This section provides an overview for capillaries as well as their applications and principles. Also, please take a look at the list of 12 capillary manufacturers and their company rankings.
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A capillary is a narrow tube in which capillary action occurs. Capillary action refers to the phenomenon in which liquid flowing into a capillary tube moves through the tube due to the surface tension of the liquid and wetting against the inner wall of the capillary tube.
This phenomenon is caused by intermolecular forces between the liquid and the surrounding solid surface (e.g., glass). Specifically, if the diameter of the capillary is small enough, the equilibrium between the surface tension of the liquid and the adhesive force between the liquid and the container wall is disrupted, acting to propel the liquid.
This is also the principle behind the wetting phenomenon of paintbrushes, thin tubes, porous materials such as paper and plaster, and sand, a very familiar natural phenomenon. Other examples: plants, for example, suck up water from their roots and transport it to cells throughout the body, and capillary action is also involved in part in this mechanism.
Capillary action is very practical in our daily lives. Let us consider its basic mechanism from the operation of wiping water off with a kitchen towel.
The force of attraction between similar substances, such as between water molecules, is called cohesion. On the other hand, the force of attraction between substances of different properties, such as between the fine fibers of a kitchen towel and water molecules, is called adhesive force.
If the adhesive force is greater than the cohesive force, the water molecules wet the surface of the kitchen towel fibers and, as a result, are drawn into the space between the fibers. The drawn water molecules are then attracted to each other by the water molecules they come in contact with due to the action of the cohesive force.
As a result, the leading water molecules pull the following water molecules into the spaces between the fibers of the kitchen towel. Because of this difference in cohesive and adhesive forces, water soaks into the kitchen towel at a constant rate.
Thin-layer chromatography is an instrument used in chemistry experiments in which a thin film of silica gel or other stationary phase is applied to a glass or aluminum plate to separate complex compounds. Capillary action is also successfully used in this analytical technique.
Specifically, this analytical technique involves immersing one end of a thin layer of a sample spot in a solvent, which causes the solvent to move from the bottom to the top of the thin layer plate through a gap in the stationary phase.
Capillary electrophoresis is an analytical method for separating trace components by injecting an electrolytic sample solution into a fused silica capillary tube and electrophoresis.
Compared to chromatography such as HPLC or electrophoresis, capillary electrophoresis is an analytical method suitable for detecting components from a small amount of sample because it consists of a capillary, which has a very small volume of separation, and generally requires only about 100 ml of sample.
In this method, the capillary is first immersed in an electrolyte buffer solution with an anode and a cathode connected at each end. The inner wall of the capillary contains silanol groups (-SiOH), which ionize upon contact with the buffer solution and carry a negative charge.
This negative charge attracts positively charged substances from the buffer solution, forming an electric double layer on the inner wall surface. Applying a voltage in this state causes the positive charge in the mobile phase outside the electric double layer to move toward the cathode. This generates a flow of mobile phase called electroosmotic flow.
In capillary electrophoresis, the positively charged material is the first to be detected because it moves quickly toward the cathode. Neutral and negative substances that would not move to the cathode due to their electrical properties alone also move to the cathode side and are detected by the detector because of the electroosmotic flow generated in the capillary.
Generally, capillaries with an inner diameter of 20 to 100 μm are used. The larger the inner diameter, the higher the detection sensitivity, allowing even trace elements to be detected with high sensitivity. On the other hand, a smaller inner diameter improves resolution.
Capillaries (thin and narrow tubes) immersed in water form a meniscus. The curvature of this meniscus becomes larger (i.e., the radius of curvature becomes smaller) the narrower the tube. The curvature causes a pressure difference at the interface between the liquid and gas. A liquid with a sharp contact angle (e.g., water on glass) forms a concave meniscus, so the liquid pressure below the meniscus is less than atmospheric pressure.
Thus, the water in the tube is driven from its initial position by the greater pressure of the water outside the tube (i.e., water at atmospheric pressure below the horizontal air-water interface) to rise through the tube at the same level.
The upward motion stops when the pressure difference between the water inside the tube and the water below the plane outside the tube is counteracted by the hydrostatic pressure exerted by the water column inside the capillary tube.
This is why the water level inside the tube is higher than the water outside the tube, and water molecules are able to absorb around the tube and stay at that water level in contradiction to their weight.
Capillaries are made by heating the center of a glass tube or other long, thin glassware with a gas burner to soften it, then quickly removing it from the flame and stretching it vigorously with both hands.
Immediately after the glass capillary is stretched, it is still hot, so the area around the heated part should be cooled down well. After confirming that the glass capillary has returned to room temperature, use a cutting tool, such as an ampoule cutter, to cut off any unnecessary portions.
Finally, adjust it to a length that is easy to use and store it in an appropriate container to prevent it from breaking. In addition to glass tubes, Pasteur pipettes can be prepared in place of glass tubes. Since cutting with glass and burns may occur, protective equipment should be worn and care should be taken when working with the pipette.
*Including some distributors, etc.
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Selco Products was founded in 1958 and is headquartered in Reno, Nevada. The ISO 9001:2008-certified company is a global manufacturer of customized electronic components. The company serves several markets including medical, HVAC, electronics, and aerospace OEMs and end-user companies, leveraging JIT delivery, Kansan, and engineering support. Electronics offerings include thermistors, temperature controls, panel instruments, and signal converters. Non-electronics include buttons, knobs, and other accessories used in digital and analog electronics controls.
Oshwin Overseas, established in 1999, is a manufacturer and supplier based in Mumbai, India, specializing in high nickel alloy and stainless steel products. The company produces stainless steel seamless pipes and tubes that increase durability due to the absence of weak points, reduce the risk of corrosion, and allow more efficient flows due to smoother internal surfaces. Other products include instrument fittings, instrumentation valves, buttweld fittings, and flanges. These products are utilized in diverse industries such as medical, structural engineering, oil and gas, marine, and aerospace. Oshwin Overseas holds ISO 45001:2018 and ISO 14001:2015 certifications.
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