact angle affects the capillary suction, which in turn affects the distribution and movement of water in the soil, the shear strength and stability of the soil.

[0005] At present, the contact angle can be obtained through experimental methods such as the sessile drop method, and can also be obtained through empirical force fields (CLAYFF, CVFF, COMPASS, PCFF and other force fields) and molecular dynamics simulation fitting. In experiments and molecular dynamics simulations, the contact angle is usually obtained by fitting the contour curve of the droplet. That is, the relevant parameters are determined by curve fitting the outer contour of the water droplet, and then the contact angle is calculated.

[0006] There are many methods for curve fitting, such as circular curves, elliptical curves, etc. However, the boundary contour needs to be determined manually during fitting. Different curves will lead to different water contact angle results each time. Therefore, the method of testing water contact angle by curve fitting is prone to errors. In addition, the current contact angle test and molecular dynamics simulation are measured on relatively smooth surfaces, while in real soil, the soil surface is often rough, but the current tests and simulations cannot well characterize the wettability of rough surfaces. Summary of the invention

[0007] In view of the deficiencies in the prior art, the present invention provides a method for calculating the wettability of a rough surface of a mineral. The present invention can avoid the systematic error caused by the rough surface, thereby accurately characterizing the surface wettability of the mineral after a fire.

[0008] The technical solution of the present invention is: a method for calculating the wettability of a rough surface of a mineral, comprising the following steps:

[0009] S1) obtaining the calcined mineral crystal models of various soil minerals at different temperatures through reaction molecular dynamics simulation;

[0010] S2), constructing a water drop model, obtaining a calcined mineral-water drop model, and performing wettability simulation on the surface of the calcined mineral crystal;

[0011] S3), obtaining the surface tension coefficient of the water drop and the adhesion work of the water drop / mineral interface through the water drop model and the calcined mineral-water drop model;

[0012] S4) The contact angle of the rough surface is calculated by the Young-Drupé equation to characterize the surface wettability of the mineral after high temperature.

[0013] Preferably, in step S1), the construction of the calcined mineral crystal model specifically comprises the following steps:

[0014] S11), importing the cell models of various minerals into Materials Studio software respectively, and performing supercell processing on the mineral cell models to obtain large-scale models of various minerals;

[0015] S12), performing energy minimization processing on the large-scale models of the minerals through LAMMPS to obtain a mineral model with a reasonable density distribution;

[0016] S13), calcining simulation is performed on the mineral model, and the mineral model obtained in step S12) is respectively established along the positive and negative directions of the Z axis. The vacuum layer is set at the top and bottom The distillation layer is used to remove the water molecules formed by mineral dehydroxylation under high temperature. The mineral model is heated to 378K, 473K, 573K, 673K, 773K and 873K at a constant rate of 10K / ps, and the simulation is carried out for 3ns at this temperature until the total energy of the system tends to be stable, and the water molecules formed by mineral dehydroxylation are removed in the process.

[0017] S14), and then the temperature is reduced from the specified temperature to 298K at a constant rate of 10K / ps, and a simulation is performed at 298K for 1ns until the total energy of the system tends to be stable again, and finally an accurate calcined mineral crystal model is obtained.

[0018] Preferably, in step S12), during the energy minimization process, the relaxation of the NVE ensemble with a duration of 100 ps, ​​the NVT ensemble with a duration of 100 ps and a temperature of 298 K, and the NPT ensemble with a duration of 100 ps, ​​a temperature and a pressure of 298 K and 1 atm respectively are performed.

[0019] Preferably, in step S13), the calcination simulation of the mineral model is carried out under the NVT ensemble, the integration step is selected to be 0.25 fs, and the temperature is maintained at 298 K by the Nosé-Hoover method, the temperature damping constant is set to 1 fs to control the heating and cooling rates to 10 K / ps, and the temperature damping constant is set to 100 fs to maintain a constant temperature; periodic boundaries are set in the X and Y directions, and an elastic wall is set at the Z direction boundary.

[0020] Preferably, in step S13), during the calcination simulation of the mineral model, the Reax / c module in the LAMMPS code is used to perform molecular dynamics simulation, and the H / O / Si / Al / Na / Mg reaction force field is selected to define the interaction between the atoms of the mineral.

[0021] Preferably, in step S14), the calcined mineral crystal model includes a calcined kaolinite crystal model, a calcined Na-montmorillonite crystal model, a calcined illite crystal model and a calcined quartz crystal model.

[0022] Preferably, in step S2), the construction of the water drop model specifically includes the following steps:

[0023] S211), constructing a water box model;

[0024] S212), set up around the water box model In the vacuum layer, after energy minimization, the temperature of the system was controlled at 296K, 298K and 300K respectively using the Nosé-Hoover method; 1ns was simulated under the NVT ensemble with a time step of 0.25fs, and the water droplet model was obtained by simulating 1ns.

[0025] Preferably, in step S2), the calcined mineral-water drop model is obtained by merging the water drop model and the calcined mineral crystal model in Lammps.

[0026] Preferably, in step S2), in order not to affect the diffusion movement of water droplets, the calcined mineral model is subjected to a 2a×2b×1c supercell.

[0027] Preferably, in step S2), during the wettability simulation, the Reax / c module in the LAMMPS code is used to perform molecular dynamics simulations, and the H / O / Si / Al / Na / Mg reaction force field is selected to define the interactions between the atoms of the minerals and water molecules. The simulations are all performed under the NVT ensemble, with an integration step of 0.25 fs. The temperature is maintained at 298 K using the Nosé-Hoover method, and the boundaries are set to three-dimensional periodicity.

[0028] Preferably, in step S2), the calcined mineral-water drop model includes: calcined kaolinite-water drop model, Na-montmorillonite-water drop model, calcined illite-water drop model, and calcined quartz-water drop model.

[0029] Preferably, in step S3), the water drop surface tension coefficient γ l The calculation formula is:

[0030]

[0031] Where ΔE is the increased surface free energy of the liquid, which is calculated by the total energy difference between the water drop model and the water box model at equilibrium; ΔS is the increased surface area of ​​the liquid, which is calculated by the surface area difference between the water drop model and the water box model at equilibrium.

[0032] As a preferred embodiment, in step S3), the water droplet / mineral interface adhesion work W a The calculation formula is:

[0033]

[0034] In the formula, E total is the total energy; E water and E soild are the energies of water and mineral after separation; A is the contact area of ​​the water droplet / mineral interface.

[0035] Preferably, in step S4), the contact angle of the rough surface is calculated by the Young-Drupé equation, specifically:

[0036] S41), screening water molecules in contact with the substrate and obtaining the coordinates of these water molecules;

[0037] S42), calculating the contact area by using Alpha Shape algorithm;

[0038] S42), the contact angle θ is calculated by the Young-Dupré equation, that is:

[0039] W a =γ l (1+cosθ);

[0040] Where W a is the adhesion work of water droplet / mineral interface; γ l is the surface tension coefficient of the water droplet.

[0041] The beneficial effects of the present invention are:

[0042] 1. The present invention can obtain an accurate mineral crystal model after high temperature through reaction molecular dynamics, which can better reflect the influence of high temperature on its surface wettability;

[0043] 2. The present invention calculates the surface tension coefficient of the water droplet and the adhesion work of the water droplet / mineral interface respectively, and then accurately calculates the contact angle through the Young-Drupé equation. It can not only quickly characterize the wettability of the mineral surface, but also reduce the error caused by the contour curve fitting method, while avoiding the systematic error caused by the rough surface, so that the water contact angle of the rough surface can be accurately calculated, and then the rough surface wettability of the mineral after the fire can be accurately characterized. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a flow chart of the method of the present invention;

[0045] Figure 2 Schematic diagram of the modeling process of the calcined kaolinite crystal model in an embodiment of the present invention;

[0046] Figure 3 This is a structural diagram of the calcined kaolinite crystal model after simulation of the present invention;

[0047] Figure 4 Schematic diagram of modeling a water droplet model in an embodiment of the present invention;

[0048] Figure 5 This is a schematic structural diagram of the calcined kaolinite-water drop model in an embodiment of the present invention;

[0049] Figure 6 A schematic diagram of calculating the surface tension coefficient in an embodiment of the present invention;

[0050] Figure 7 Schematic diagram of calculation of adhesion work at water droplet / mineral interface in an embodiment of the present invention;

[0051] Figure 8 Schematic diagram of measuring contact angle using the edge profile method in an embodiment of the present invention. DETAILED DESCRIPTION

[0052] The specific implementation of the present invention will be further described below in conjunction with the accompanying drawings:

[0053] like Figure 1As shown, the present invention provides a method for calculating the wettability of a rough surface of a mineral, comprising the following steps:

[0054] S1) obtaining the calcined mineral crystal models of various soil minerals at different temperatures through reaction molecular dynamics simulation;

[0055] The construction of the calcined mineral crystal model specifically includes the following steps:

[0056] S11), importing the cell models of various minerals into Materials Studio software respectively, and performing supercell processing on the mineral cell models to obtain large-scale models of various minerals;

[0057] S12), using LAMMPS to perform energy minimization on the large-scale model of the mineral, relaxation of the NVE ensemble with a duration of 100 ps, ​​the NVT ensemble with a duration of 100 ps and a temperature of 298 K, and the NPT ensemble with a duration of 100 ps, ​​a temperature of 298 K and a pressure of 1 atm, respectively; and obtain a mineral model with a reasonable density distribution;

[0058] S13), calcining simulation is performed on the mineral model, and the mineral model obtained in step S12) is respectively established along the positive and negative directions of the Z axis. The vacuum layer is set at the top and bottom The distillation layer is used to remove the water molecules formed by mineral dehydroxylation under high temperature. The mineral model is heated to 378K, 473K, 573K, 673K, 773K and 873K at a constant rate of 10K / ps, and the simulation is carried out for 3ns at this temperature until the total energy of the system tends to be stable, and the water molecules formed by mineral dehydroxylation are removed in the process.

[0059] S14), and then the temperature is reduced from the specified temperature to 298K at a constant rate of 10K / ps, and a simulation is performed at 298K for 1ns until the total energy of the system tends to be stable again, and finally an accurate calcined mineral crystal model is obtained.

[0060] As preferred in this embodiment, in step S13), during the simulation of mineral model calcination, the Reax / c module in the LAMMPS code is used for molecular dynamics simulation, and the H / O / Si / Al / Na / Mg reaction force field is selected to define the interaction between the atoms of the mineral. The simulation of mineral model calcination is carried out under the NVT ensemble, the integral step is selected to be 0.25fs, and the temperature is maintained at 298K by the Nosé-Hoover method, the temperature damping constant is set to 1fs to control the heating and cooling rate to 10K / ps, and the temperature damping constant is set to 100fs to maintain constant temperature; the X and Y directions are set as periodic boundaries, and the Z direction boundary is set as an elastic wall.

[0061] As a preferred embodiment of the present invention, in step S14), the calcined mineral crystal model includes a calcined kaolinite crystal model, a calcined Na-montmorillonite crystal model, a calcined illite crystal model and a calcined quartz crystal model. Figure 2 and 3 Shown are the modeling process of the calcined kaolinite crystal model and the calcined kaolinite crystal model after simulation.

[0062] S2), constructing a water drop model, and simulating the wettability of the calcined mineral crystal surface to obtain a calcined mineral-water drop model;

[0063] In this embodiment, Figure 4 As shown, the construction of the water drop model specifically includes the following steps:

[0064] S211), constructing a water box model;

[0065] S212), set up around the water box model The vacuum layer, after energy minimization, uses the Nosé-Hoover method to control the temperature of the system at 296K, 298K and 300K respectively; simulates 1ns in the NVT ensemble with a time step of 0.25fs, and obtains a water droplet model. In this embodiment, the size of the water box model is There are 1000 water molecules with a density of 1 g / cm 3 .

[0066] Then, the calcined mineral-water drop model is obtained by merging the water drop model and the calcined mineral crystal model in LAMMPS. The calcined mineral-water drop model obtained in this embodiment includes: calcined kaolinite-water drop model, Na-montmorillonite-water drop model, calcined illite-water drop model, and calcined quartz-water drop model.

[0067] In this embodiment, the surface minerals in the four-layer mineral model obtained by calcination are first retained. In order not to affect the diffusion movement of water droplets, the surface minerals before and after calcination are subjected to 2a×2b×1c supercells. In this embodiment, kaolinite is taken as an example. After supercell processing of the surface minerals before and after calcination, a kaolinite-water droplet model and a calcined kaolinite-water droplet model are obtained; wherein, there are 20408 atoms in the kaolinite-water droplet model, and there are 13765 to 18229 atoms in the calcined kaolinite-water droplet model; the structure is See the structure diagram Figure 5 shown.

[0068] Moreover, during the wettability simulation, the Reax / c module in the LAMMPS code was used for molecular dynamics simulation, and the H / O / Si / Al / Na / Mg reaction force field was selected to define the interaction between the atoms of minerals and water molecules. The simulations were all carried out under the NVT ensemble, with an integration step of 0.25 fs. The temperature was kept at 298 K using the Nosé-Hoover method, and the boundaries were set to three-dimensional periodicity.

[0069] S3), obtaining the surface tension coefficient of the water drop and the adhesion work of the water drop / mineral interface through the water drop model and the calcined mineral-water drop model;

[0070] In this embodiment, in order to calculate the surface tension coefficient γ of the water droplet l In this example, a water box with 1000 water molecules is used as the object to construct the existence and non-existence of The water box in the vacuum layer was minimized in LAMMPS, and the simulation lasted for 1 ns in the NVT ensemble with a time step of 0.25 fs. The temperature of the system was controlled at 296 K, 298 K, and 300 K respectively using the Nosé-Hoover method. The surface tension coefficient of the water droplet γ l The calculation formula is:

[0071]

[0072] Wherein, ΔE is the increased surface free energy of the liquid, which is calculated by the total energy difference between the water drop model and the water box model at equilibrium; ΔS is the increased surface area of ​​the liquid, which is calculated by the surface area difference between the water drop model and the water box model at equilibrium; the surface area of ​​the water drop model is fitted by the Alpha Shape algorithm.

[0073] like Figure 7 As shown, the water droplet / mineral interface adhesion work W a The calculation formula is:

[0074]

[0075] In the formula, E total is the total energy; E water and E soild are the energies of water and mineral after separation; A is the contact area of ​​the water droplet / mineral interface.

[0076] S4) Calculate the contact angle of the rough surface by the Young-Drupé equation, and then characterize the surface wettability of the mineral after high temperature, specifically:

[0077] S41), screening water molecules in contact with the substrate and obtaining the coordinates of these water molecules;

[0078] S42), calculating the contact area by using Alpha Shape algorithm;

[0079] S42), the contact angle θ is calculated by the Young-Dupré equation, that is:

[0080] W a =γ l (1+cosθ);

[0081] Where W a is the adhesion work of water droplet / mineral interface; γ l is the surface tension coefficient of the water droplet.

[0082] In this embodiment, the water contact angle of the calcined kaolinite surface is taken as an example. In order to verify the accuracy of the calculated contact angle, the curve is also measured by the edge profile method. Figure 8 As shown, during the measurement, first find the line segment of the local water / kaolinite contact in the left figure, then find the outer contour line segment of the water droplet in the right figure, and then calculate the angle between the two line segments. Finally, the average value of the line segment angles is used as the measured contact angle.

[0083] The final contact angle calculated by the Young–Dupré equation and the contact angle measured are shown in Table 1

[0084] Table 1 Calculated contact angle and measured contact angle

[0085]

[0086] As can be seen from Table 1, there is a certain gap between the contact angle calculated by the Young–Dupré equation and the contact angle measured. The difference is caused by the large surface roughness of kaolinite after high temperature, which hinders the diffusion of water droplets on the kaolinite surface. In addition, the water droplet contour is not full, the atomic distribution is very scattered, and its contact boundary is artificially calibrated, which leads to many subjective factors in the contact angle measurement process. The contact angle calculated by the Young–Dupré equation can avoid the systematic error caused by the rough surface, thereby accurately characterizing the wettability of the rough surface of the mineral after the fire.

[0087] The above embodiments and descriptions are only for illustrating the principles and best embodiments of the present invention. Without departing from the spirit and scope of the present invention, the present invention may be subject to various changes and improvements, all of which fall within the scope of the present invention to be protected.

Claims

1. A method for calculating the wettability of a rough mineral surface, characterized in that: The following steps are involved: S1) obtaining the calcined mineral crystal models of various soil minerals at different temperatures through reaction molecular dynamics simulation; S2), constructing a water drop model, obtaining a calcined mineral-water drop model, and performing wettability simulation on the surface of the calcined mineral crystal; S3), obtaining the surface tension coefficient of the water drop and the adhesion work of the water drop / mineral interface through the water drop model and the calcined mineral-water drop model; S4) The contact angle of the rough surface is calculated by the Young-Drupé equation to characterize the surface wettability of the mineral after high temperature.

2. A method for calculating the wettability of a rough mineral surface according to claim 1, characterized in that: In step S1), the construction of the calcined mineral crystal model specifically includes the following steps: S11), importing cellular models of various minerals into Materials Studio software, and performing supercell processing on the mineral cellular models to obtain large-scale models of various minerals; S12), performing energy minimization processing on the large-scale models of the minerals through LAMMPS to obtain a mineral model with a reasonable density distribution; S13), calcining simulation is performed on the mineral model, and the mineral model obtained in step S12) is respectively established along the positive and negative directions of the Z axis. The vacuum layer is set at the top and bottom The distillation layer is used to remove the water molecules formed by mineral dehydroxylation under high temperature. The mineral model is heated to 378K, 473K, 573K, 673K, 773K and 873K at a constant rate of 10K / ps, and the simulation is carried out for 3ns at this temperature until the total energy of the system tends to be stable, and the water molecules formed by mineral dehydroxylation are removed in the process. S14), and then the temperature is reduced from the specified temperature to 298K at a constant rate of 10K / ps, and a simulation is performed at 298K for 1ns until the total energy of the system tends to be stable again, and finally an accurate calcined mineral crystal model is obtained.

3. A method for calculating the wettability of a rough surface of a mineral according to claim 2, characterized in that: In step S12), during the energy minimization process, the NVE ensemble with a duration of 100 ps, ​​the NVT ensemble with a duration of 100 ps and a temperature of 298 K, and the NPT ensemble with a duration of 100 ps, ​​a temperature and a pressure of 298 K and 1 atm are relaxed respectively.

4. A method for calculating the wettability of a rough mineral surface according to claim 2, characterized in that: In step S13), the calcination simulation of the mineral model is carried out under the NVT ensemble, the integration step is selected as 0.25fs, and the temperature is maintained at 298K by the Nosé-Hoover method, the temperature damping constant is set to 1fs to control the heating and cooling rates to 10K / ps, and the temperature damping constant is set to 100fs to maintain constant temperature; periodic boundaries are set in the X and Y directions, and an elastic wall is set at the Z direction boundary.

5. A method for calculating the wettability of a rough surface of a mineral according to claim 2, characterized in that: In step S14), the calcined mineral crystal model includes a calcined kaolinite crystal model, a calcined Na-montmorillonite crystal model, a calcined illite crystal model and a calcined quartz crystal model.

6. A method for calculating the wettability of a rough mineral surface according to claim 1, characterized in that: In step S2), the construction of the water drop model specifically includes the following steps: S211), constructing a water box model; S212), set up around the water box model In the vacuum layer, after energy minimization, the temperature of the system was controlled at 296K, 298K and 300K respectively using the Nosé-Hoover method; 1ns was simulated under the NVT ensemble with a time step of 0.25fs, and the water droplet model was obtained by simulating 1ns.

7. A method for calculating the wettability of a rough mineral surface according to claim 6, characterized in that: In step S2), the calcined mineral-water drop model is obtained by merging the water drop model and the calcined mineral crystal model in LAMMPS.

8. A method for calculating the wettability of a rough mineral surface according to claim 7, characterized in that: In step S2), during the wettability simulation, molecular dynamics simulation was performed using the Reax / c module in the LAMMPS code, and the H / O / Si / Al / Na / Mg reaction force field was selected to define the atomic interactions between minerals and water molecules. The simulations were all performed under the NVT ensemble, with an integration step of 0.25 fs. The temperature was maintained at 298 K using the Nosé-Hoover method, and the boundaries were set to three-dimensional periodicity.

9. A method for calculating the wettability of a rough mineral surface according to claim 1, characterized in that: In step S3), the water drop surface tension coefficient γ l The calculation formula is: Where △E is the increased surface free energy of the liquid, which is calculated by the total energy difference between the water drop model and the water box model at equilibrium; △S is the increased surface area of ​​the liquid, which is calculated by the surface area difference between the water drop model and the water box model at equilibrium; The water droplet / mineral interface adhesion work W a The calculation formula is: In the formula, E total is the total energy; E water and E soild are the energies of water and mineral after separation; A is the contact area of ​​the water droplet / mineral interface.

10. A method for calculating the wettability of a rough mineral surface according to claim 9, characterized in that: In step S4), the contact angle of the rough surface is calculated by the Young-Drupé equation, specifically: S41), screening water molecules in contact with the substrate and obtaining the coordinates of these water molecules; S42), calculating the contact area by using Alpha Shape algorithm; S42) The contact angle θ is calculated by the Young–Dupré equation, namely: W a =c l (1+cosθ); Where W a is the adhesion work of water droplet / mineral interface; γ l is the surface tension coefficient of the water droplet.

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