Apparatus and method for measuring surface tension

Abstract

The present invention relates to an apparatus and method for measuring surface tension. More particularly, the present invention relates an apparatus and method for measuring surface tension through an electrical scheme which is simpler and has improved accuracy compared to a conventional optical scheme.

Claims

1. A device for measuring surface tension, which measures a magnitude of surface tension between a conductive fluid and a nonconductive fluid by using an electric characteristic, the device comprising: a substrate; an electrode formed on the substrate; a dielectric layer formed at an upper side of the electrode; a container containing the conductive fluid to cover the electrode; and a voltage applying unit for applying a voltage between the electrode and the conductive fluid, wherein the nonconductive fluid is provided in the container to be located on the electrode inside the conductive fluid so that the dielectric layer is interposed between the nonconductive fluid and the electrode, and wherein capacitance according to a geometric shape of the nonconductive fluid is measured to measure a contact angle and surface tension.

2. A device for measuring surface tension, which measures a magnitude of surface tension between a conductive fluid and a nonconductive fluid by using an electric characteristic, the device comprising: a substrate; an electrode formed on the substrate; a dielectric layer formed at an upper side of the electrode; a container containing the nonconductive fluid to cover the electrode, the conductive fluid being provided in the container to be located on the electrode inside the nonconductive fluid so that the dielectric layer is interposed between the conductive fluid and the electrode; and a voltage applying unit for applying a voltage between the electrode and the conductive fluid, wherein capacitance according to a geometric shape of the conductive fluid is measured to measure a contact angle and surface tension.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram showing an existing method for measuring surface tension.

(2) FIG. 2 is a diagram showing a device for measuring surface tension according to the present disclosure.

(3) FIG. 3 is a diagram for illustrating the principle of applying the device for measuring surface tension according to the present disclosure.

(4) FIG. 4 is a diagram showing a device for measuring surface tension according to the present disclosure.

(5) FIG. 5 is a diagram for illustrating the principle of applying the device for measuring surface tension according to the present disclosure.

(6) FIG. 6 is a diagram for illustrating an effect of the device for measuring surface tension according to the present disclosure.

(7) FIG. 7 is a diagram for illustrating an effect of the device for measuring surface tension according to the present disclosure.

(8) FIG. 8 is a diagram showing an embodiment of the device for measuring surface tension according to the present disclosure.

(9) FIG. 9 is a diagram showing another embodiment of the device for measuring surface tension according to the present disclosure.

MODE FOR CARRYING OUT THE INVENTION

(10) Hereinafter, the present disclosure will be described in detail with reference to accompanying drawings.

(11) For better understanding of the present disclosure, any features of the present disclosure already known in the art will not be described in detail here. The following embodiments are just for better understanding of the present disclosure and not intended to limit the scope of the present disclosure. Therefore, any equivalents having the same function as the present disclosure will also fall within the scope of the present disclosure.

(12) In the following explanation, the same reference sign indicates the same component, and details of well-known features and techniques may be omitted.

(13) FIG. 2 is a diagram showing a device for measuring surface tension according to the present disclosure.

(14) According to an embodiment of the present disclosure, a device for measuring surface tension (hereinafter, also referred to as a surface tension measuring device) measures a magnitude of surface tension between a conductive fluid 30 and a nonconductive fluid 20 by using an electric characteristic.

(15) Even though it is described in the following embodiment that the nonconductive fluid 20 is included in the conductive fluid 30 in a drop form, but the same principle may also be applied even though the nonconductive fluid 20 is included in a drop form in the conductive fluid 30 on the contrary.

(16) According to the present disclosure, the surface tension measuring device includes a substrate 11, an electrode 12 formed on the substrate 11, a dielectric layer 13 formed at an upper side of the electrode 12, a container 14 containing the conductive fluid 30 to cover the electrode 12, and a voltage applying unit 40 for applying a voltage between the electrode 12 and the conductive fluid 30.

(17) Even though the electrode is depicted as a flat electrode, the present disclosure is not limited thereto, and various kinds of electrodes having a V shape, a cylindrical shape, a rectangular well shape or the like may also be used.

(18) However, a dielectric layer may be removed so that the electrode 12 is directly exposed to the container.

(19) The nonconductive fluid 20 is provided in the container 14 to be located on the electrode 12 inside the conductive fluid 30 so that the dielectric layer 13 is interposed between the nonconductive fluid 20 and the electrode 12.

(20) Capacitance according to a geometric shape of the nonconductive fluid 20 is measured to measure a contact angle and surface tension.

(21) The nonconductive fluid 20 is provided to be entirely included in the conductive fluid 30, and there is a pressure difference in a medium between the nonconductive fluid 20 and the conductive fluid 30 due to different surface energies. In this case, the pressure difference forms a curvature to keep a balance on the surface, and the nonconductive fluid 20 has a partial spherical shape as shown in FIG. 2.

(22) In the present disclosure, when the nonconductive fluid 20 forms a contact angle in the conductive fluid 30 and has a partial spherical shape, impedance including capacitance according to the shape is obtained, and a contact angle is calculated therefrom. Generally, in the case of FIG. 2, Equation 3 below is established.

(23) $\begin{array}{cc}\cos =\cos {}_Y+\frac{\mathrm{.Math.}}{2d}{V}^2& \mathrm{Equation}3\end{array}$

(24) where represents a contact angle changed by electro-wetting when a voltage is applied to a fluid, Y represents a contact angle in an equilibrium state, represents a dielectric constant, represents surface tension, V represents an applied voltage, and d represents a thickness of dielectric material.

(25) This will be described in more detail below.

(26) FIG. 3 shows a figure of the nonconductive fluid 20 which forms a partial spherical shape. Even though FIG. 3 shows a general 3-phase relation, the same concept may also be applied when a liquid is applied to the nonconductive fluid 20, a gas is applied to the conductive fluid 30, and a solid is applied to the dielectric layer or the electrode.

(27) A radius R of the entire sphere changes according to the contact angle of the nonconductive fluid 20, and accordingly an area of a circular shape contacting the dielectric layer also changes. At this time, a radius of the circle of the contact surface is defined as r.

(28) In this case, Equation 4 below is established from the geometrical shape information. In addition, capacitance according to the contact surface of the nonconductive fluid 20 also establishes Equation 4 below. Here, V represents a volume of the nonconductive fluid, A represents an area of a surface of the nonconductive fluid which is in contact with the dielectric layer, c represents a dielectric constant, d represents a thickness of the dielectric layer, and R and are shape values depicted in FIG. 3.

(29) $\begin{array}{cc}R=\frac{3V^{1/3}}{\left(2+\cos \right){\left(\begin{array}{cc}1& \cos \end{array}\right)}^2}A=R^2{\sin }^{2}C=\frac{\mathrm{.Math.}A}{d}& \mathrm{Equation}4\end{array}$

(30) If Equation 4 is arranged, the contact value may also be expressed as a function of capacitance like Equation 5 below.
=f(Cap.)Equation 5

(31) In the measuring device of FIG. 2 or 4, if a voltage is applied to the electrode 12 and the fluid, the device may be simplified into a circuit of FIG. 5. Rw represents resistance by the fluid, and Cd represents capacitance by the dielectric material.

(32) In this case, the surface tension measuring device may further include a current measuring unit for measuring a current flowing between the electrode 12 and the fluid in the container 14, and a calculating unit for calculating a contact angle and a magnitude of surface tension by using the voltage applied by the voltage applying unit 40 and the current measured by the current measuring unit.

(33) At this time, the calculating unit measures capacitance by using Equation 6 below with the applied voltage and the measured current as input values. Here, Z represents impedance, V represents a voltage, I represents a current, R represents resistance of the fluid, C represents capacitance, represents frequency of power, and represents a phase angle.

(34) As obvious from the following equation, the voltage and current used in the present disclosure should be AC voltage and current having a magnitude and phase.

(35) $\begin{array}{cc}Z=\frac{V}{I}=\frac{V/\underset{\_}{{}_v}}{I/\underset{\_}{{}_i}}=R_w-j\frac{1}{C_d}=Z/\underset{\_}{}={}_v-{}_i={\tan }^{-1}\frac{-1}{R_{w_{|}}{C}_d}& \mathrm{Equation}6\end{array}$

(36) By obtaining the capacitance through Equation 6, a contact angle may be obtained if the capacitance is applied to Equation 4 above.

(37) After obtaining the contact angle, the contact angle may be applied to Equation 3 to obtain surface tension. However, an equilibrium contact angle is just a constant and may be treated as an unnecessary value in calculation, and thus Equation 3 may be primarily differentiated to induce Equation 7 below. Here, V represents an applied voltage, and represents a contact angle at this time.

(38) $\begin{array}{cc}=\frac{\mathrm{.Math.}}{d\frac{\cos }{V}}.Math.V& \mathrm{Equation}7\end{array}$

(39) The calculating unit includes an algorithm of Equation 7, and performs the above calculation by using the obtained contact angle as an input value. The surface tension obtained through this calculation advantageously has less noise.

(40) In addition, Equation 3 may be secondarily differentiated to induce Equation 8 below. Here, V represents an applied voltage, and represents a contact angle at this time.

(41) $\begin{array}{cc}=\frac{\mathrm{.Math.}}{d\frac{{}^2\cos }{V^2}}& \mathrm{Equation}8\end{array}$

(42) Also, the calculating unit includes an algorithm of Equation 8, and performs the above calculation by using the obtained contact angle as an input value. The surface tension obtained through this calculation has noise more or less, but there is substantially no surface charging effect, advantageously.

(43) In addition, the surface tension may also be obtained using Equation 9 below. At this time, after different voltages are applied, contact angles for the voltages are compared to obtain more accurate surface tension. Here, 1 represents a contact angle when a voltage V1 is applied, and 2 represents a contact angle when a voltage V2 is applied.

(44) 0$\begin{array}{cc}=\frac{\left(V_1^2-V_2^2\right)}{2d\left(\cos {}_1-\cos {}_2\right)}& \mathrm{Equation}9\end{array}$

(45) The calculating unit may selectively use any one of Equations 7 to 9. Though not depicted in the figures, in the calculating unit, the algorithm described above is generally previously set as a combination of a CPU and a memory, and the calculating unit receives an input value and mechanically performs a calculation by using the preset algorithm. This is obvious to those skilled in the art and will not be described in detail here.

(46) The current measuring unit is also not depicted in the figures but obvious to those skilled in the art.

(47) FIGS. 6 and 7 show measurement results of the measuring device of the present disclosure, from which it may be found that the measuring device of the present disclosure ensures good accuracy.

(48) In the present disclosure, particularly, a coating layer 15 made of hydrophobic material is further formed on the dielectric layer 13. The coating layer 15 allows the nonconductive fluid 20 present inside the conductive fluid 30 to be located at an upper side of the coating layer 15. By doing so, the fluid subject to measurement is automatically positioned.

(49) FIG. 8 shows another embodiment of the present disclosure. This embodiment is directed to a surface tension measuring device for measuring surface tension of a plurality of nonconductive fluids 20 simultaneously by using an electric characteristic. The surface tension measuring device includes a substrate 11, a plurality of flat electrodes 121, 122 spaced apart from each other on the substrate 11, a dielectric layer 13 formed at an upper side of the electrode 12, a container containing the conductive fluid 30 to cover the electrode 12, and a voltage applying unit 40 for applying a voltage between the electrode 12 and the conductive fluid 30, wherein the nonconductive fluid is provided in the container 14 to be located on the electrode inside the conductive fluid 30 so that the dielectric layer 13 is interposed between the nonconductive fluid 20 and the electrode 12, and wherein capacitance according to a geometric shape of the nonconductive fluid is measured to measure a contact angle and surface tension.

(50) The principle of this embodiment is identical to the former embodiment, but the plurality of electrodes are provided to be electrically separated from each other. The electrodes are individually connected to electrode lines 161, 162 for applying voltages, respectively.

(51) FIG. 9 shows another application of the present disclosure. The surface tension measuring device of this embodiment measures surface tension of the conductive fluid 30. Here, the conductive fluid 30 is located on the electrode 12 in a drop form, and a voltage is directly applied to the conductive fluid 30 so that capacitance varying depending on an area of a contact surface according to the contact angle of the conductive fluid 30 is measured to obtain surface tension. This embodiment is different from the former embodiment in the point that only a single conductive fluid exposed to a gas state is used instead of two kinds of conductive and nonconductive fluids. Its principle is substantially identical to the former embodiment and thus not described in detail here.

(52) A method for measuring surface tension according to the present disclosure includes a first step of locating a nonconductive fluid 20 in a conductive fluid 30 to form a contact angle with respect to an electrode 12, a second step of applying a voltage to the electrode 12 and the conductive fluid 30 and measuring a current flowing between the electrode 12 and the conductive fluid 30 while changing the applied voltage, a third step of calculating capacitance according to the contact angle of the nonconductive fluid 20 by using Equation 6, a fourth step of measuring a contact angle by using Equation 4, and a fifth step of measuring a magnitude of surface tension by using Equation 7 or 8.

(53) In this specification, the expression forming a contact angle with respect to an electrode has a meaning including not only a direct contact onto the electrode but also a contact to a dielectric layer (or, an insulating layer) if the dielectric layer is formed on the upper surface of the electrode. In other words, the same structure as described above may be applied hereto.

(54) In addition, a method for measuring surface tension according to the present disclosure may include a first step of locating a conductive fluid 30 in a nonconductive fluid 20 to form a contact angle with respect to an electrode 12, a second step of applying a voltage to the electrode 12 and the conductive fluid 30 and measuring a current flowing between the electrode 12 and the conductive fluid 30 while changing the applied voltage, a third step of calculating capacitance according to the contact angle of the conductive fluid 30 by using Equation 6, a fourth step of measuring a contact angle by using Equation 4, and a fifth step of measuring a magnitude of surface tension by using Equation 7 or 8.

(55) Details of this embodiment are substantially identical to the former embodiment and thus not described in detail here.

(56) The term comprise, include or have used herein means that any component can be provided therein, unless otherwise stated, and thus should be interpreted as being capable of further including other components, instead of excluding other components. All terms including technical terms or scientific terms have the same meanings as being generally understood by those skilled in the art of the present disclosure, unless otherwise defined. Terms generally used as defined in dictionaries should be interpreted as being in agreement with the context of relevant descriptions, and should not be interpreted too ideally or excessively formally, unless otherwise defined in the present disclosure.