JP2011021979A - Method for measurement of surface tension of very small amount of liquid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure the surface tension of an extremely small amount of a liquid to be measured. <P>SOLUTION: A method is provided for measuring the surface tension or wet index of an extremely small amount of the liquid to be measured using the needle driven by a drive unit. In the method, the tip of the needle is brought into contact with the liquid to be measured and subsequently moved in the direction separated from the liquid to be measured to measure the force required for separating the tip of the needle from the liquid to be measured and the surface tension or wet index of the liquid to be measured is calculated from the measured force value. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for measuring the surface tension or wetting index of a minute amount of liquid.

  The surface tension is a general physical property at the gas-liquid interface and has been widely measured. Examples of the measuring method include a capillary ascending method, a dropping method, a Whlhelmy method, a du Nouy ring method (hanging ring method), and a maximum bubble pressure method.

  In the following, γ is the surface tension, r is the capillary radius, ρ is the density of the liquid, g is the acceleration of gravity, m is the mass of the droplet, θ is the contact angle, l is the contact length, and K is the capillary constant. Respectively.

In the capillary ascending method, the surface tension is obtained according to the following formula from the height h that rises in the capillary when the capillary mouth is immersed in the liquid.
2πrγ = πr ² hρg
In the dropping method, the surface tension is determined according to the following equation from the number of drops when dropping from the mouth of a circular tube and the specific gravity of the liquid.
mg = 2πrγ
In the hanging ring method, a platinum small ring is hung horizontally and brought into contact with the liquid surface, and the surface tension is obtained from the force f acting when the small ring is pulled up according to the following equation.
f = 4πrγ
In the maximum bubble pressure method, the surface tension is obtained from the maximum pressure P when bubbles generated by slowly injecting gas from the capillary into the liquid are separated from the tip of the capillary according to the following equation.
Pr = K2γ cos θ
In the Wilhelmy method, a metal plate or the like is immersed in a liquid, and the surface tension is determined from the resultant surface tension and the resultant force F of the buoyancy f according to the following equation.
F = mg + γcos θ · l−f

  All of these methods are established measurement methods and have high reliability. However, since these methods require a certain amount of the liquid to be measured, they cannot be applied when the liquid to be measured is small.

  In recent years, as a measuring method with a very small amount of liquid to be measured, Christenson et al. Reported a disk lifting method (Non-Patent Document 1). There, about 0.2 μl of three types of liquid droplets to be measured (ethanol, ethylene glycol, water) are inserted between 2 cm diameter silica disks polished in a convex shape, and the distance between the two disks is gradually increased. It is shown that there is a correlation between the distance at which the attractive force between the disks suddenly decreases and the surface tension of the liquid to be measured.

J. of Colloid and Interface Science, 232, pp408-409, (2000)

  The problem to be solved by the present invention is to provide a method capable of measuring the surface tension or wetting index of a liquid even when the amount of liquid to be measured is very small.

  The present inventors have used, for example, a force curve function of a scanning probe microscope (hereinafter, sometimes referred to as “SPM”), in a standard liquid with a known surface tension or wetting index. By comparing with the measurement results, a method for measuring the surface tension or the wetting index of a very small amount of liquid to be measured was found, and further research was advanced to arrive at the present invention.

  That is, the present invention is a method of measuring surface tension or wetting index using a needle driven by a driving unit, and the tip of the needle is brought into contact with the liquid to be measured and then moved away from the liquid. And measuring the force required to disengage the tip of the needle from the liquid to be measured, and obtaining the surface tension or wetting index of the liquid to be measured from the measured value.

  According to the present invention, even when only a very small amount of liquid to be measured is obtained, the surface tension or the wetting index of the liquid to be measured can be measured. For example, it is useful when the liquid to be measured is derived from a specific living body or a specific small amount of adhering liquid.

It is a schematic diagram which shows an example of the force curve of SPM. 3 is a graph showing measurement results in Example 1. 6 is a graph showing measurement results in Example 2.

  The present invention is a method for measuring surface tension or wetting index using a needle driven by a drive unit, and after contacting the tip of the needle with the liquid to be measured, it is moved away from the liquid, This is a measurement method in which the force required to disengage the tip of the needle from the liquid to be measured is measured, and the surface tension or wetting index of the liquid to be measured is obtained from the measured value.

  As the force required for the tip of the needle to detach from the liquid to be measured, the value immediately before detachment is preferable, and usually takes the maximum value at that point, but as long as the surface tension or wetting index is required, It is not limited.

  As an example of the needle used in the present invention, in the SPM probe, a needle fixed to the tip of a cantilever can be mentioned. The contact-detachment between the tip of the cantilever and a minute amount of liquid can be observed, and the force required for the removal. Any other shape of needle may be used as long as it can be measured.

  The shape of the tip of the needle used in the present invention is not particularly limited as long as it is easy to observe contact and detachment with a minute amount of liquid. For example, a conical shape may be mentioned, but the apex angle does not need to be an acute angle, and may be 60 °, for example. The needle material used in the present invention is not particularly limited as long as it can observe contact and separation between the needle material and a small amount of liquid. For example, carbon nanotubes (CNT) can be used.

  Here, the “tip” is a portion where the needle comes into contact with the liquid to be measured, and does not necessarily have to be at the end of the needle. When measuring the surface tension of a very small amount of liquid to be measured, it is important to reduce the size of the part that is involved in the measurement. It is preferable to reduce the size. For example, a needle having a diameter of about several tens of nanometers, which is the diameter of the needle used in SPM, can be used.

  In addition, such a needle is attached to a cantilever-shaped member in SPM, which is a preferred embodiment. However, the needle is smoothly moved so that contact between the tip of the needle and the liquid to be measured is separated. If there is no trouble in measuring the force required to observe and disengage, it may be attached to another member.

  When the needle is driven by a drive unit via a spring mechanism, for example, a cantilever, the force required for the tip of the needle to move away from the liquid to be measured is the force when the tip of the needle is separated from the liquid to be measured. It can be calculated from the position and the spring constant of the spring mechanism. For example, when the spring mechanism is a cantilever, the deflection of the cantilever is obtained from the position when the tip of the needle is separated from the liquid to be measured, and the tip of the needle is detached from the liquid to be measured from the degree and the spring constant. You can calculate the force required to do it. However, the spring constant is used when the displacement and restoring force of the tip of the needle can be approximated to a linear function, and it is preferable to approximate with a higher order function depending on the required accuracy.

  The driving unit for driving the needle can be cited as a suitable mechanism used in SPM, but is not limited thereto, and has a required driving accuracy, and the tip of the needle is detached from the liquid to be measured. The mechanism of the drive unit is not limited as long as it has the accuracy necessary to measure the force required for the operation.

  As described above, it is preferable to design the needle used in the present invention, the shape of the tip thereof, its attachment mode and driving method according to the SPM mechanism, and it is particularly preferable to use the SPM itself. Hereinafter, taking a case where SPM is used as an example, one of the preferred embodiments of the present invention will be described in more detail.

  SPM is one of surface shape measuring devices. Many models can measure force curves as standard. Here, the force curve means that the probe is moved up and down by setting the upper and lower limits of the distance between the probe (the cantilever is attached to the probe and the needle is attached to the cantilever) and the liquid to be measured. A curve in which the relationship between the probe-sample distance and the force acting on the cantilever (amplitude attenuation) is plotted. The force curve is often measured by the contact mode, but is not limited to this.

  In the present invention, the measurement principle such as the surface tension of the liquid to be measured is explained as follows using the schematic diagram of the force curve shown in FIG. 1 when SPM is used. In this figure, the horizontal axis represents the movement of the probe including the cantilever mounting portion, and the vertical axis represents the force received by the tip of the needle.

  Initially, when the probe is moved as in 1, the tip of the needle approaches the solid surface on which the liquid to be measured is placed, but until the solid surface A is reached, the tip of the needle does not receive any force. When the probe is further pushed after reaching the surface A, the cantilever to which the needle is attached operates as a spring as shown in 2, and the tip of the needle receives a force proportional to the moving distance.

  When the probe is released after a certain distance has been pushed in, the same line is drawn as shown in 3 but the tip of the needle remains on the liquid surface due to the surface tension of the liquid to be measured even after passing the solid surface A. Without getting away from it, this time it receives attraction.

  When point C is reached, the pulling force by the spring of the cantilever becomes equal to the pulling force due to the surface tension of the liquid to be measured, and the tip of the needle moves away from the surface. Thereafter, the force applied to the tip of the needle suddenly returns to zero (D) as indicated by 4. After that, since the tip of the needle is not subjected to force, a locus like 5 is drawn.

Here, the distance AD until the tip of the needle is separated varies depending on the surface tension of the liquid to be measured. For example, the surface tension or the wetting index of the liquid to be measured can be obtained by measuring the distance AD using a liquid having a known surface tension and comparing it with the same.
In this way, the surface tension or the wetting index can be easily obtained even with a sample of a very small amount of liquid to be measured (for example, 100 nl).

  The liquid to be measured by the method of the present invention is not particularly limited, but depending on the measurement conditions, there is a concern about evaporation or curing of the liquid to be measured, so it is desirable to set an appropriate measurement time and measurement environment. For example, when there is a concern about evaporation, the circumference of the liquid to be measured is filled with the vapor.

  In the measuring method of the present invention, the moving speed and measuring time of the needle are not particularly limited as long as the force required to disengage the tip of the needle from the liquid to be measured can be measured. However, since the liquid to be measured is very small, it should be set in consideration of the evaporation effect.

[Example 1]
Olympus cantilever OTR-8 was attached to SPM Nanoscope IIIa manufactured by Digital Instruments, and the force curve was determined in contact mode. Since the surface to be measured needs to be clean and flat, a mica peeling surface was adopted.
Pure water and ethylene glycol were used as liquid samples, and the measurement was performed in the air. The SPM probe was wetted with each solution, and the distance at which the attractive force rapidly decreased was obtained on the obtained force curve.

  More specifically, 100 nl of the liquid to be measured was dropped on the mica plate using a micropipette or the like. While observing with an optical microscope or the like, the tip of the needle was lowered to the very end of the liquid to be measured. In the case of Nanoscope IIIa, since the needle is held from the right side, the tip of the needle is lowered to the right end of the droplet. When the tip of the needle comes into contact with the droplet, a part of the droplet adheres to it and becomes spherical. When that state was reached, the probe was manually raised, and the tip of the needle was moved to a location where the mica plate was not wet. The position was set immediately on the right side in order to minimize the influence of evaporation of the liquid to be measured by shortening the moving distance of the tip of the needle. This operation is to optimize the amount of the liquid to be measured. Thereafter, force curve measurement was performed again. This operation was repeated as many times as necessary.

  Table 1 shows the measurement results of the surface tension (reference values) of each liquid and the probe moving distance when the tip of the SPM needle is separated from the surface of the liquid to be measured. As shown in FIG. 2, a good correlation was observed between the surface tension of each liquid and the moving distance of the probe (ie, the deflection of the cantilever) when the tip of the needle was separated.

[Example 2]
The same measurement as in Example 1 was performed by changing the liquid to be measured. That is, three types of liquid mixture for wet tension test (No. 40.0, 60.0, 67.0) manufactured by Wako Pure Chemical Industries, Ltd. were used as liquids to be measured. Table 2 shows the measurement results of the wetting index of each liquid and the movement distance of the probe when the tip of the SPM needle is separated from the surface of the liquid to be measured. As shown in FIG. 3, a correlation was observed between the wetting index of the liquid to be measured and the moving distance of the probe when the probe was separated (that is, the deflection of the cantilever).

  According to the present invention, the surface tension and the wetting index can be measured even with a small amount of liquid to be measured. For example, it can be used at various industrial sites such as the management process and various research processes when producing various reagents.

Claims (6)

  A method of measuring surface tension or wetting index using a needle driven by a driving unit, wherein the tip of the needle is brought into contact with the liquid to be measured, and then moved away from the liquid, so that the tip of the needle Is a measurement method that measures the force required to release the liquid from the liquid to be measured, and calculates the surface tension or wetting index of the liquid to be measured from the measured value.   The force required for the needle to be driven by the drive unit via the spring mechanism and the tip of the needle to be separated from the liquid to be measured is calculated from the position when the tip of the needle is separated from the liquid to be measured and the spring constant. The measurement method according to claim 1 to be obtained.   The measuring method according to claim 1 or 2, wherein a scanning probe microscope is used.   The measurement method according to claim 1, wherein the measurement target liquid is placed on a mica plate and measured.   The measuring method according to claim 1, wherein a substance having a known surface tension is used as a standard.   The measurement method according to claim 5, wherein a wetting index standard solution is used as the substance having a known surface tension.

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