Capillary Tube: Uses, Working Principle, Size & Applications

You’ve probably seen them around but never knew what they were called. Those little glass tubes that always seem to come with things like thermometers and barometers are capillary tubes, and they play a bigger role than you might think. Coming in all shapes and sizes, capillary tubes may be tiny but they have some huge applications across many fields, from analytical chemistry to HVAC systems.

In this article, we’ll break down everything you need to know about capillary tubes. You’ll learn how they work, why they’re designed the way they are, and the many ways they make our lives easier even though we barely notice them. Who knew such small glass tubes could do so much? Read on to have your mind blown about the power of capillary action.

What Is a Capillary Tube?

Capillary tubes are very thin tubes composed of a rigid material, such as plastic or glass, that allow liquid to flow up into them against gravity in a process known as capillary action.

A capillary tube is a narrow tube with a very small internal diameter, typically 1mm or less. They’re useful for moving liquids through small spaces. How do they work? Capillary action. The surface tension of the liquid and the adhesion between the liquid and tube create an upward force that counteracts gravity.

How Does a Capillary Tube Work?

Capillary tubes take advantage of capillary action, where liquid moves through narrow spaces. As the liquid molecules stick to the capillary tube walls, surface tension pulls the molecules up the tube. The narrower the tube, the higher the liquid will rise.

This works because water molecules are attracted to each other (cohesion) and to the walls of the capillary tube (adhesion). Since the tube is narrow, the surface tension effect is large compared to gravity and is able to lift the water. The height the water reaches depends on the tube radius and the liquid’s surface tension and density.

Capillary action is what allows plants to draw water from their roots up to their leaves and is used in many applications like chromatography, heat pipes, lab equipment, and more. Understanding capillary flow is key to manipulating fluids at a small scale.

Why is it called a Capillary Tube?

Ever wonder why these narrow tubes are called capillary tubes? It’s because they have an internal diameter similar in size to human capillaries, the tiny blood vessels that connect your arteries and veins. Capillary tubes rely on capillary action, where liquid flows through narrow spaces without the assistance of any external forces like gravity.

The small size of capillary tubes, usually 0.5 to 2 millimeters, allows liquids to flow upwards against gravity due to the adhesive forces between the liquid and the tube material. As the liquid flows up, the surface tension forms a meniscus, curving the liquid surface. The narrower the tube, the higher the liquid will rise.

While capillary tubes are simple in design, they have many useful applications like transferring liquids between containers, siphoning, and priming pumps. Their small size and ability to draw liquid upwards make them ideal for precision fluid control in medical, engineering and scientific equipment.

Types of Capillary Tube

One type of capillary tube is an internal restricted tube. It has an internal restriction that reduces the flow area, which increases pressure drop and reduces refrigerant flow.

Fixed-length capillary tubes have a set length and diameter designed for a specific refrigeration system. They provide a constant rate of flow regardless of pressure conditions.

Pulsating capillary tubes use an accumulator to create pressure pulses. An on/off solenoid valve is opened and closed rapidly, sending pressure pulses through the tube. This improves refrigerant flow.

Thermostatic expansion valve (TXV) capillary tubes use a temperature-sensitive element to control refrigerant flow based on demand. They provide good temperature control and efficiency.

Adjustable capillary tubes have an adjustable element that can control refrigerant flow. A needle valve is used to manually adjust the size of the opening. This allows you to control the flow and optimize the system.

Capillary tubes come in a variety of sizes and materials for different applications. The size and length depends on factors like desired flow rate, pressure drop, and system capacity. Common materials are copper, aluminum, and stainless steel.

Applications and Uses of Capillary Tubes

These are some of the uses and applications of Capillary Tube:

Capillary tubes are used in Thermometers

A capillary tube has a very small internal diameter, allowing for capillary action to draw a liquid up and into the tube. This property is used in thermometers, where a temperature-sensitive liquid will rise in the capillary tube as the temperature increases.

They are used in barometers

Capillary tubes are also used in barometers to measure atmospheric pressure. As the pressure changes, it causes the level of liquid in the capillary tube to rise or fall. By measuring the height of the liquid column, the pressure can be determined.

Used in medical tests

Capillary tubes are used for collecting blood samples for medical tests like blood glucose monitoring or hemoglobin measurement. The small size of the tube allows for collection of just a drop or two of blood.

Chromatography

Capillary tubes are used in some chromatography experiments and applications. The small diameter provides a large surface area to volume ratio, which is useful for chromatographic separations.

Gas chromatography

In gas chromatography, a capillary column is used to separate the components of a gas mixture. The small diameter provides a large surface area, which helps separate molecules based on their interaction with the column coating.

High performance liquid chromatography

Capillary tubes are also used in high performance liquid chromatography (HPLC) as the separation column. The small size allows for high pressure and efficient separations.

Microfluidics

Capillary tubes are used in microfluidic devices to handle and manipulate very small volumes of fluids. Their small size is ideal for microfluidic applications.

Capillary electrophoresis

Capillary electrophoresis uses narrow-bore capillary tubes to separate molecules based on their electrophoretic mobility. The capillary tube acts as the separation channel in these systems.

Analytical Chemistry

Capillary tubes are commonly used in analytical chemistry for applications like gas chromatography, liquid chromatography, and electrophoresis. Their narrow bore provides high surface area to volume ratio, increasing the sensitivity of analysis.

Microfluidics:

The small size of capillary tubes allows precise control of small volumes of fluids. They are integral components of microfluidic devices used in biotechnology and bioengineering applications.

Fiber Optics

Capillary tubes with an internal reflective coating are used as optical fibers to transmit light signals over long distances. Their small diameter allows light to be guided through total internal reflection.

Inkjet Printers

The nozzles in inkjet printers contain capillary tubes to draw ink from a reservoir and spray it onto the paper with precision. The capillary effect and surface tension control the flow of ink through the nozzles.

Heat Pipes

Capillary tubes are used as wicks in heat pipes to transport heat from an evaporator to a condenser section through a working fluid. The wick draws liquid back to the evaporator through capillary action.

As you can see, capillary tubes find applications in various fields due to their ability to control flow rates precisely and effectively transfer or filter liquids. Their simple structure and low cost make them suitable for many industrial uses.

Capillary Tube Sizes and Dimensions

Capillary tubes come in a variety of sizes to suit different applications. The most common sizes are 0.5mm, 0.8mm, 1.0mm and 1.5mm in diameter. These tiny tubes have an inside diameter of just 0.25mm to 1.2mm.

The size used depends on the amount of fluid flow needed and the purpose of the capillary tube in the system. For example, a 0.5mm tube would be suitable for a small medical device, while a 1.5mm tube may be needed in an AC system to allow for the proper flow rate of refrigerant.

In some cases, multiple capillary tubes of different sizes are used together in a single system to achieve the desired pressure drop and fluid flow for optimal performance. The dimensions and sizing of these miniature components is carefully calculated by engineers to precisely control the flow of liquids and gases.

What Happens When a Capillary Tube is Placed in Water?

When a capillary tube is immersed in water, several events happen due to capillary action. The narrow bore of the tube causes the water molecules to adhere to the tube walls and each other, creating surface tension. This pulls the water upwards inside the tube. At the same time, adhesion between the water and tube creates a meniscus, curving the water surface. The strength of these forces depends on the tube’s material and diameter.

In a glass tube, the water will rise to a certain height due to strong adhesion. In plastic, the rise is less due to weaker adhesion. The smaller the tube, the higher the water climbs due to increased surface tension effects. This capillary action continues until the weight of the water column equals the surface tension forces, at which point equilibrium is reached. Removing the tube from the water causes the column to break at the top meniscus.

The capillary action allows the tube to spontaneously absorb and transport the water despite gravity. This mechanism drives many natural and technological processes, including the transport of water and nutrients in plants and the operation of many medical diagnostics tests like pregnancy tests.

Why Does Water Rise in a Capillary Tube but Mercury Falls in the Same Tube?

The key reason water rises in a capillary tube is due to adhesion and surface tension. Water molecules stick to the glass surface (adhesion), and also to each other (surface tension), creating an upward pull. The narrow tube magnifies this effect, raising the water.

On the other hand, mercury does not adhere or bond to the glass surface. Its surface tension pulls molecules together, but since it lacks adhesion to the glass, this results in a net downward pull in the narrow tube. The mercury is also much denser than water, so gravity has a stronger effect, causing it to sink.

Though the tube is the same, the properties of the liquids lead to opposite behaviors. Adhesion and surface tension overcome gravity for water, while gravity and lack of adhesion win out for mercury. The tube simply amplifies the forces already present in the liquids.

What is the Formula for the Rise of a Capillary Tube?

The capillary rise formula expresses the height to which a liquid will rise in a narrow tube. It depends on several factors:

Surface tension refers to the cohesive forces between molecules in a liquid. The higher the surface tension, the higher the capillary rise. Viscosity measures a liquid’s resistance to flow. More viscous liquids experience less capillary action. The radius of the tube also matters, the narrower the tube, the higher the rise.

Formula:

h = 2 γ cos θ ρ g R

Where:

h = Height of liquid rise (capillary rise)

γ = Surface tension of the liquid

ρ = Density of the liquid

g = Acceleration due to gravity

r = Radius of the capillary tube

θ = The angle of contact

The formula shows that as the tube radius decreases, the capillary rise increases. And as surface tension increases and density and viscosity decrease, the height also goes up. Understanding the factors that drive capillary action helps in choosing the right tubes and liquids for your needs.

What Happens If a Capillary Tube is Too Short?

If a capillary tube is too short for its intended purpose, it won’t be able to produce the necessary pressure drop required for flow control or fluid transportation. The capillary action relies on the adhesion between the liquid molecules and the tube wall to overcome gravity and move the liquid upwards. If the tube is too short, there won’t be enough surface area for the adhesive forces to act upon the liquid, so the meniscus won’t rise high enough.

In refrigeration systems, a capillary tube that’s too short won’t be able to properly expand and decompress the refrigerant. This can reduce the cooling capacity and efficiency of the unit. The capillary tube has to be sized correctly based on factors like the type of refrigerant, desired pressure drop, and required refrigeration effect. An improperly sized capillary tube is one of the most common causes of poor performance or failure in small refrigeration systems.

For the transportation of liquids, a capillary tube that’s too short won’t be able to lift the liquid to the necessary height due to insufficient capillary action. The liquid won’t rise high enough in the tube and won’t reach its intended destination or application. The height that a liquid will rise in a capillary tube depends on the surface tension of the liquid and the adhesion between the liquid and tube wall. If the tube is too short, these forces won’t be enough to overcome gravity and lift the liquid.

What are the Limitations of Capillary Tube?

Capillary tubes, while useful for certain applications, have some downsides to consider:

1. Limited flow rate – The narrow bore of a capillary tube restricts the amount of fluid that can pass through it. This limits its usefulness for high volume applications.
2. Prone to clogging – The small diameter makes capillary tubes susceptible to clogging from contaminants or debris in the fluid. Regular cleaning and filtration is required to prevent blockages.
3. Pressure drop – Fluid moving through the narrow capillary tube experiences a drop in pressure. This can be problematic for systems that require consistent high pressure.
4. Requires precision – The capillary effect that allows these tubes to function requires a precise internal diameter and perfectly round shape. Any imperfections can disrupt the capillary action.
5. Limited material options – Only certain materials like glass, steel and plastics can be fabricated into capillary tubes with the necessary dimensional precision. This limits choices for chemical compatibility.
6. Fragile – Because of their narrow diameter, capillary tubes made of glass and certain plastics can be fragile and prone to breaking. Care must be taken in their handling and installation.
7. Prone to contamination – The high surface area to volume ratio of capillary tubes means chemicals and contaminants have more opportunities to interact with the tube walls. This can be an issue for applications requiring high purity.

In summary, while capillary tubes have important uses where their unique properties are required, their limitations mean they are not suitable as a one-size-fits-all solution. The specifics of your application should be carefully considered to determine if a capillary tube is the right choice.

Capillary Tube FAQs

These are some of the frequently asked questions about Capillary Tube:

Do capillary tubes come in different sizes?

Yes, capillary tubes come in a range of sizes to suit different applications. The size is specified by the internal diameter, usually ranging from 0.1mm up to 5mm. The most common sizes are 0.5mm, 1mm and 2mm. The size used depends on factors like the viscosity of the fluid and flow rate required.

What materials are used to make capillary tubes?

Capillary tubes are made from materials like glass, stainless steel, copper, and plastics such as PTFE or PEEK. The material chosen depends on factors such as chemical compatibility, temperature, and flexibilty. Glass and stainless steel are popular choices as they have a high chemical resistance. Plastics may be used for corrosive fluids.

How does a capillary tube work?

A capillary tube works by using capillary action to passively draw a liquid through the tube. The small internal diameter of the tube results in strong surface tension and adhesion of the liquid to the tube walls. This causes the liquid to creep up the tube, even against gravity. The flow rate through a capillary tube depends on the viscosity and surface tension of the liquid, and the length and diameter of the tube.

What are common applications of capillary tubes?

Capillary tubes have many applications, including:

• Sample injection in gas chromatography
• Filtration and separation
• Microfluidic devices
• Inkjet printer nozzles
• Medical testing equipment
• Thermostats

Conclusion

Capillary tubes may seem small and insignificant, but they play a huge role in our everyday lives. From helping plants take up water to being an essential component in AC and refrigeration systems, these tiny tubes make modern life possible.