GB2092758A

GB2092758A - Process and Apparatus for Determining the Characteristics of a Drop of a Liquid Required for Calculating the Surface Tension of the Liquid

Info

Publication number: GB2092758A
Application number: GB8103713A
Authority: GB
Original assignee: NAT IRANIAN OIL CO; Shell Internationale Research Maatschappij BV; NATIONAL IRANIAN OIL Co
Current assignee: NAT IRANIAN OIL CO; Shell Internationale Research Maatschappij BV; NATIONAL IRANIAN OIL Co
Priority date: 1981-02-06
Filing date: 1981-02-06
Publication date: 1982-08-18
Also published as: GB2092758B

Abstract

Method and apparatus for determining the characteristics of a drop of a liquid required for calculating the surface tension of the liquid, wherein the drop of the liquid is formed at the free end of a wire. The apparatus comprises a vessel 1, a wire 2 having a free end installed in the interior of the vessel 1, means 11, 12 for forming a drop of liquid at the free end of the wire 2, and means e.g. a camera and light source 9, 10 for determining certain characteristics of the drop of the liquid. Liquid may be introduced through pipe 11 to immerse the end of wire 2 while gas under pressure is introduced by pipe 12. When the liquid level is lowered below the end of wire 2, drops form thereon. Liquid may be supplied to the wire by a capillary tube or by raising the wire from a liquid bath. Several wires of different diameters carried by a rod supplied with liquid by a capillary tube may be used to obtain the optimum drop shape. The number of drops plus volume of drops falling from a wire in a certain time may be measured. <IMAGE>

Description

SPECIFICATION Process and Apparatus for Determining the Characteristics of a Drop of a Liquid Required for Calculating the Surface Tension of the Liquid The invention relates to a process and an apparatus for use in the calculation of the surface tension of a liquid. More in particular the invention relates to a process and an apparatus for determining the characteristics of a drop of a liquid required for calculating the surface tension of the liquid.

The surface tension (also called interfacial tension) of a liquid is the force between two adjacent portions of the surface of the liquid per unit length of the common boundary-line of said portions.

Many methods are known for determining the surface tension of a liquid and some well known methods will be briefly discussed hereinafter.

In the "capillary rise" method the surface tension is determined from the rise of the liquid in a vertical capillary tube, which is partly immersed in the liquid to be tested. The "sessile drop" method is a method wherein the shape of a drop of liquid resting on a smooth horizontal surface, is used for the determination of the surface tension.

Other well known methods are the "ring" method and the "vertical plate" method. In the "ring" method the deformation of the surface of a liquid occurring when pulling a horizontal ring out of the liquid, is determined. This deformation is an indication of the value of the surface tension of the relevant liquid. In the "vertical plate" method the downward force exerted by a liquid on a vertical plate partly immersed therein is determined. The surface tension of the liquid is calculated from the value of this force and the dimensions of the plate.

Commonly applied methods for determining the surface tension are the "falling drop" method and the "pendant drop" method. The present invention now relates to a process and an apparatus for use in the calculation of the surface tension according to one of the two lastmentioned methods.

In both of these methods a free hanging drop forms the base for the calculation of the surface tension.

In the "pendant drop" method the surface tension is calculated from the values of certain dimensions of the free hanging drop. The values of these dimensions are measured at the moment that the hanging drop is stable and has a rotary symmetrical shape. In this method it is important that the hanging drop is stable at least for the period necessary for observing the drop to measure the values of the particular dimensions.

In the "falling drop" method, also called the "drop volume" method, the value of the volume of the drop as it falls is determined for calculating the surface tension. A requirement in the "falling drop" method is that the drop is formed sufficiently slowly. Experiments have shown that in case the drop is formed too fast, the drop will fall before reaching its maximum volume, resulting in inaccuracies in the calculated values of the surface tension.

Up to now, the drop for use in the "pendant drop" method and the "falling drop" method is formed at the open ended, usually cylindrical, tip of a capillary tube by supplying liquid in a controlled manner at the other end of said tube.

The use of such capillary tube gives good results when determining surface tensions of relatively high values. Problems are, however, encountered when measuring surface tensions of low values, such as values below about 5 mN/m. These problems have two main causes. Firstly, a liquid with a low surface tension can form only small drops. Secondly, upon leaving the bore of the tip of a capillary tube, a liquid having a low surface tension will first spread over the end of the tip before forming a drop. The drop eventually formed adheres to the tip end and not only to the liquid in the bore of the tip. For forming drops of a liquid with a low surface tension, it is therefore not sufficient to apply tips with small bores, but it is necessary that the end surfaces of the tips are small. To obtain small tip ends it is known to apply conically shaped tips.However, it is very difficult and very costly to reduce the tip diameter to such a small value that surface tensions below a value of about 0.5 mN/m can be determined by means of the capillary tube technique.

An object of the invention is to provide a process enabling the determination of surface tensions of very low values (below about 0.5 mN/m) in a very easy manner.

To this end the process according to the invention is characterized in that a drop of the liquid to be tested is formed at the free end of a wire Since the diameter of a wire can be much smaller than the diameter of a capillary tip, the lower limit of the surface tension which can be determined is considerably reduced by the proposed process.

Another object of the invention is to provide an apparatus for use in the process according to the invention. To this end the apparatus according to the invention comprises a vessel, a wire having a free end installed in the interior of the vessel, means for forming a drop of a liquid at the free end of the wire, and means for determining the characteristics of the drop of liquid required for calculating the surface tension of the liquid.

The invention will now be described in more detail with reference to the accompanying drawing showing an embodiment of the invention.

The drawing shows a closed vessel 1, able to withstand high pressures and high temperatures and therefore suitable in determining surface tensions under extreme conditions, such as may occur in subterranean formations. A free hanging metal wire 2 is arranged in the interior of the vessel 1. The wire 2 is secured to the vessel such as by welding. The vessel 1 consists of a lower part 3 and an upper part 4, being secured to each other in a fluid-tight manner by means of flanges 5 and 6 being bolted together. In the side wall of the vessel 1 a window 7 is arranged for observing the interior of the vessel by means of a camera 9. Said camera 9 cooperates with a light source 10 in front of a window 8 arranged in the side wall of the vessel 1 opposite the window 7.

Conduits 11 and 12, provided with valves 13 and 14, respectively, communicate with the interior of the vessel 1, for supplying or removing fluids to or from the interior of the vessel 1.

For determining the surface tension of a liquid in a gas at a predetermined pressure and temperature by means of the apparatus shown in tt drawing the following process is applied.

,ia conduits 11 and valve 13, vessel 1 is filled with the liquid to be tested to such a level that at least the lower end of the wire 2 is wetted by the liquid. The liquid is pressurized by supplying the gas under pressure to the interior of the vessel 1 via conduit 12 and valve 14. For forming a drop of liquid at the lower end of the wire 2, the level of the liquid inside the vessel 1 is subsequently lowered by draining part of the liquid from the vessel 1 via conduit 11 and valve 13 until the liquid level is below the lower end of the wire 2.

Gas is simultaneously supplemented to the vessel 1 for maintaining a predetermined pressure therein. Subsequently the drop that is formed at the end of the win 2 is photographed by the camera 9 and the picture thus obtained is used for determining those characteristic dimensions of the drop which are required for calculating the surface tension of the liquid in the gaseous surrounding.

Instead of a fixedly mounted wire and a variable level of liquid inside the vessel, alternative arrangements may be chosen for wetting the wire to form a pendant drop of liquid.

In an alternative arrangement the level of liquid in the interior of the vessel is maintained at a constant level and the wire is mounted In a vertically displaceable manner. To this end the upper end of the wire is for example secured to a rod extending through an opening in the upper part 4 of the vessel 1. The rod is arranged to be vertically displaceable through said opening. By moving the rod relative to the upper part 4 of the vessel 1, the wire is displaced vertically.

In another alternative arrangement to wet the wire 2, liquid is supplied to the wire in a controlled manner via a capillary tube extending into the interior of the vessel 1. The upper end of the wire 2 may be secured to the capillary tube, for example by suitable clamping means.

The first-mentioned arrangements can only be used for carrying out the "pendant drop" method whereas the last-mentioned arrangement is applicable for carrying out the "pendant drop" method as well as the "falling drop" method.

In the latter method the number of drops falling from the wire 2 are counted by a counter, which is for example connected to a photoelectric cell cooperating with a light source for observing the drops. The volume of the drops falling from the wire 2 may be measured by determining the volume of liquid supplied to the wire 2 via the capillary tube. From said number and said volume of the drops the surface tension of the liquid may be calculated in a manner known per se.

When determining the surface tension on the base of the "pendant drop" method, it may be advantageous to provide the vessel 1 with a plurality of wires 2, having various diameters. By wetting each of the wires, drops of various shapes will be formed. The drop with the most suitable shape for determining the characteristics required for calculating the surface tension may then be chosen for this calculation. The plurality of wires may be secured to a thin rod, which rod in his turn may be secured to a capillary tube such that liquid supplied through the capillary tube will flow along the rod to each of said wires. To determine surface tensions with values of below 0.5 mN/m the diameter of the wires should not exceed' 1 mm.

It will be. appreciated that the invention is not restricted to the application of metal wires.

Smooth thin threads or fibres of material, other than metal, which can be wetted with the liquid to be tested may also advantageously be applied.

The expression "wire" as used in the present specification and claims includes these alternatives.

Further, it is observed that in case the liquid for forming the drop at the end of the wire has a lower density than the fluid surrounding the drop, the free end of the wire should be arranged to point upwards.

Moreover, the invention is not restricted to the arrangement of the wire inside a closed vessel.

For example, when surface tensions of liquids bordered by air at atmospheric conditions are to be calculated, an open vessel may also advantageously be applied.

A further advantage of a wire over a capillary tube is that a drop formed at the end of a wire will come earlier into a thermodynamic equilibrium with its environment than a drop formed at the tip of a capillary tube. It is observed that the correct value of the surface tension can only be determined when the drop is in thermodynamic equilibrium with its environment.

Claims

1. Method for determining the characteristics of a drop of a liquid required for calculating the surface tension of the liquid, wherein the drop of the liquid is formed at the free end of a wire.

2. Method according to claim 1, wherein the wire has a diameter not exceeding 1.0 mm.

3. Apparatus for carrying out the method according to claim 1, comprising a vessel, a wire having a free end installed in the interior of the vessel, means for forming a drop of liquid at the free end of the wire, and means for determining certain characteristics of the drop of the liquid.

4. Apparatus according to claim 3, wherein the wire has a diameter not exceeding 1.0 mm.

5. Apparatus according to claim 3 or 4, wherein the means for forming the drop of liquid at the free end of the wire comprises a capillary tube for supplying the liquid to the wire.

6. Apparatus according to claim 3 or 4, wherein the free end of the wire is vertically displaceable relative to the vessel.

7. Apparatus according to any one of the claims 3-6, wherein a plurality of wires having a free end varying in diameter and each having a free end are installed in the interior of the vessel.

8. Apparatus according to any one of the claims 3-7, wherein the vessel is closed and is provided with means for supplying a fluid to the interior of the vessel.

9. Apparatus according to claim 8, wherein the vessel is provided with means for controlling the level of the fluid in the interior of the vessel.

1 0. Means for determining the characteristics of a drop of a liquid required for calculating the surface tension thereof, substantially as described in the specification with reference to the drawing.

11. Apparatus for determining certain characteristics of a drop of a liquid for calculating the surface tension of the liquid substantially as described in the specification with reference to the drawing.