CN116236924A - Microbial cellulose-based composite oil-water separation film - Google Patents
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- CN116236924A CN116236924A CN202310272522.6A CN202310272522A CN116236924A CN 116236924 A CN116236924 A CN 116236924A CN 202310272522 A CN202310272522 A CN 202310272522A CN 116236924 A CN116236924 A CN 116236924A
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- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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- B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
- B01D71/06—Organic material
- B01D71/08—Polysaccharides
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- B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
- B01D67/0002—Organic membrane manufacture
- B01D67/0023—Organic membrane manufacture by inducing porosity into non porous precursor membranes
- B01D67/0025—Organic membrane manufacture by inducing porosity into non porous precursor membranes by mechanical treatment, e.g. pore-stretching
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Abstract
The invention discloses a microbial cellulose-based composite oil-water separation membrane, belonging to the field of composite membranes; the method comprises the steps of soaking the microbial cellulose hydrogel into a shaping agent solution to enable the shaping agent to fully diffuse, primarily extruding and shaping, and then soaking the shaping agent into a cross-linking agent solution to enable the shaping agent molecules to fully cross-link in the microbial cellulose hydrogel, so as to prepare the microbial cellulose-based composite hydrogel; then cutting pore channels by adopting a laser cutting technology, and compositing the microbial cellulose baseThe hydrogel is prepared into an oil-water separation membrane with a macroscopic net structure. In air, the contact angle of the surface of the separating film with water is 0 o The contact angle between the surface of the separation membrane and oil under water is 150 o . The separation membrane prepared by the method is suitable for treating oily wastewater mixtures generated by oil stain leakage in life, industry, sea and the like.
Description
Technical Field
The invention relates to the field of composite membranes, in particular to a microbial cellulose-based composite oil-water separation membrane.
Background
In life, oil extraction, chemical production and other processes, a large amount of oily wastewater can be generated, and oil-water separation treatment of the oily wastewater becomes a problem to be solved urgently. The prior art for treating the oily wastewater comprises modes of distillation water removal, standing, clarification, extraction and the like. In addition, in recent years, membrane separation technology has been receiving attention because of its advantages such as low cost, high separation efficiency, easy operation, and the like.
In the prior art, chinese patent publication No. CN113441015B discloses a microbial cellulose-agarose composite hydrogel base oil water separation membrane and a preparation method thereof, and the prepared separation membrane is soft and easy to deform and is not beneficial to cutting slits.
Disclosure of Invention
The invention aims to provide a microbial cellulose-based composite oil-water separation membrane, which is prepared from microbial cellulose hydrogel.
The invention adopts the following technical scheme: the invention provides a microbial cellulose-based composite oil-water separation membrane, which is prepared by the following steps: impregnating the sheet microbial cellulose hydrogel with water to fully swell the sheet microbial cellulose hydrogel; then sequentially dipping the materials by adopting a shaping agent solution and a cross-linking agent solution; finally, opening holes on the membrane to prepare the microbial cellulose-based composite oil-water separation membrane.
Further, the sheet-like microbial cellulose hydrogel is impregnated with water and then digested with an alkali solution.
Further, the alkali solution is any one of sodium hydroxide, potassium hydroxide and calcium hydroxide.
Further, the shaping agent is a high molecular polysaccharide substance containing hydroxyl groups.
Further, the shaping agent is any one of sodium alginate, chitosan and prolamin.
Further, the cross-linking agent solution is an aqueous solution containing any one of divalent cations of Ca2+, mg2+ and Cu2+.
Further, after the molding agent solution is immersed and before the cross-linking agent solution is immersed, the molding agent adhered to the surface of the hydrogel is scraped off, and the thickness of the hydrogel is extruded to be close to that of the initial microbial cellulose hydrogel sheet.
And further, opening holes on the membrane by using a laser cutting technology to prepare the microbial cellulose-based composite oil-water separation membrane.
Further, a laser cutting technology is used for cutting the film in a cross mode in two directions, a plurality of slits are formed, the cutting depth of each time is 55-70% of the film thickness, and a flow channel is formed at the intersection point of the slits.
The invention has the beneficial effects that: the method comprises the steps of soaking the microbial cellulose hydrogel into a shaping agent solution to enable the shaping agent to fully diffuse, primarily extruding and shaping, and then soaking the shaping agent into a cross-linking agent solution to enable the shaping agent molecules to fully cross-link in the microbial cellulose hydrogel, so as to prepare the microbial cellulose-based composite hydrogel; and then cutting pore channels by adopting a laser cutting technology, and preparing the microbial cellulose-based composite hydrogel into the oil-water separation film with a macroscopic net structure. In air, the contact angle of the surface of the separation membrane with water is 0 degrees, and the contact angle of the surface of the separation membrane with oil under water is 150 degrees. The separation membrane prepared by the method is suitable for treating oily wastewater mixtures generated by oil stain leakage in life, industry, sea and the like.
Drawings
FIG. 1 shows a representation of a microbial cellulose hydrogel sheet after impregnation with water.
Fig. 2 is a microbial cellulose hydrogel sheet without a shaping agent and a cross-linking agent soaking.
Fig. 3 is a microbial cellulose hydrogel sheet impregnated with a shaping agent and a cross-linking agent.
FIG. 4 is a schematic structural diagram of a microbial cellulose-based composite oil-water separation membrane prepared after laser cutting; in the figure, 1-flow channel, 2-slit.
FIG. 5 is a sample diagram of the microbial cellulose-based composite oil-water separation membrane prepared in the present application.
Fig. 6 is an optical microscope image of fig. 5.
Fig. 7 is an electron microscope image of fig. 5.
FIG. 8 is an electron micrograph of a microbial cellulose hydrogel.
FIG. 9 is a separation membrane prepared in comparative example 1.
Description of the embodiments
The following describes the technical scheme of the present invention in detail by means of specific examples, but the content of the present invention is not limited to the following examples only. The experimental methods used in the following examples are conventional methods unless otherwise specified. Materials, reagents and the like used in the following examples are commercially available unless otherwise specified.
The invention provides a microbial cellulose-based composite oil-water separation membrane, and the preparation method thereof comprises the following steps:
step 1, soaking the sheet microbial cellulose hydrogel in water to fully swell the sheet microbial cellulose hydrogel, so that the nano holes in the material are fully stretched, and the sheet microbial cellulose hydrogel is convenient for entering shaping agent molecules in subsequent pore channels. Preferably, deionized water is replaced for multiple times in the soaking process, sugar on the surface of the hydrogel is primarily washed away, and the problem that the sugar is gelatinized and adhered to the microbial cellulose hydrogel sheet due to high temperature in the subsequent alkaline boiling process is avoided. The microbial cellulose hydrogel used in the application is prepared by microbial fermentation, the cellulose purity is 99%, the polymerization degree is 2000-8000, the water holding capacity is 99.2-99.7%, the nano pore size is 1-200nm, and the thickness is preferably more than 1mm. An illustration of a microbial cellulose hydrogel sheet after impregnation with water according to the present application is shown in fig. 1.
It should be noted that the pure microbial cellulose hydrogel sheet used in the present application does not have the ability to separate oil from water, and water cannot pass through the hydrogel sheet. The microbial cellulose hydrogel sheet with a thickness of more than 1mm is selected because, according to tests, the separating membrane with a thickness of less than 1mm has poor pressure-bearing capacity, and can not bear the pressure of water in the actual use process, thereby causing the breakage of the separating membrane.
Soaking with water, preferably 2-6% alkali solution, and soaking in 80-100deg.C oil bath for 4-6 hr; the alkali solution can be any alkali solution which is not easy to decompose under heating, such as sodium hydroxide, potassium hydroxide, calcium hydroxide, etc. The microbial bacteria remained in the microbial cellulose hydrogel can reproduce again under a specific environment, and the oil-water separation film can be decomposed; the impurities such as microbial cells, saccharides and proteins remained in the hydrogel sheet can be removed by alkaline boiling, so that the problem is avoided. In addition, the existence of alkali ions can cause the reduction of the oil-water separation capability of the oil-water separation film under different acid-base environments, so deionized water is preferably used for cleaning again after alkali boiling to remove alkali ions, and the microbial cellulose hydrogel is cleaned to pH 6-8. It should be noted that this step is an optional operation, and is not necessarily performed.
And 2, preparing a shaping agent solution. The shaping agent used in the application refers to high molecular polysaccharide substances, and has good biocompatibility; specifically usable shaping agents include sodium alginate, chitosan, prolamin, and the like; the shaping agent is dissolved in deionized water to prepare a solution with the mass fraction of 0.5-5%.
The mass fraction of the shaping agent in the application determines the viscosity of the solution, and the higher the mass fraction is, the higher the viscosity of the solution is, and the mobility of the shaping agent molecules is poor, which can lead to the difficulty of the shaping agent molecules entering the inside of the microbial cellulose hydrogel; on the contrary, if the mass fraction is too low, the molding agent molecules in the unit volume of the solution are fewer, and the water molecules are more, so that the water molecules entering the microbial cellulose hydrogel are large in proportion, the molding agent molecules are small in proportion, and the molding is poor. Therefore, experiments prove that the mass fraction of the shaping agent in the shaping agent solution is preferably controlled to be 0.5-5%.
In addition, the molecular structure of the shaping agent selected in the application contains a large number of hydroxyl groups, so that the material has super-hydrophilic and underwater super-oleophobic characteristics; meanwhile, the molecular cross-linking can be generated under specific conditions to form gel, and the gel has good sol-gel property so as to solve the problem that the microbial cellulose hydrogel is too soft and easy to deform.
And 3, preparing a cross-linking agent solution. The crosslinking agent used in the present application is an aqueous solution containing any one of divalent cations such as ca2+, mg2+, cu2+, and the like, and can intermolecular crosslink the polysaccharide molding agent to form a gel. Specifically, any of chlorate, sulfate, and nitrate such as CaCl2, mgSO2, and Cu (NO 3) 2 can be used. When specifically configured, the mass fraction of the crosslinker solution is preferably higher than 2%; if the mass fraction is too low, insufficient crosslinking of the molecules of the molding agent may result, failing to form a gel.
And 4, preparing the microbial cellulose-based composite hydrogel sheet.
Firstly, the microbial cellulose hydrogel treated in the step 1 is fully immersed in the shaping agent solution prepared in the step 2, and the immersion time is preferably more than 8 hours. In the dipping process, the shaping agent can diffuse into the microbial cellulose hydrogel, and the diffusion rate and the maximum diffusion amount of the shaping agent into the microbial cellulose hydrogel can be influenced due to different viscosity caused by different concentration of the shaping agent solution, and the insufficient diffusion of the shaping agent molecules into the hydrogel can be caused by too short soaking time, so that the soaking time is determined by the concentration and the viscosity of the shaping agent solution.
Thereafter, the following operations are preferably performed: taking out the fully impregnated microbial cellulose hydrogel, removing the shaping agent adhered to the surface layer by using a scraper, and extruding by using a uniform-speed scraper extruder to enable the thickness of the microbial cellulose hydrogel to be close to that of the initial microbial cellulose hydrogel sheet.
The reason for extrusion with the blade and extruder is as follows: the surface of the pure cellulose hydrogel sheet is uneven, and the lower surface of the low-power microscope is uneven; meanwhile, the shaping agent solution has certain viscosity, and a large amount of solution can adhere to the surface layer of the hydrogel, so that the thickness of the flaky gel can be increased and uneven; if the surface is not cleaned, a sandwich structure is formed on the upper part and the lower part of the hydrogel sheet after the subsequent soaking and crosslinking agent is crosslinked, the formed sandwich structure prevents oil-water separation, and the oil-water separation efficiency is greatly reduced. The surface concave part of the microbial cellulose hydrogel piece is guaranteed not to store a large amount of molding agent solution through extrusion, and meanwhile, the thickness of the composite hydrogel piece can be accurately controlled to be consistent and uniform.
Spreading the extruded microbial cellulose hydrogel sheet in a glass container with a smooth and flat bottom, slowly and uniformly adding the cross-linking agent solution prepared in the step 3 until the solution submerges the spread microbial cellulose hydrogel sheet for gelation; in this process, the shaper molecules form an interpenetrating network with the microbial cellulose hydrogel. The soaking time is determined by the gel rate and the gel skeleton strength, and the preferable soaking time is 10-18h. The cross-linking is formed in the microbial cellulose hydrogel, so that the problem that the microbial cellulose hydrogel is too soft and is easy to deform is solved, and the microbial cellulose-based composite hydrogel sheet with higher mechanical strength and difficult deformation is prepared, thereby providing a favorable guarantee for preparing a filtering membrane with stable shape and high separation efficiency. In addition, the crosslinked microbial cellulose-based composite hydrogel sheet has better anti-pulling capability and does not generate swelling phenomenon. The microbial cellulose hydrogel sheet without the shaper and cross-linker soaked is shown in fig. 2, and the microbial cellulose hydrogel sheet with the shaper and cross-linker soaked is shown in fig. 3.
It should be noted that, the hydrogel piece after soaking the shaping agent solution is extruded and preliminarily shaped, the shaping agent solution entering the hydrogel piece is not formed into gel yet, the shaping agent is unstable in the hydrogel piece, and the hydrogel piece is prevented from being impacted by the crosslinking agent solution when the crosslinking agent solution is added, so that the crosslinking agent solution is added at a relatively slow speed at a uniform speed, and the impact is reduced.
And 5, forming a through hole below the millimeter level on the microbial cellulose-based composite hydrogel sheet prepared in the step 4 to prepare the microbial cellulose-based composite oil-water separation film.
In a specific embodiment, a laser cutting technique is used to uniformly cut a plurality of parallel slits in a two-dimensional plane of the composite hydrogel sheet at a laser power of 50-95%, the slit width being about 0.2mm and the cutting depth being 55-70% of the thickness of the composite hydrogel sheet. Then, the composite hydrogel sheet is rotated by 90 degrees, and is cut at the same laser power in the vertical direction of the first cutting; because the cutting depth of the two times reaches more than 50%, a flow passage is formed at the intersection point of the two mutually perpendicular slits, and the section size of the flow passage is about 0.04mm < 2 > measured. The separation efficiency of the microbial cellulose-based composite hydrogel-based oil-water separation membrane is regulated by changing the width of the cutting gap so as to change the size of the separation membrane flow pore canal and the distance between the cutting gaps so as to regulate the density of the separation membrane flow pore canal. FIG. 4 is a schematic structural diagram of a microbial cellulose-based composite oil-water separation membrane obtained after laser cutting; FIG. 5 is a sample diagram of the microbial cellulose-based composite oil-water separation membrane. Fig. 6 is an optical microscope image of fig. 5.
Fig. 7 is an electron microscopic view of the microbial cellulose-based composite oil-water separation film prepared in the present application, and fig. 8 is an electron microscopic view of the microbial cellulose hydrogel. As can be seen by comparison, the molding agent of the present application was successfully compounded into the microbial cellulose hydrogel sheet.
Example 1
The preparation method of the microbial cellulose-based composite oil-water separation membrane comprises the following steps:
step 1, pretreatment of microbial cellulose hydrogel: the microbial cellulose hydrogel with the thickness of 1mm is immersed in deionized water for 20 hours, the deionized water is replaced for 3 times in the immersing process, 2% sodium hydroxide solution by mass fraction is used after the immersing, and the microbial cellulose hydrogel is immersed in an 80 ℃ oil bath for 4 hours. And (3) washing again by using deionized water after alkaline boiling, and washing the microbial cellulose hydrogel to pH 6-8.
Step 2, preparation of a shaping agent solution: 1g of sodium alginate solid was dissolved in 199mL of deionized water so that the mass fraction of the molding agent solution was 0.5%.
Step 3, preparation of a cross-linking agent solution: dissolving 4g of CaCl2 solid in 196mL of deionized water to obtain a cross-linking agent solution with the mass fraction of 2%;
Step 5, preparing a microbial cellulose-based composite oil-water separation membrane: and (3) uniformly cutting slits with the spacing of 2mm and the width of 0.03mm on a two-dimensional plane of the composite hydrogel sheet by using a laser cutting technology, wherein the cutting depth is 55-70% of the thickness of the hydrogel sheet. After cutting, the gel sheet is rotated clockwise by 90 degrees, slits with the same spacing, width and depth are repeatedly cut in the vertical direction of the first cutting by the same laser power, through holes are formed at the connecting points of the two mutually perpendicular slits, and the size of the section of the pore canal is 0.0009mm < 2 >.
And (3) carrying out experiments by utilizing an oil-water separation device, immersing the obtained separation membrane in water, taking out, clamping the separation membrane in the middle of a clamp, enabling the mixed solution of the n-hexane dyed by oil red and water to contact with the separation membrane through an upper feeding glass tube, intercepting the n-hexane at the upper end of the separation membrane, and enabling the water to flow into a lower collector through the separation membrane to realize oil-water mixture separation. Through detection, the separation rate reaches 46560L/(m 2. H), and the separation efficiency is 99.9%.
In addition, the contact angle between the surface of the separation membrane prepared by the method and water in the air is 0 degree; the contact angle of the surface of the separation membrane with oil under water is 150 degrees.
Example 2
The preparation method of the microbial cellulose-based composite oil-water separation membrane comprises the following steps:
step 1, pretreatment of microbial cellulose hydrogel: the microbial cellulose hydrogel with the thickness of 3mm is immersed in deionized water for 24 hours, the deionized water is replaced for 4 times in the immersing process, and 4% sodium hydroxide solution with the mass fraction is used for immersing in an 80 ℃ oil bath for 4 hours. And (3) washing again by using deionized water after alkaline boiling, and washing the microbial cellulose hydrogel to pH 6-8.
Step 2, preparation of a shaping agent solution: 2g of sodium alginate solids were dissolved in 198mL of deionized water such that the mass fraction of the molding agent solution was 1%.
Step 3, preparation of a cross-linking agent solution: 8g CaCl2 solids were dissolved in 192mL deionized water to give a crosslinker solution with a mass fraction of 4%.
Step 5, preparing a microbial cellulose-based composite oil-water separation membrane: and (3) uniformly cutting slits with the spacing of 1.5mm and the width of 0.05mm on a two-dimensional plane of the composite hydrogel sheet by using a laser cutting technology, wherein the cutting depth is 55-70% of the thickness of the hydrogel sheet. After cutting, the gel sheet is rotated clockwise for 90 degrees, slits with the same spacing, width and depth are repeatedly cut in the vertical direction of the first cutting by the same laser power, through holes are formed at the connecting points of the two mutually perpendicular slits, and the size of the section of the pore canal is 0.0025mm2.
And (3) carrying out experiments by utilizing an oil-water separation device, immersing the obtained separation membrane in water, taking out, clamping the separation membrane in the middle of a clamp, and respectively carrying out oil-water separation tests on various oil-water mixtures such as n-hexane, petroleum ether, vegetable oil, gasoline, diesel oil and the like. The separation rate after film formation reaches 46560L/(m 2. H), and the separation efficiency is 99.9%.
Example 3
The preparation method of the microbial cellulose-based composite oil-water separation membrane comprises the following steps:
step 1, pretreatment of microbial cellulose hydrogel: the microbial cellulose hydrogel with the thickness of 10mm is immersed in deionized water for 36 hours, the deionized water is replaced for 5 times in the immersing process, and 6 mass percent of sodium hydroxide solution is used for immersing in an oil bath pot with the temperature of 90 ℃ for 6 hours. And (3) washing again by using deionized water after alkaline boiling, and washing the microbial cellulose hydrogel to pH 6-8.
Step 2, preparation of a shaping agent solution: 10g of sodium alginate solids were dissolved in 190mL of deionized water such that the mass fraction of the molding agent solution was 5%.
Step 3, preparation of a cross-linking agent solution: 20g CaCl2 solids were dissolved in 180mL deionized water to give a crosslinker solution with a mass fraction of 10%.
Step 5, preparing a microbial cellulose-based composite oil-water separation membrane: and (3) uniformly cutting slits with the spacing of 2mm and the width of 0.2mm on a two-dimensional plane of the composite hydrogel sheet by using a laser cutting technology, wherein the cutting depth is 55-70% of the thickness of the hydrogel sheet. After cutting, the gel sheet is rotated clockwise for 90 degrees, slits with the same spacing, width and depth are repeatedly cut in the vertical direction of the first cutting by the same laser power, through holes are formed at the connecting points of the two mutually perpendicular slits, and the size of the section of the pore canal is 0.04mm < 2 >.
And (3) carrying out experiments by utilizing an oil-water separation device, immersing the obtained separation membrane in water, taking out the separation membrane and clamping the separation membrane in the middle of a clamp, enabling mixed liquid of edible peanut oil and water to contact the separation membrane through an upper feeding glass tube, intercepting oil at the upper end of the separation membrane, and enabling water to flow into a lower collector through the separation membrane to realize oil-water mixture separation. The separation rate after film formation reaches 47980L/(m 2. H), and the separation efficiency is 99.6%.
Comparative example 1
The difference from example 1 is that comparative example 1 was not immersed in the molding agent and crosslinking agent solution. As shown in fig. 9, the separation membrane prepared in comparative example 1 was not subjected to soaking molding by using a molding agent and a crosslinking agent solution, and the microbial cellulose hydrogel was too soft to deform, and had poor mechanical strength, and the slit cut by laser was not uniform in width, so that oil-water separation was not performed.
The oil-water separation membrane prepared by the method is suitable for treating oily wastewater mixtures generated by domestic, industrial and marine oil stain leakage and the like, can realize separation of oily extraction mixtures in industry, and has higher separation efficiency compared with the traditional separation method. When the oil-water separation membrane is used for separating an oil-water mixture, the oil-water mixture is firstly pre-wetted by water, and then is contacted with the separation membrane for separation after being fully wetted, oil is trapped by the separation membrane on one side of the separation membrane, and water passes through the separation membrane to reach the other side of the separation membrane, so that the separation of the oil-water mixture is realized. Through detection, the separation membrane has good separation effect on single oil components such as n-hexane, benzene and derivatives thereof, petroleum ether, chloroform and the like and various oil components such as vegetable oil, animal oil, gasoline, diesel oil, petroleum and the like.
The above examples are only illustrative of the invention and are not intended to be limiting of the embodiments. Other variations in various forms will be apparent to those of ordinary skill in the art in view of the foregoing description. And obvious variations thereof are contemplated as falling within the scope of the invention. Finally, it is also noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
Claims (9)
1. The microbial cellulose-based composite oil-water separation membrane is characterized by being prepared by the following steps: impregnating the sheet microbial cellulose hydrogel with water to fully swell the sheet microbial cellulose hydrogel; then sequentially dipping the materials by adopting a shaping agent solution and a cross-linking agent solution; finally, opening holes on the membrane to prepare the microbial cellulose-based composite oil-water separation membrane.
2. The microbial cellulose-based composite oil-water separation membrane according to claim 1, characterized in that: the sheet-like microbial cellulose hydrogel is impregnated with water and then digested with an alkali solution.
3. The microbial cellulose-based composite oil-water separation membrane according to claim 2, characterized in that: the alkali solution is any one of sodium hydroxide, potassium hydroxide and calcium hydroxide.
4. The microbial cellulose-based composite oil-water separation membrane according to claim 1, characterized in that: the shaping agent is a macromolecular polysaccharide substance containing hydroxyl groups.
5. The microbial cellulose-based composite oil-water separation membrane according to claim 4, characterized in that: the shaping agent is any one of sodium alginate, chitosan and prolamin.
6. The microbial cellulose-based composite oil-water separation membrane according to claim 1, characterized in that: the cross-linking agent solution contains Ca 2+ 、Mg 2+ 、Cu 2+ An aqueous solution of any one of divalent cations.
7. The microbial cellulose-based composite oil-water separation membrane according to claim 1, characterized in that: after the molding agent solution is immersed and before the cross-linking agent solution is immersed, the molding agent adhered to the surface of the hydrogel is scraped off, and the thickness of the hydrogel is extruded to be close to that of the initial microbial cellulose hydrogel sheet.
8. The microbial cellulose-based composite oil-water separation membrane according to claim 1, characterized in that: and opening holes on the membrane by using a laser cutting technology to prepare the microbial cellulose-based composite oil-water separation membrane.
9. The microbial cellulose-based composite oil-water separation membrane according to claim 8, characterized in that: and cutting the film by using a laser cutting technology in a cross manner twice to form a plurality of slits, wherein the depth of each cut is 55-70% of the film thickness, and a circulating pore canal is formed at the intersection point of the slits.
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