RU2244288C1 - Method of measurement of surface tension coefficient and static and dynamic wetting angles - Google Patents

Abstract

FIELD: chemical, paint-and-varnish and food-processing industries.

SUBSTANCE: proposed method includes pumping liquid through two similar capillaries made in form of circular passages of different length, determination of volumetric flow of this liquid and pressure differential at capillary ends and viscosity and yield point.

EFFECT: enhanced accuracy of measurements; facilitated procedure of determination of viscosity and yield point.

2 dwg

Description

The invention relates to the field of measuring equipment and may find application in various industries, for example, in the chemical, paint and varnish and food industries.

The surface tension of the liquid and the contact angles of wetting are the most important indicators characterizing the processes and phenomena occurring at the interface between liquid - solid - gas.

There is a method of determining the coefficient of surface tension of a liquid and the static contact angle of wetting of a solid surface by the method of “lying” drops (AD Zimon. Adhesion of liquids and wetting. M., Chemistry, 1974, S. 52-55), which consists in determining the shape and the size of the droplet lying on the plate, using optical systems, such as a microscope.

The main disadvantage of the “lying” drop method is the difficulty in accurately determining the parameters of the drop, and, consequently, the low accuracy of the method. In addition, this method does not allow to determine the dynamic contact angles of wetting a solid surface with a moving fluid.

A known method for determining the coefficient of surface tension of a liquid (Surface phenomena and surface-active substances. Handbook edited by A.A. Abramov. L., Chemistry, 1984, s.167-168), including measuring the force of separation of the ring from the investigated fluid. However, this method does not allow to determine the contact angles of wetting.

A known method for determining the dynamic contact angle of wetting (B.V. Deryagin, N.V. Churaev. Wetting films. M., Nauka, 1984, pp. 93-95), based on the calculation of the specified angle using data on the velocity of the meniscus of the fluid fixed by optical means in the capillary under the influence of pressure drop and capillary forces.

The disadvantages of this method include very stringent requirements for the constancy of the inner radius of the tube and the uniformity of the inner surface.

Closest to the proposed technical essence is a method for determining the surface tension of liquids by direct weighing (RF patent No. 2154265, class G 01 N 13/02, 1998 - prototype), including weighing the reference body in air, immersion in half and full immersion in the test fluid and calculating the surface tension coefficient of the fluid.

The disadvantages of the method include the fact that it is applicable only to liquids that completely wet the surface, that is, having a statistical contact angle θ _article = 0. In addition, this method does not allow to determine the dynamic contact angle, that is, in the case of fluid movement relative to a solid surface.

The technical result of the invention consists in the possibility of separately determining the coefficient of surface tension of the liquid and the static and dynamic contact angles of wetting of the solid surface during one measurement cycle and improving the accuracy of measurement of these characteristics.

This result is achieved due to the fact that in the method of measuring the coefficient of surface tension of a liquid and static and dynamic contact angles of contact, including weighing the test material in air and in the test liquid, followed by calculating the coefficient of surface tension of the liquid, with the plate of the test material in the lower its parts attach two mustaches 1-2 mm long, and they are directed vertically downwards, then the plate is cleaned of contamination, and after weighing the plate into the air number of weighings carried out: a) in the moment of contact with the liquid under investigation plate;

c) at the moment preceding the rupture of the liquid film, when lowering the cell with the liquid at a minimum speed; c) at the time of passage of the meniscus of fluid in the middle of the plate, when lifting the cuvette with fluid at a constantly given speed; and d) immediately before the plate is completely immersed in the liquid, wherein the surface tension coefficient of the liquid is determined by the formula:

the static contact angle is calculated from the ratio:

and dynamic contact angle - using the formula:

where A is the width of the plate; B is the plate thickness;

G ₀ - the reading of the weighing device when weighing the plate in air;

G ₁ - the indication of the weighing device at the moment of contact of the test plate with liquid;

G ₂ - the indication of the weighing device at the time preceding the rupture of the liquid film;

G ₃ - the indication of the weighing device at the time of passage of the meniscus of fluid in the middle of the plate;

G ₄ - the reading of the weighing device immediately before the complete immersion of the plate in the liquid.

Figure 1 presents a diagram of a device for implementing the proposed method for measuring the surface tension coefficient of a liquid and static and dynamic contact angles of wetting of a solid surface, where 1 is an engine control unit; 2 - electric motor; 3 - block kinematics; 4 - thermostatic cell; 5 - weighing device; 6 - a tripod; 7 - electronic block amplification and signal conversion; 8 - unit for recording readings of the weighing device.

Figure 2 presents the timing diagram of the readings of the registration unit of the device for determining the coefficient of surface tension of the liquid and the static and dynamic contact angles of wetting of a solid surface: τ - time; G is the reading of the registration unit of the weighing device.

The proposed method is as follows.

A plate of the test material is pre-prepared. For this, two whiskers are attached to the lateral ends of the plate, due to which a liquid film is formed at the lower end when the plate is removed from the liquid. The mustache is directed vertically downwards so that the width of the resulting liquid film is equal to the width of the plate. For measurements, a length of 1-2 mm is sufficient. After that, the plate is thoroughly cleaned, since contamination can introduce a significant error in the measurement of not only the contact angle, but also the surface tension. Then, a cuvette with the test liquid is installed on the site. The liquid is thermostatically controlled by appropriate means (not shown in FIG. 1). A plate of the test material is suspended from the weighing device, the plate is weighed in air and the first reading G _{0 is} recorded. After that, using the drive mechanism for moving the cuvette with a minimum speed, they are lifted up until the plate touches the liquid. The movement is stopped and the readings G _{1 are} measured. The drive mechanism is turned on and the cell with liquid is lowered with a minimum speed. At the moment preceding the rupture of the liquid film, the reading of the weighing device is recorded - G ₂ . With the help of a speed dial, the desired lifting speed of the cell is set. Turn on the mechanism for lifting the cell with the liquid. When passing the meniscus of the liquid, in the middle of the plate, the readings of the weighing device are fixed - G ₃ . Just before the plate is completely immersed in the liquid, the last reading of the weighing device, G _{4, is} recorded.

Claims (1)

A method of measuring the coefficient of surface tension of a liquid and static and dynamic contact angles of contact, comprising weighing the test material in air and in the test fluid, followed by calculating the coefficient of surface tension of the liquid, characterized in that two whiskers 1- are attached to the plate of the test material in its lower part 2 mm, and they are directed vertically downwards, then the plate is cleaned of contamination, and after weighing the plate in air, a number of weighings are carried out: a) at the moment of the study of the test plate with liquid; c) at the moment preceding the rupture of the liquid film, when lowering the cell with the liquid at a minimum speed; c) at the moment of passage of the meniscus of the liquid in the middle of the plate when lifting the cell with the liquid at a constantly given speed; and d) immediately before the plate is completely immersed in the liquid, wherein the surface tension coefficient of the liquid is determined by the formula:

the static contact angle is calculated from the ratio:

and dynamic contact angle - using the formula:

where A is the width of the plate; B is the plate thickness; G ₀ - readings of the weighing device when weighing the plate in air; G ₁ - readings of the weighing device at the moment of contact of the test plate with liquid; G ₂ - readings of the weighing device at the time preceding the rupture of the liquid film; G ₃ - readings of the weighing device at the time of passage of the meniscus of the liquid in the middle of the plate; G ₄ - readings of the weighing device immediately before the complete immersion of the plate in the liquid.

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