KR20160131654A

KR20160131654A - Ultra hydrophobic membrane produced by oxygen plasma irradiation and chemical treatment and production method therefor

Info

Publication number: KR20160131654A
Application number: KR1020150064545A
Authority: KR
Inventors: 이슬기; 박민수; 김진호
Original assignee: 주식회사 에코니티
Priority date: 2015-05-08
Filing date: 2015-05-08
Publication date: 2016-11-16

Abstract

The present invention relates to a method for semipermanently superhydrophobizing polyolefin-based or fluorine-based hydrophobic polymeric precision filtration membranes. The present invention further relates to a superhydrophobic separation membrane produced by the method. More specifically, the present invention relates to a method for making a surface of separation membranes superhydrophobic, while maintaining properties of existing materials such as strong chemical resistance, durability, and high tensile strength. The present invention further relate to a superhydrophobic separation membrane produced by the method. To this end, hydrophobic polymeric separation membranes may undergo oxygen plasma treatment and chemical treatment by carrying out reactions with superhydrophobic compounds.

Description

TECHNICAL FIELD [0001] The present invention relates to an ultra-hydrophobic separation membrane produced by an oxygen plasma irradiation and a chemical treatment,

The present invention relates to a method of semi-hydraulically hydrating polyolefin-based or hydrophobic hydrophobic polymer microfiltration membranes and a super-hydrophobic separation membrane produced by such a method. More specifically, the present invention relates to a method of micro- A method of imparting more hydrophobic super hydrophobicity to the surface of the separator while maintaining the characteristics such as strong chemical resistance, durability and high tensile strength of the existing material by performing a chemical treatment to react with the ultra-hydrophobic separator It is about.

Seawater desalination technology is the only way to cope with water shortage by desalinating seawater that exists in the world indefinitely without affecting the water quality. Therefore, there is a need for technology development as the only alternative in countries where there is absolutely no freshwater resources. In this seawater desalination technology, a variety of seawater desalination techniques such as evaporation method, determination method, reverse osmosis method, and electrodialysis method are applied, but new technology is required due to high energy cost. The most talked about technique is membrane distillation.

Membrane distillation is one of the separation processes using a separation membrane. The vapor pressure difference due to the temperature difference is used as a driving force. The vapor that has evaporated from the hot feed water flowing in contact with the separator passes through the pores of the separator to obtain condensed pure water. Removal rates of up to 100% when removing non-volatile and mineral-containing feed water. The membrane is used as a hydrophobic porous membrane that is not wetted with water. Hydrophobic polymer membranes such as PE, PP, PVDF and PTFE can be used as the material. However, PTFE materials are mainly used as the membrane used in the membrane distillation method. Korean Patent Laid-Open Publication No. 2014-0073731 discloses a multi-layered PTFE membrane membrane distillation membrane in which fine pore control is performed to remove particulate pollutants and fine pores of the external active layer are finely adjusted to be used as a filtration layer To improve backwash efficiency.

On the other hand, hydrophobicity is one of the important factors directly affecting the operation of the membrane distillation membrane. As the hydrophobic membrane once wetted, it takes a lot of energy and time to remove the brine and scale in the membrane pore, Absolutely necessary. However, PTFE membranes have a disadvantage of high production cost. Therefore, various techniques for miniaturizing existing membranes have been developed. As the micro-hydration techniques for such membranes, surface coating, There have been developed techniques for various chemical treatments such as water repellency treatment after primer treatment and surface treatment of water repellent adhesive, and such prior arts have been devised in order to solve the problems of the prior art in which a base layer of a nanofiber structure including a base polymer and a fluorine- Korean Patent No. 1422918, which relates to a hydrophobic membrane. However, since the surface coating and water repellent adhesive treatment are remarkably inferior in abrasion resistance, the water repellent treatment after the primer treatment is inefficient due to high production cost, In addition to PE, PP, PVDF The second hydrophobic membrane material given to technology for manufacturing the membrane is applied to the membrane distillation is a situation that has not yet been made up properly.

Korean Patent Publication No. 2014-0073731 Korean Patent No. 1422918

In order to solve the problems of the prior art as described above, the present invention provides a separation membrane which can be applied to a membrane distillation method by imparting super hydrophobic property to a conventional hydrophobic separation membrane material such as PE, PP, PVDF and the like.

In addition, in the present invention, it is desired to provide the most preferable method for reducing production cost and energy consumption in the case of imparting such super-hydrophobicity. That is, the present invention provides a super-hydrophobic separation membrane that minimizes the phenomenon of pore-wetting by water in the separation membrane by significantly reducing the surface energy by introducing an ultra-hydrophobic substance into the PE, PP, and PVDF separation membranes, which are economical materials compared to PTFE.

According to an aspect of the present invention, A plasma irradiating step of irradiating the separation membrane with plasma; And a super-hydrophobic separation step of separating the separation membrane, which has undergone the plasma irradiation step, with a super-hydrophobic compound.

The separation membrane may be a hydrophobic separation membrane, and the hydrophobic separation membrane may be either a polyolefin separation membrane or a fluorine separation membrane.

The superhydrophobic compound may be a fluorine-containing chlorosilane compound, a perfluoro methacrylate compound or a perfluorosulfonic acid compound, more preferably a perfluoroalkylsilane compound such as trichloro (1H, 1H, 2H, 2H-perfluorooctyl 2H, 2H-perfluorooctyl) silane or triethyl (trifluoromethyl) silane, more preferably trichloro (1H, 1H, 2H, (2H, 2H-perfluorooctyl) silane). As the perfluoromethacrylate compound, perfluorooctyl methacrylate (1H, 1H-perfluorooctyl methacrylate) is preferable and as the perfluoro sulfonyl chloride, heptadecafluoro-1-octanesulfonyl chloride chloride is preferred.

The plasma irradiation step may be performed in the presence of oxygen gas, the amount of oxygen gas to be introduced may be 100 cc / min to 500 cc / min, the plasma irradiation power may be 100 W to 500 W, 60 seconds to 300 seconds.

Preferably, the surface of the separation membrane is hydrosilated with a solution of an ultra-hydrophobic compound at a concentration of 0.05 to 0.5% by weight.

The ultra-hydrophobic separation membrane manufactured according to the production method of the present invention has a water drop contact angle of 120 ^° or more and a liquid inflow pressure of the second distilled water of 3 bar or more.

The present invention provides a method of completely hydrolyzing a surface of a separation membrane by plasma-treating the surface of a hydrophobic separation membrane such as PE, PP, PVDF, etc., and reacting the hydrophobic separation membrane with a superhydrophobic compound after introducing a hydroxyl group. These hydrophobic membranes have the advantage of reducing the wetting phenomenon of the pore due to the salt water and pressure on the surface of the superhydrophobic membrane and by relaxing the scale phenomenon in the pore, thereby extending the repair period of the membrane distillation membrane.

In addition, the hydrophobic separation membrane produced according to the production method of the present invention does not change the inherent physical properties of the separation membrane and is advantageous in that the hydrophobic property can be imparted by a relatively simple process. In addition, the ultra- And has a long lifetime due to abrasion resistance. Thus, the method of manufacturing a microhydrolytic membrane using such a conventional hydrophobic membrane is the most suitable method for manufacturing a membrane-dedicated membrane.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process diagram showing a step of micro-hydrating a surface of a separation membrane according to a preferred embodiment of the present invention. FIG.
FIG. 2 is a result of measurement of the contact angle of the ultra-hydrated PVDF membrane according to a preferred embodiment of the present invention.
FIG. 3 is a measurement result of a liquid inflow pressure (LEP) of a hydrosilated PVDF membrane according to a preferred embodiment of the present invention.
FIG. 4 is a result of X-ray photoelectron spectroscopy (XPS) of a PVDF membrane obtained by hydrosilation according to a preferred embodiment of the present invention.
5 is a scanning electron microscope (SEM) photograph of a PVDF membrane that has been hydrosilated according to a preferred embodiment of the present invention.
FIG. 6 is a graph showing the poresize distribution of a hydrosilated PVDF membrane according to a preferred embodiment of the present invention.
FIG. 7 is a graph illustrating a result of measuring the permeability of a PVDF membrane obtained by hydrosilylation according to a preferred embodiment of the present invention.

The present invention provides a separator comprising: a separator preparing step for preparing a separator; A plasma irradiating step of irradiating the separation membrane with plasma; And a super-hydrophobic separation step of separating the separation membrane, which has undergone the plasma irradiation step, with a super-hydrophobic compound.

First, a hydrophobic separation membrane such as PE, PP, or PVDF can be used as the separation membrane. Such a separation membrane can be manufactured by a generally known manufacturing method, and a PVDF separation membrane, which is one of the most preferred examples, can also be manufactured by a general separation membrane production method.

Next, the step of irradiating the separation membrane with plasma is as follows. The prepared PVDF membrane is impregnated with a solvent such as distilled water, ethanol, methanol, acetone, etc. to remove diluents and impurities in the surface and pores. The separator is thoroughly dried in an oven at 40 to 50 ° C for 24 hours or more to ensure that there is no moisture in the separator. The separation membrane is placed in a plasma treatment apparatus and examined under the following conditions. The feed gas is supplied at a flow rate of 100 cc / min to 500 cc / min with an irradiation power of 100 W to 500 W and an irradiation time of 60 to 300 seconds. It is preferable to maximize the plasma irradiation effect by minimizing the contact between the separation membrane and the separation membrane during the plasma treatment, and to conduct the treatment under conditions of atmospheric humidity of 50% or less.

The micro-hydration step of the separation membrane that reacts the separation membrane with the super-hydrophobic compound is as follows. The separation membrane having been subjected to the plasma irradiation step is immersed in isopropanol (IPA), and then immersed in a 0.05 to 0.5% solution of a super-hydrophobic compound for 30 minutes to 3 hours to react. Ensure that there is no air trap between the separators during impregnation so that there are no uncoated areas and the temperature is maintained between 20 and 25 ° C. The separator is dried in a desiccator so that it is not exposed to direct sunlight, and the drying time is preferably 4 to 12 hours.

The superhydrophobic compound used herein may be a fluorine-containing chlorosilane compound, a perfluoro methacrylate compound or a perfluorosulfonic acid compound, more preferably a perfluoroalkylsilane compound such as trichloro (1H, 1H, 2H, 2H- 1H, 1H, 2H-perfluorooctyl) silane or triethyl (trifluoromethyl) silane, more preferably trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane). As the perfluoromethacrylate compound, perfluorooctyl methacrylate (1H, 1H-perfluorooctyl methacrylate) is preferable and as the perfluoro sulfonyl chloride, heptadecafluoro-1-octanesulfonyl chloride chloride is preferred.

Particularly, the superhydrophobic compound is more preferably a perfluorinated alkylsilane, and the number of alkyl groups can be variously controlled. The ultrahydro hydratable compound is preferably a substance having a large amount of CF bonds because the CF bond is a very stable bond among the chemical bonds and the reactivity is low and the affinity with water is very low so that super hydrophobicity can be given. Any chemical compound having a perfluoroalkyl group and a chlorosilane group at the same time can be used so that the chemical bonding with the separation membrane for giving hydrophobicity can utilize the chlorosilane group having excellent reactivity with the -OH group. More preferably, trichloro (1H, 1H, 2H-2H-perfluorooctyl) silane (trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane) may be used.

A trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane, which is a preferred embodiment, has the following structure (1).

&Lt; Formula 1 >

The above separation membrane preparation step; A plasma irradiating step of irradiating the separation membrane with plasma; And a micro-hydration step of separating the separation membrane, which has undergone the plasma irradiation step, with an ultra-hydrophobic compound, is briefly described in FIG.

1, oxygen radicals are formed on the surface of the separator by plasma irradiation in the presence of oxygen, and the oxygen radicals formed are called hydroxy groups by immersion of isopropanol (IPA) or the like. Therefore, a large number of -OH groups are introduced on the surface of the separator. Such a surface modification reaction can be similarly applied to a polyolefin separator containing PE, PP, or a fluorine separator including PVDF. Thus, in the separation membrane in which many -OH groups are introduced on the surface, the chlorosilane group of the hyper-hydroscopic compound, chlorosilane group, reacts with the -OH group in the super hydration step in which the superhydrophobic compound is reacted with the following super- And is introduced into the surface. In this reaction, the reaction between a large number of -OH groups on the surface and the chlorosilane group can proceed in the form of a kind of crosslinking reaction, and the surface of the separator has a siloxane form (-Si-O-Si-O-Si-) Can be connected. Therefore, a kind of coating film is formed on the surface of the separation membrane with a perfluorocompound, and the surface of the separation membrane is made hydrophobic.

The contact angles of PVDF membranes before and after treatment were measured to confirm hydrophobicity of the hydrolysis membranes. The contact angle is a measure of the affinity of water for the material. The higher the contact angle, the lower the affinity for water. The longer the exposure time of the plasma, the higher the contact angle of the hydrophobic material, even at the same concentration, due to the higher number of hydroxyl groups that can react with the hydrophobic material. Also, the higher the concentration of hydrophobic substance, the higher the contact angle and the maximum value of 140 °, which is higher than the PTFE material (130 ~ 135 °) (see Table 2).

In addition, the liquid inlet pressure (LEP) was measured to assess how well the hydrosilated membrane was suitable for membrane distillation systems. When the surface becomes hydrophobic, the surface tension is reduced and the LEP is increased. However, if the pore walls are not hydrophobic and hydrophobic surfaces are hydrophobic, the contact angle is high, but the LEP remains at the original level of the membrane. The higher the plasma exposure time and the higher the hydrophobic substance, the higher the LEP. This is similar to the tendency for the contact angle to rise, which indicates that both the plasma irradiation method and the hydrophobic material immersion method can treat not only the surface but also the wall in the pore (see Table 2).

Thus, the contact angle of the droplet was 120 ° or more and the liquid inflow pressure of the second distilled water was 3 bar or more in the super-hydrophobic separation membrane manufactured according to the manufacturing method of the present invention.

The surface super hydration method of the present invention can be applied not only to a hollow fiber membrane but also to a flat membrane, and it can be applied to various kinds of polymers used for the preparation of the membrane and the porosity of the membrane, And temperature and the like, which is an economical and effective method.

Hereinafter, the present invention will be described in more detail with reference to the following examples. However, the scope of the present invention should not be construed as being limited thereto, and these examples should be construed to facilitate understanding of the present invention.

&Lt; Example 1 >

The PVDF membrane was washed and impregnated with ethanol or methanol for 2 hours to remove impurities and diluents in the membrane surface and pores. The separated membrane was thoroughly dried for 24 hours or more in an internal circulating oven at 40 to 50 ° C to completely remove water in the separator. The separator was placed in a plasma processing apparatus, and oxygen was supplied at 200 cc / min. Plasma was irradiated for 20 seconds. After the plasma irradiation, the PVDF membrane was immersed in isopropanol (IPA), and then trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane (trichloro ) 0.05 wt% 1, 4-dioxane solution for 1 hour and reacted. The separator was dried in a desiccator for 8 hours.

&Lt; Example 2 >

The procedure of Example 1 was repeated except that the plasma irradiation time for the separation membrane was 40 seconds.

&Lt; Example 3 >

The procedure of Example 1 was repeated except that the plasma irradiation time for the separation membrane was 60 seconds.

The procedure of Example 1 was repeated except that the plasma irradiation time for the separation membrane was 80 seconds.

&Lt; Example 5 >

The procedure of Example 1 was repeated except that the plasma irradiation time for the separation membrane was 100 seconds.

&Lt; Example 6 >

Except that a solution having a concentration of trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane (trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane) as a superhydrophobic compound was used Was prepared in the same manner as in Example 3.

&Lt; Example 7 >

Except that a solution having a concentration of trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane (trichloro (1H, Was prepared in the same manner as in Example 3.

&Lt; Example 8 >

The PVDF membrane was prepared in the same manner as in Example 1, except that the PVDF membrane was not subjected to the plasma irradiation and the hydrothermal reaction.

The experimental conditions of Examples 1 to 8 and Comparative Examples are summarized in Table 1.

Plasma irradiation time (sec) Concentration of superhydrophobic compound (% by weight) Example 1 20 0.05 Example 2 40 0.05 Example 3 60 0.05 Example 4 80 0.05 Example 5 100 0.05 Example 6 60 0.1 Example 7 60 0.15 Example 8 60 0.2 Comparative Example 0 0

&Lt; Evaluation of physical properties of separator &

1. Measurement of water drop contact angle

A tensiometer (Sigma 701, KSV Co., Finland) equipped with a dynamic contact angle measurement method was used to measure the water wettability and water affinity of the membrane surface. It is a method to calculate the angle of the interface between the surface of the separation membrane and distilled water by immersing the hollow fiber membrane in the container containing the distilled water. Since the contact angle may cause errors depending on the chemical nonuniformity and coarseness of the film surface, in the experiment, the error range was measured within a range of not exceeding the maximum of ± 2 ° by analyzing 10 times or more.

2. Measurement of Liquid Inlet Pressure (LEP)

In order to measure the liquid inflow pressure, the storage tank with the pressurizing device connected to the pressurizing device for filling the solution in the lower part of the membrane was filled with water and the storage tank was pressurized and the module was manufactured by potting 5 membranes on an acrylic tube having a length of 50 mm and a diameter of 15 mm Respectively. The liquid inlet pressure was filled with the solution to the bottom including the module before the measurement, and then the valve was closed to prevent the solution from escaping to the storage tank. In the experiment, secondary distilled water was used as the feed solution. A heat exchange system was installed in the feed solution storage tank to maintain the feed solution temperature at 25 ° C. The liquid inlet pressure was increased stepwise by 0.1 bar at intervals of 10 seconds and the pressure at the time of distilled water was measured as the liquid inlet pressure.

The contact angle of the water drops and the liquid inflow pressure of Examples 1 to 8 and Comparative Examples were measured and are summarized in Table 2.

Contact angle (°) Liquid inlet pressure (bar) Example 1 106.8 2.63 Example 2 119.7 3.08 Example 3 121.4 3.17 Example 4 119.7 3.22 Example 5 124.3 3.21 Example 6 128.3 3.29 Example 7 131.4 3.43 Example 8 134.6 3.57 Comparative Example 101.7 2.27

As can be seen from the above Table 2, the contact angles and the liquid inflow pressure of the PVDF membranes of Examples 1 to 8 of the present invention were significantly increased as compared with Comparative Examples in which the micro hydration treatment was not performed. According to a preferred embodiment of the present invention, the contact angle of the microhydrolytic modified PVDF membrane was greatly increased to 134 ° as compared with 101 ° of the comparative example, and the liquid inflow pressure (LEP) was significantly increased to 3.43 bar compared with 2.27 of the comparative example.

FIG. 2 is a graph showing the contact angles of the ultra-hydrated PVDF membrane according to a preferred embodiment of the present invention, measured according to the concentration of the hydrosilated compound and the plasma exposure time. The plasma contact angle is a measure of the affinity of water for the material. The higher the contact angle, the lower the affinity for water. The longer the exposure time of the plasma, the higher the contact angle of the hydrophobic material, even at the same concentration, due to the higher number of hydroxyl groups that can react with the hydrophobic material. However, when the exposure time of the plasma was more than 40 seconds, there was no further increase, and when the concentration of the superhydrophilic compound was large, the contact angle was larger. Also, the higher the concentration of the hydrophobic substance, the higher the contact angle, , Which is evaluated as having a smaller number of PTFE material (130 ~ 135 °) than the PTFE material (130 ~ 135 °).

FIG. 3 is a graph showing the results of measurement of the liquid inflow pressure (LEP) of the ultra-hydrated PVDF membrane according to a preferred embodiment of the present invention, in accordance with the concentration of the ultra-hydrated compound and the plasma exposure time. In other words, as a result of measuring the liquid inflow pressure (LEP) to evaluate the suitability of the hydrophobed membrane for application to membrane distillation systems, the surface tension becomes smaller and the LEP is increased when the surface becomes hydrophobic. However, if the pore walls are not hydrophobic and hydrophobic surfaces are hydrophobic, the contact angle is high, but the LEP remains at the original level of the membrane. The longer the plasma exposure time, the higher the LEP is. This suggests that the contact angle rise tendency is similar, and it can be proved that both the plasma irradiation method and the hydrophobic material immersion method can treat not only the surface but also the wall in the pore.

FIG. 4 is a graph showing the XPS spectra of XPS spectroscopy (XPS) spectra of the hydrosilated PVDF membranes according to a preferred embodiment of the present invention. . The XPS data represented in FIG. 3 corresponds to F1s spectra and the amount of electrons generated from the 1s orbitals of fluorine (F) per sample is measured and shown. Since the ultra-water-soluble thin film material is a CF-bonded structure, it is possible to quantitatively measure the degree of introduction of the hydrophobic substance through the amount of fluorine. As can be seen from FIG. 3, the amount of fluorine after treatment is considerably increased compared with that before surface treatment. It can be seen that the concentration of the super-hydrophobic material is significantly increased at a concentration of 0.1 wt.%, 0.2 wt.% Compared to 0.05 wt.%. The results are similar to those of LEP and contact angle measurements. From the correlation between the above three experiments, it was found that the concentration of the super-hydrophobic compound is preferably 0.1 to 0.2 wt%.

FIG. 5 is a scanning electron microscope (SEM) photograph (magnification of 500 times) of a PVDF membrane that has been hydrosilated in accordance with a preferred embodiment of the present invention. The surface of the PVDF membrane (a) before treatment and the surface of the PVDF membrane after treatment ((b) 0.05 wt%, (c) 0.1 wt%, (d) 0.2 wt% ultrahydrophilic compound treatment membrane) were examined with a scanning electron microscope , It can be confirmed that the structural characteristics of the film surface are not changed by the micro hydration treatment of the PVDF separation membrane of the present invention.

FIG. 6 shows the pore size distribution of a hydrosilated PVDF membrane according to a preferred embodiment of the present invention. The pore size is analyzed and expressed in order to precisely measure the physical property change. PMIC porometry was used for pore analysis and Galwick was used as wetting liquid. Mean pore size of the membrane and bubble point pore size, the largest pore size, were measured. The pore size before coating was similar to that after coating. As a result, it can be confirmed that the pore size is greatly reduced after the coating or the membrane pore clogging of the membrane pores does not occur.

FIG. 7 is a graph illustrating the permeability of the PVDF membrane before and after the surface treatment of the PVDF membrane according to a preferred embodiment of the present invention as a result of measuring the permeability of the PVDF membrane according to the present invention . Membrane distillation permeability was measured according to the temperature difference between the raw water and the filtrate, and the experiment was conducted to confirm the improvement of the permeability according to the surface treatment. The membrane distillation permeability increased as the temperature difference between raw water and filtrate water increased, but the permeability was not affected by hydrophobization. It can be said that hydrophobicization only contributes to improvement of the durability between the membrane distillation processes of the membrane.

As described above, it can be seen that the hydrophobicity improved separation membrane produced according to the production method of the present invention does not change the inherent physical properties of the separation membrane and can be imparted with super hydrophobicity by a simple process at a low cost.

Claims

A separation membrane preparation step for preparing a separation membrane;
A plasma irradiating step of irradiating the separation membrane with plasma;
And a micro-hydration step of separating the separation membrane, which has undergone the plasma irradiation step, with a super-hydrophobic compound.

The method according to claim 1,
Wherein the separation membrane is a hydrophobic separation membrane.

The method of claim 2,
Wherein the hydrophobic separation membrane is one of a polyolefin separation membrane and a fluorine separation membrane.

The method according to claim 1,
Wherein the super-hydrophobic compound is a fluorine-containing chlorosilane-based compound.

The method according to claim 1,
Wherein the plasma irradiation step is performed in the presence of oxygen gas.

The method according to claim 1,
Wherein the amount of oxygen gas injected in the plasma irradiation step is 100 cc / min to 500 cc / min.

The method according to claim 1,
Wherein the plasma irradiation power of the plasma irradiation step is 100 W to 500 W.

The method according to claim 1,
Wherein the plasma irradiation time of the plasma irradiation step is 60 seconds to 300 seconds.

The method according to claim 1,
Wherein the step of micro-hydrating the separation membrane comprises micro-hydrating the surface of the separation membrane with a solution of a super-hydrophobic compound at a concentration of 0.05 to 0.5% by weight.

A method for manufacturing a semiconductor device, which is manufactured by a method according to any one of claims 1 to 9,
Wherein a super-hydrophobic group is introduced on the surface so that the contact angle of the droplet is 120 DEG or more and the liquid inflow pressure of the second distilled water is 3 bar or more.