Device and method for measuring free energy of solid-liquid interface of hydrophobic thin-layer porous fiber membrane

Abstract

The invention discloses a device and a method for measuring solid-liquid interface free energy of a hydrophobic thin-layer porous fiber membrane, a micro pump is adopted to convey micro liquid to be attached to a horizontal fiber membrane through a capillary tube, the fiber membrane is vertically moved downwards to enable liquid drops to be separated from the capillary tube, stable solid-liquid interface contact is formed, and the free energy of the hydrophobic thin-layer porous fiber membrane is measured. Pressure disturbance and free energy change of a solid-liquid interface caused by manual movement of the liquid drops are avoided; a digital optical microscope is used for measuring a plurality of geometric contours in the circumferential direction of a liquid drop, two orthogonal liquid drop contour curvature radiuses are adopted for calculating the pressure in the vertex of the liquid drop, the surface normal stress of the liquid drop is obtained according to the height of the liquid drop, and the solid-liquid interface free energy anisotropy caused by the complex porous structure of the fiber membrane is quantitatively represented; a surface tension resultant force on a liquid drop contour is obtained by adopting a surface tension integration method, and the surface free energy of a solid-liquid interface of the fiber membrane is calculated according to a force balance condition of the liquid drop on a gas-solid-liquid three-phase contact line.

Description

Technical Field

[0001] The present invention belongs to the technical field of free energy measurement, and particularly relates to an apparatus and method for measuring the solid-liquid interface free energy of a hydrophobic thin-layer porous fiber membrane. Background Art

[0002] Hydrophobic thin-layer porous fiber membranes are commonly used filtration and separation materials in the industrial field, and the solid-liquid interface free energy has an important impact on the filtration and separation performance. The traditional method uses the apparent contact angle to characterize the solid-liquid interface free energy. The fiber membrane has a typical rough and porous structure, and the complex pore structure makes the liquid drop in a state of partial wetting with the membrane. It is very difficult to measure microscopic parameters such as the wetting area and wetting depth. The surface free energy of the fiber membrane is not only related to the physical properties of the material itself, but also closely related to microscopic parameters such as the pore structure and wetting depth. The apparent contact angle formed by the liquid drop on the fiber membrane has the characteristics of randomness and anisotropy, and will also change significantly with the liquid drop size and wetting state. Therefore, a single measurement of the apparent contact angle cannot accurately evaluate the solid-liquid interface free energy of the fiber membrane. The complex porous structure of the fiber membrane will also cause a significant change in the solid-liquid interface free energy, which is significantly different from the surface of a smooth material. Summary of the Invention

[0003] In order to solve the deficiencies of the prior art and achieve the purpose of accurately characterizing the anisotropy phenomenon of the solid-liquid interface free energy caused by the complex porous structure of the fiber membrane, the present invention adopts the following technical solutions:

[0004] A method for measuring the solid-liquid interface free energy of a hydrophobic thin-layer porous fiber membrane, comprising the following steps:

[0005] Step 1: Around the center point of the liquid drop, collect a group of liquid drop images on the fiber membrane and extract the liquid drop contour;

[0006] Step 2: Calculate the internal pressure of the liquid drop based on the liquid drop contour, and calculate the normal contact stress between the liquid drop and the fiber membrane at the solid-liquid interface according to the internal pressure of the liquid drop and the liquid drop height;

[0007] Step 3: Combine the normal contact stress to perform a numerical integration of the surface tension on the contour curves of a group of liquid drops to obtain the resultant force of the surface tension of the liquid drop and the fiber membrane in the horizontal direction and the vertical direction at the three-phase contact point. Since the liquid drop is in a force equilibrium state in the horizontal direction, the resultant force of the surface tension in the horizontal direction is equal to the solid-liquid interface force of the fiber membrane. By dividing the resultant force of the surface tension in the horizontal direction by the radius from the three-phase contact point to the center of the liquid drop, the surface free energy of the fiber membrane solid-liquid interface is obtained. The three-phase contact point is located on the "gas-solid-liquid" three-phase contact line of the liquid drop.

[0008] Further, in step 1, two sets of orthogonal droplet profile curves are collected. In step 2, based on the two sets of orthogonal droplet profile curves, the radius of curvature of the droplet vertex is calculated, and from the radius of curvature of the vertex, the internal pressure p0 of the droplet formed by the surface tension of the droplet is calculated using the following formula:

[0009]

[0010] where σ0 represents the normal stress of the droplet vertex surface in the normal direction, and γ LV represents the liquid surface tension coefficient, and r1 and r2 respectively represent the radii of curvature of the two orthogonal profiles of the micro-droplet vertex.

[0011] Further, in step 2, from the internal pressure p0 of the droplet vertex and the height h from the droplet vertex to the surface of the fiber membrane, the normal stress σ n of the droplet surface is calculated using the following formula:

[0012] σ n = p0 + ρgh

[0013] where ρ represents the droplet density and g represents the acceleration due to gravity.

[0014] Further, in step 3, the surface free energy γ * SL of the solid-liquid interface between the droplet and the fiber membrane is calculated using the following formula:

[0015]

[0016] where F cx represents the resultant force in the horizontal direction, θ τ represents the angle between the tangent of the droplet profile and the horizontal plane, x represents the horizontal distance from the droplet profile to the droplet center, r represents the droplet radius, α represents the circumferential angle of the droplet profile, Δα represents the step size of the rotation angle, and r Ω represents the radius from the three-phase contact point to the droplet center.

[0017] An apparatus for measuring the free energy of the solid-liquid interface of a hydrophobic thin-layer porous fiber membrane includes a droplet profile extraction module, a droplet surface normal stress calculation module, and a surface free energy generation module. By using the method for measuring the free energy of the solid-liquid interface of the hydrophobic thin-layer porous fiber membrane, the free energy of the solid-liquid interface of the fiber membrane is obtained.

[0018] Further, the droplet profile extraction module includes a micro pump, a capillary, a fiber membrane, a rotary micro stage, a vertical micro stage, and a microscope camera;

[0019] The micro pump transports the liquid to form a droplet at the outlet of the capillary. The droplet adheres to the horizontally placed fiber membrane. The vertical micro stage is moved downward to detach the droplet from the capillary, forming a contact between the solid-liquid interface of the droplet and the fiber membrane;

[0020] Adjust the rotary micro - motion platform at regular angular intervals, and use a microscopic camera to photograph the droplets to obtain multiple groups of contour dimensions of the droplets along the circumferential direction.

[0021] Furthermore, the microscopic camera is composed of two cameras with perpendicular axes for photographing, and two orthogonal droplet contour curves are photographed to obtain two orthogonal droplet contour curves.

[0022] The droplet surface normal stress calculation module calculates the radius of curvature of the droplet vertex through two orthogonal droplet contour curves, and calculates the internal pressure p0 of the droplet formed by the surface tension of the droplet through the radius of curvature of the vertex. The formula is as follows:

[0023]

[0024] Among them, σ0 represents the surface tension at the droplet vertex, γ LV represents the liquid surface tension coefficient, and r1 and r2 respectively represent the radii of curvature of two orthogonal contours of the micro - droplet vertex.

[0025] Furthermore, the droplet surface normal stress calculation module calculates the droplet surface normal stress σ n from the internal pressure p0 at the droplet vertex and the height h from the droplet vertex to the surface of the fiber membrane. The formula is as follows:

[0026] σ n = p0 + ρgh

[0027] Among them, ρ represents the droplet density, and g represents the acceleration due to gravity.

[0028] Furthermore, for the surface free energy generation module, the surface free energy γ * SL of the solid - liquid interface between the droplet and the fiber membrane has the following formula:

[0029]

[0030] Among them, F cx represents the resultant force in the horizontal direction, θ τ represents the angle between the tangent of the droplet contour and the horizontal plane, x represents the horizontal distance from the droplet contour to the droplet center, r represents the droplet radius, α represents the circumferential angle of the droplet contour, Δα represents the angular step, and r Ω represents the radius from the three - phase contact point to the droplet center.

[0031] The advantages and beneficial effects of the present invention are as follows:

[0032] The present invention designs a capillary tube for transporting liquid droplets, which keeps the static pressure of the liquid droplets in contact with the fiber membrane constant during each test, avoiding the pressure disturbance and the change of the free energy of the solid-liquid interface caused by manually moving the liquid droplets; the present invention designs a device for circumferentially rotating the liquid droplets, measures multiple sets of liquid droplet profiles at fixed angular intervals, and quantitatively characterizes the anisotropy of the free energy of the solid-liquid interface caused by the complex porous structure of the fiber membrane; the present invention uses two orthogonal radii of curvature of the liquid droplet profile to calculate the internal pressure at the vertex of the liquid droplet, and obtains the normal stress on the surface of the liquid droplet according to the height of the liquid droplet; the present invention uses the surface tension integration method to obtain the resultant force of the surface tension on the liquid droplet profile, and calculates the surface free energy of the solid-liquid interface of the fiber membrane according to the force balance condition at the three-phase contact line of the liquid droplet. Description of the Drawings

[0033] Figure 1 It is a schematic structural diagram of the device according to an embodiment of the present invention.

[0034] Figure 2 It is a flowchart of the method according to an embodiment of the present invention.

[0035] Figure 3a It is a schematic surface structure diagram of the micro-liquid droplet profile in an embodiment of the present invention.

[0036] Figure 3b It is a diagram of the numerical integration result of the surface tension along the micro-liquid droplet profile in an embodiment of the present invention. Detailed Embodiment

[0037] The following further describes the detailed embodiment of the present invention with reference to the accompanying drawings. It should be understood that the detailed embodiment described herein is only used to illustrate and explain the present invention, and is not used to limit the present invention.

[0038] As Figure 1 shown, a device for measuring the free energy of the solid-liquid interface of a hydrophobic thin-layer porous fiber membrane includes a liquid droplet profile extraction module, a liquid droplet surface normal stress calculation module, and a surface free energy generation module.

[0039] The liquid droplet profile extraction module includes a micro-pump 1, a capillary tube 2, a fiber membrane 3, a rotary micro-stage 5, a vertical micro-stage 6, a high-definition microscope camera 7, a synchronizer 8, and a computer 9; the micro-pump 1 transports a small amount of liquid to form a liquid droplet 4 at the outlet of the capillary tube 2, the liquid droplet 4 adheres to the horizontally placed fiber membrane 3, the vertical micro-stage 6 is moved downward to separate the liquid droplet 4 from the capillary tube 2, forming a stable solid-liquid interface contact between the liquid droplet 4 and the fiber membrane 3; the rotary micro-stage 5 is adjusted at regular angular intervals, and the high-definition microscope camera 7 takes pictures of the liquid droplet 4 to obtain multiple sets of contour dimensions of the liquid droplet 4 in the circumferential direction; the high-definition microscope camera 7 has two axes perpendicular to each other for taking pictures, taking two orthogonal contour curves of the liquid droplet 4, and calculating the radius of curvature of the vertex of the micro-liquid droplet 4 from the contour curves.

[0040] Droplet surface normal stress calculation module, obtaining the internal pressure p0 of the droplet formed by surface tension according to the Young-Laplace equation, as shown in the following formula (1); calculating the droplet surface normal stress σ at the solid-liquid interface between the droplet 4 and the fiber membrane 3 from the internal pressure p0 at the vertices of the droplet 4 and the height of the droplet 4. n , as shown in the following formula (2).

[0041] Surface free energy generation module, performing numerical integration of the surface tension on a set of contour curves of the droplet 4, as shown in the following formula (3), to obtain the resultant force F of the surface tension at the three-phase contact point between the droplet 4 and the fiber membrane 3 in the horizontal direction. cx and the resultant force F in the vertical direction. cy ; in the horizontal direction, the droplet 4 is in a state of force balance, so the horizontal force F of the surface tension. cx is equal to the solid-liquid interface force of the fiber membrane 3, and the surface free energy γ of the solid-liquid interface of the fiber membrane 3 is calculated from the following formula (4). * SL .

[0042] As Figure 2 shown, a method for measuring the free energy of the solid-liquid interface of a hydrophobic thin-layer porous fiber membrane includes the following steps:

[0043] Step 1: Droplet transportation and measurement of multiple sets of droplet contours along the circumferential direction to obtain micro-droplet images.

[0044] On the hydrophobic fiber membrane, the formation of the solid-liquid interface mainly depends on the microporous structure, the surface energy of the solid and liquid materials, and the contact pressure. The internal pressure and velocity of the droplet when it contacts the fiber membrane have an important impact on the formation of the solid-liquid interface. In order to keep the internal pressure of the droplet constant in each experiment, the present invention designs a capillary infusion device, as Figure 1 shown.

[0045] The capillary is vertically fixed above the fiber membrane, and a micro amount of liquid is transported to the capillary outlet by a micro pump to form a stationary micro-droplet. The internal pressure formed by the surface tension of the micro-droplet is balanced with the hydrostatic pressure of the capillary liquid. Continuing to slowly pump the liquid, the micro-droplet gradually grows and contacts the fiber membrane to form a solid-liquid interface. During this process, the contact pressure of the solid-liquid interface is always the hydrostatic pressure of the capillary liquid (p = ρgh). Thus, the contact pressure of the solid-liquid interface in the experiment can be accurately adjusted by the height of the capillary, and this device solves the disturbance of the contact pressure of the solid-liquid interface caused by manual dropping and moving the droplet.

[0046] Horizontally rotate the micro-motion platform at a certain rotation angle every other time, and take micro-droplet images with a high-definition microscope camera. The droplet contour is obtained by using an image edge recognition algorithm, and the micro-droplet contour size is obtained according to the calibration of the camera pixels and the actual size. The change of the droplet contour along the circumferential direction can reflect the change of the free energy of the solid-liquid interface caused by the change of the pore structure of the fiber membrane.

[0047] Step 2: Calculation of the pressure inside the droplet

[0048] When the droplet contacts the fiber membrane, a solid-liquid interface is formed, which causes a change in the interfacial energy and changes the pressure inside the droplet. Thus, the pressure inside the droplet needs to be re-measured. Traditional contact pressure measurement methods will damage the micro-droplet. The present invention proposes a non-contact measurement method for the pressure inside the micro-droplet based on the radius of curvature.

[0049] From two sets of orthogonal micro-droplet profile curves, the radius of curvature at the vertex of the micro-droplet is calculated. According to the Young-Laplace equation, the pressure p0 inside the droplet formed by the surface tension is obtained, as shown in Equation (1).

[0050]

[0051] where p0 is the pressure inside the vertex of the micro-droplet, with the unit of Pa; σ0 is the normal stress of the surface at the vertex of the micro-droplet, with the unit of Pa; γ LV is the liquid surface tension coefficient, with the unit of mN / m; r1 and r2 are the radii of curvature of the two orthogonal profiles at the vertex of the micro-droplet, with the unit of m.

[0052] From the pressure p0 inside the vertex of the micro-droplet and the height of the micro-droplet, the normal stress σ n of the surface of the micro-droplet is calculated, as shown in Equation (2).

[0053] σ n = p0 + ρgh (2)

[0054] where ρ is the density of the micro-droplet, kg / m 3 ; g is the acceleration due to gravity, with the unit of m / s 2 ; h is the height from the vertex of the micro-droplet to the surface of the fiber membrane, with the unit of m.

[0055] Step 3: Calculation of the free energy of the solid-liquid interface

[0056] Numerical integration of the surface tension is performed on a set of profile curves of the micro-droplet, as shown in Equation (3), to obtain the resultant force F cx in the horizontal direction and the resultant force F cy in the vertical direction at the three-phase contact point between the micro-droplet and the fiber membrane.

[0057] In the horizontal direction, the micro-droplet is in a state of force balance. Therefore, the horizontal force F cx of the surface tension is equal to the solid-liquid interface force of the fiber membrane. The surface free energy γ * SL of the solid-liquid interface of the fiber membrane 4 is calculated from Equation (4).

[0058]

[0059] where θτ is the angle between the tangent of the droplet profile and the horizontal plane, in rad; x is the horizontal distance from the droplet profile to the droplet center, in m; r is the droplet radius, in m; α is the circumferential angle of the droplet profile, and Δα represents the step size of the rotation angle, in rad.

[0060]

[0061] Among them, r Ω is the radius from the three-phase contact point to the droplet center, in m.

[0062] In an embodiment of the present invention, first, start the micro pump to transport a small amount of liquid to the capillary outlet to form micro droplets; the micro droplets contact the fiber membrane to form a stable solid-liquid interface; vertically move the micro platform downward to separate the micro droplets from the capillary; horizontally rotate the micro platform along the circumferential direction to capture the images of the micro droplets; identify the droplet profile curve from the micro droplet images; calculate the internal pressure at the vertex of the micro droplet, the normal contact stress of the solid-liquid interface, the resultant force in the horizontal direction of the three-phase contact line, and the free energy of the solid-liquid interface in sequence. Among them, the numerical integration result of the surface tension along the micro droplet profile is as Figure 3a , Figure 3b shown,

[0063] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for measuring the solid-liquid interface free energy of a hydrophobic thin porous fiber membrane, characterized in that The steps include: Step 1: Collect a set of droplet images on the fiber membrane around the center point of the droplet and extract the droplet contour; Step 2: Calculate the internal pressure of the droplet based on the droplet profile, and calculate the normal contact stress between the droplet and the fiber membrane at the solid-liquid interface according to the internal pressure of the droplet and the droplet height; Step 3: Perform numerical integration of the surface tension on the contour curves of a group of droplets in combination with the normal contact stress to obtain the resultant force of the surface tension in the horizontal direction between the droplets and the fiber membrane at the three-phase contact point. The surface free energy of the solid-liquid interface of the fiber membrane is obtained by the ratio of the resultant force of the surface tension in the horizontal direction to the radius from the three-phase contact point to the droplet center.

2. The method for measuring the solid-liquid interfacial free energy of a hydrophobic thin porous fiber membrane according to claim 1, characterized in that: In step 1, two sets of orthogonal droplet profile curves are collected. In step 2, the vertex curvature radius of the droplet is calculated through the two sets of orthogonal droplet profile curves. The internal pressure p0 of the droplet formed by the surface tension of the droplet is calculated through the vertex curvature radius. The formula is as follows: Where σ0 represents the normal stress on the droplet vertex surface, γ LV represents the surface tension coefficient of the liquid, r1 and r2 represent the curvature radii of the two orthogonal contours of the microdroplet vertex, respectively.

3. The method for measuring the solid-liquid interfacial free energy of a hydrophobic thin porous fiber membrane according to claim 1, characterized in that: In step 2, the normal stress σ on the droplet surface is calculated from the pressure p0 inside the droplet vertex and the height h from the droplet vertex to the fiber membrane surface. n , the formula is as follows: s n =p0+ρgh Where ρ represents the droplet density and g represents the gravitational acceleration.

4. The method for measuring the solid-liquid interfacial free energy of a hydrophobic thin porous fiber membrane according to claim 3, characterized in that: In step 3, the surface free energy γ of the solid-liquid interface between the droplet and the fiber membrane * SL The formula is as follows: Among them, F cx represents the horizontal force, θ τ represents the angle between the tangent line of the droplet contour and the horizontal plane, x represents the horizontal distance from the droplet contour to the droplet center, r represents the droplet radius, α represents the circumference angle of the droplet contour, Δα represents the turning angle step, and r Ω represents the radius from the three-phase contact point to the droplet center.

5. A device for measuring the solid-liquid interface free energy of a hydrophobic thin layer porous fiber membrane, comprising a droplet profile extraction module, a droplet surface normal stress calculation module and a surface free energy generation module, characterized in that: The method for measuring the solid-liquid interface free energy of a hydrophobic thin-layer porous fiber membrane as described in claim 1 is used to obtain the solid-liquid interface free energy of the fiber membrane.

6. The device for measuring the solid-liquid interfacial free energy of a hydrophobic thin porous fiber membrane according to claim 5, characterized in that: The droplet profile extraction module comprises a micro-motion pump (1), a capillary (2), a fiber membrane (3), a rotary micro-motion platform (5), a vertical micro-motion platform (6), and a microscope camera (7). The micro-motion pump (1) transports liquid to form droplets at the outlet of the capillary (2), and the droplets adhere to the horizontally placed fiber membrane (3). The vertical micro-motion platform (6) moves downward to make the droplets detach from the capillary (2), forming a solid-liquid interface contact between the droplets and the fiber membrane (3); The rotating micro-motion platform (5) is adjusted at certain rotation angles, and the droplet is photographed by a microscope camera (7) to obtain multiple groups of contour dimensions of the droplet along the circumferential direction.

7. The device for measuring the solid-liquid interfacial free energy of a hydrophobic thin porous fiber membrane according to claim 5, characterized in that: The microscope camera (7) is configured to shoot two sets of orthogonal droplet profile curves with two axes perpendicular to each other, so as to obtain two sets of orthogonal droplet profile curves; The droplet surface normal stress calculation module calculates the droplet vertex curvature radius through two sets of orthogonal droplet contour curves, and calculates the internal pressure p0 formed by the droplet surface tension through the vertex curvature radius. The formula is as follows: Where σ0 represents the surface tension at the droplet vertex, γ LV represents the surface tension coefficient of the liquid, r1 and r2 represent the curvature radii of the two orthogonal contours of the microdroplet vertex, respectively.

8. The device for measuring the solid-liquid interfacial free energy of a hydrophobic thin porous fiber membrane according to claim 5, characterized in that: The droplet surface normal stress calculation module calculates the droplet surface normal stress σ from the pressure p0 inside the droplet vertex and the height h from the droplet vertex to the fiber membrane surface. n , the formula is as follows: s n =p0+ρgh Where ρ represents the droplet density and g represents the gravitational acceleration.

9. The device for measuring the solid-liquid interfacial free energy of a hydrophobic thin porous fiber membrane according to claim 8, characterized in that: The surface free energy generation module, the surface free energy γ of the solid-liquid interface between the droplet and the fiber membrane * SL The formula is as follows: Among them, F cx represents the horizontal force, θ τ represents the angle between the tangent line of the droplet contour and the horizontal plane, x represents the horizontal distance from the droplet contour to the droplet center, r represents the droplet radius, α represents the circumference angle of the droplet contour, Δα represents the turning angle step, and r Ω represents the radius from the three-phase contact point to the droplet center.

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