In the world of materials, there are different ways of providing new functionalities to a certain piece or object. The clearest case is to change the material to one that better suits our needs, but it also usually implies added complexity to the production process.
If what we want is to modify the surface properties of an object, another option is to cover said object with a thin layer of another material, or what is the same, apply a coating. With coatings we can continue to use the same base material and only a small amount of a second material is needed to provide the desired properties to the object. Despite this, it is necessary to introduce a stage in the production line in which said coating is applied, which is usually easier than changing the base material, but it still requires good planning and investment to implement it.
If we take this into account, and we want to modify the surface properties of the object, the ideal would be to be able to use the same base material and not have to add any extra processes to the production line, but is this really possible? The answer is yes, and it is based on the microstructuring of surfaces.
Surface modification by coating
Surface modification by microstructuring
What is microstructuring of surfaces?
Microstructuring of surfaces is a process by which the roughness and microscopic geometry of an object’s surface are modified to provide it with properties that it did not previously possess. The clearest example is the modification of friction, which clearly varies between a smooth and a rough surface.
With this definition, microstructuring does not seem so different from traditional surface modification processes such as chemical treatments or sanding. However, the end result is totally different. Microstructuring creates periodic patterns on the surface that completely change the properties of the material, being able to achieve with the appropriate microstructure that the surface repels water, prevents wear, or that stains do not adhere.
Microstructuring inspired by nature
Like many technological advances, microstructuring has found its inspiration in nature. Here are some examples of natural microstructures:
- The skin of certain lizards, such as the Gecko, has a microstructure that repels water and does not allow sand to adhere, which helps them in desert conditions.
- The wings of some insects also repel water thanks to their microstructure, which allows them to continue flying in conditions of high humidity, as well as in areas where water is abundant, without interfering with their flight.
- Lotus leaves have a microstructure that repels water, which makes them clean themselves of any type of dirt that may fall on them, since the water drags it off their surface.
- The shark skin, seen on a microscopic scale, has small scales that repel water very well, which helps the shark to swim faster, as the water presents less opposition to its advance.
Surface microstructuring technologies
Although we have verified that microstructures provide very useful properties in nature, how do we manage to artificially replicate them? Advances in technology in recent decades have provided us with various techniques by which it is possible to carry out microscopic-scale machining on the surface of materials:
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Injection molding or In Mold Microstructuring (IMM) for surface microstructuring
It allows all the injected parts to obtain a structure or functionality that has previously been given to the interior of the mold. To achieve this effect, the internal faces of the mold are structured with a previously designed pattern to provide the polymer with new functionality (hydrophobicity, oleophobicity, etc.) or modify a specific property (roughness, color, etc.).
This microstructure or pattern is transferred to the part during the injection process. The great advantage of this technique is that it is only necessary to apply the microstructure once, so it is economically one of the most interesting options for industrialization. One of the most important aspects of this technique is the transfer of the pattern from the mold to the part, for which it is necessary to adjust the injection parameters correctly. It is the preferred option for plastic parts. At ATRIA we have extensive experience carrying out IMM projects, providing surfaces with a multitude of new properties
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Roll to roll (R2R) for microstructuring of surfaces
It is applied in the case of having the material in coils. It can be used both with polymer films and with steel and aluminum and metal coils. In the Roll-to-Roll technique, the previously microstructured rollers through which the material is passed, transfer the microstructure thanks to the pressure (and sometimes the help of temperature) that is exerted on the material coil. It is a process that must be optimized very well for the transfer to be correct, but once implemented it does not require consumables. At ATRIA we have experience with this type of technology, carrying out various microstructuring projects using R2R.
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Surface microstructuring laser
This type of microstructuring would resemble the use of a chisel or a milling machine if we were talking about macroscopic surface modification. In the case of surface modification on a microscopic scale, one of the most widely used techniques is laser microstructuring, where chisels and burs are replaced by a beam of light concentrated in a circle of a few microns in diameter that moves along the surface. In this way, it is possible to reproduce the desired microstructure. It can be used on many materials, although its main disadvantage is usually the cycle time. This is the most commonly used technique for preparing IMM and R2R molds and rollers.
Applications and examples of surface microstructuring carried out
As we have mentioned, microstructuring the surface of a material can give it various properties. Here we show you some of these properties and their possible applications:
HYDROFOBICITY
HYDROFILICITY
SELF-CLEANING
It is the ability to repel water. With this we can keep the areas we want of our devices dry, such as touch controls. At ATRIA we have worked on various projects to provide hydrophobicity to different surfaces.
It is the ability to attract water. Any drop of water that falls on the surface expands and creates a film of water. This property is very useful in optical elements, where the presence of small drops distorts the image, but water films usually allow good visibility. At ATRIA we have worked a lot with hydrophilicity and we know its importance
It is the ability to cleanse itself. There are microstructures that do not allow dirt to adhere to its surface, causing it to slide on its own, or to be carried away by air or water.
EASY-TO-CLEAN
ANTI-BIOFILM
WATER BARRIER
It is the ability to facilitate cleaning of the surface. It is similar to the property of self cleaning, but in this case an external force is necessary to remove the dirt from the surface.
It is the ability to prevent bacterial and biofilm growth on the surface. There are certain geometries of microstructures that are similar in size to bacteria and are capable of killing them, a highly desirable aspect in medical, food or hygiene applications.
It is the ability to not let water pass through a certain area of the surface. With the appropriate microstructure, it is possible to create areas where water can flow better than others. With this, it is possible to protect, for example, areas that must always remain dry on touch devices. At ATRIA we have developed projects with water barriers that have yielded great results.
CONDENSATION AND DRYING
ANTI-WEAR
DIFFUSIVITY
TOUCH TEXTURES
It is the ability to prevent the condensation of water droplets to keep the surface dry. This is achieved through microstructures that hinder the nucleation of the water droplets and slow down their growth.
It is the ability to reduce the wear suffered by the surface. By modifying the general roughness of the part, or by making small indentations in the surface, it is possible to reduce friction and create deposits where lubricant reserves remain that help the useful life of the elements is longer.
It is the ability to regulate the amount of light that passes through a surface. In transparent materials, by applying microstructures we can regulate the degree and type of transparency of the material, being able to go from surfaces that allow the material to be completely transparent, to surfaces that completely opaque it.
It is the ability to recreate the desired texture on the surface. With this, you can make the surface soft and fluffy, or hard and fluffy, going through endless textures.
Materials on which surface microstructuring can be applied
The microstructuring of the surface can be applied to practically any type of material, although there are some with a greater industrial interest than others. Next, we show you those in which the implementation of microstructures has a more mature degree of industrial development:
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Metals
They can be microstructured directly using laser, or indirectly using techniques such as Roll to Roll (R2R). In the case of laser microstructuring, its industrial implementation has long cycle times, although with the advances of recent years this is changing rapidly, obtaining techniques with which it is possible to cover larger areas at the same time. In the case of Roll to Roll, it is necessary to treat the metals in sheet format, to be able to transfer the microstructure through the pressure exerted by the rollers.
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Polymers
They can be microstructured directly by laser, but the most widely used method is injection molding. The direct interaction of the laser contributes too much energy that tends to degrade part of the polymer, so it is not recommended in many cases. However, injection molding manages to provide the microstructure to the parts without any adverse effect on the material, since it follows its normal production process, penetrating the microstructure of the mold when the plastic is in the molten state and totally copying the microstructure.
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Films
They are mainly microstructured by Roll to Roll, since due to their manufacturing method it is not feasible to transfer the microstructure to them by molding, and since they are usually plastic, they suffer the disadvantages of direct laser interaction. In the case of films, the transfer of the microstructure through the rollers is easier than with metals, as lower pressures are required.
What do you think of our blog on surface microstructuring? Do you need to improve your material to give it self cleaning or anti bacteria properties? Do you want to find a new material that repels water? Contact us!
