A hydrophilic coating increases the wetting on a surface. In applications like µ-fluidics or anti-fog applications, it is often desirable to a have a uniform sheet of liquid. In applications in which you want to promote cleanliness or for protection against water ingress, a hydrophobic surface property which repels the liquid is desirable.
When liquid interacts with a surface, it forms a “bead” with the surface. The measurement at the interface between the liquid and the surface is called the Contact Angle. A hydrophobic contact angle is typically 90° or greater and the liquid “balls-up”. A super-hydrophobic coating has a water contact angle of 150° or greater and a sliding angle of less than 10°. Most of the time, the liquid would bounce off the surface like water off a duck.
The surface roughness is extremely important in that it amplifies any hydrophilic or hydrophobic properties. A hydrophobic surface with surface roughness will become super-hydrophobic and vise-versa, a hydrophilic coating when deposited over a rough surface will become super-hydrophilic. Note that surface roughness can also distort visual effects like haze (like a matte finish) versus smooth (like a polished finish).
Durability has and always will be a key metric for super-hydrophobic coatings. Some degree of surface roughness is required to drive the hydrophobic property to become super-hydrophobic. However, this nano-scale roughness (typically >30nm rms) can be delicate or fragile. The mechanical durability of the rough nano-structure to abrasion is the key metric. This is usually quantified by mechanical Taber (abrasion/rubbing) testing. Note that super-hydrophobic coatings generally perform very well to liquid / non-physical contact applications.
The most effective way to remove a nano-coating is through the use of an oxidizing medium like a plasma. There are other methods including UV irradiation, ozone or caustic chemical baths.
IST’s signature deposition technique is using a sub-atmospheric reactor. There are both advantages and disadvantages to the sub-atmospheric coating chamber. Advantages include complete control of the environment (ie. temperature and moisture). This means repeatability. At low pressures, the molecular mean-free path is very long which allows for the coverage into virtually any nook and cranny, down to the atomic scale. There is no chemical waste stream of organic solvents or other chemical wastes. Scale up is also possible, for business sectors, like the semiconductor IC business. The disadvantages, may be the initial capital cost of the equipment.
Most nano-coatings are typically 1-2nm in thickness. To visually discern any coating, the thickness needs to be around a quarter (¼λ) wavelength of light which essentially means that thicknesses under 100nm are invisible.
At IST, we do numerous types of coatings for various applications. The chemicals used by 100% RoHs compliant. We have coatings which are also 100% REACH compliant.
IST has tested most of our coatings per the FDA ISO:10993 Biocompatibility guidelines for Surface Devices. This includes Cytotoxicity, Sensitization and Irritation/Intracutaneous effects.
Nano-coatings are intrinsically non-conductive. However, at the thicknesses being deposited of several nanometers, they have no effect on the electrical conductivity. Another way to view this, any nanometer film <15nm can be viewed as being non-contiguous so electrical conduction is not impeded.
Yes. The nano-coating can be stripped using a plasma and then reapplied. Also, there are many films which can be classified as SAM’s (Self Assembling Monolayers). These SAM films are self-limiting so you cannot over deposit these coating as well.
The coating should last the lifetime of your device or coated substrate. Our coatings which are chemically inert like most hydrophobic films will last the lifetime of the device, unless intentionally destroyed by a plasma or UV or ozone exposure.
IST’s nano-coating are completely safe for the environment. Most of the time, all reaction by-products are also destroyed by a plasma cleaning. With the IST sub-atmospheric process, there is no wet chemical waste stream of solvents.
The EU has a mandatory phaseout of PFOA’s (Perfluorooctanoic acid) for most manufacturing applications to prevent the accumulation of “Teflon like” byproducts in the environment. While there are many PFOA chemicals which have desirable properties (ie. contact angle), IST has alternative non-PFOA coatings which meet or exceed the user’s specifications. Many of the chemicals used are TSCA approved.
Depending on the nano-coating, there is a measurable surface energy associated with the coating. This characteristic energy is measured using a Goniometer. Generally, the film is wetted with a small droplet of a known liquid and how this liquid interacts with the surface (i.e., the contact angle) is measured. As an example, if the contact angle for a water droplet measures 108°, then one would conclude the hydrophobic coating is still present. Reactive films like epoxies or amine’s or HMDS have characteristic angles like 63°, 70° etc. so a good indication of the coating can be derived.
Yes. Most nano-coating are only several molecules thick. A greasy fingerprint will be orders of magnitude thicker than most targeted coatings. Cleaning of the samples prior to nano-coatings is very important. Most of the time, a solution rinse is used ranging from a piranha acid bath or alcohol depending on the articles. Immediately prior to any nano-coating, a plasma clean pretreatment is usually performed.
Yes and No. Selective patterning of a nano-coating can be performed and dependent on the coating to be applied and the article to be coated. This is often referred to as Selective Deposition. There are many other methods as well. In a process called “Lift-Off”, a mask is applied (eg. resist, tape) to selective areas and following the coating the mask is removed along with any coating deposited on the masking material. However, since the nano-coating process is sub-atmospheric, a mask with intimate contact with the substrate is needed.
Due to the extreme thinness of the coating, metrology is specialized. A goniometer is used to measure the surface energy. An ellipsometer is often used to measure the thickness of the film but a known reference material is required. For example, a piece of polished silicon will be deposited adjacent to the real articles as a witness and then measured. An AFM (Atomic Force Microscope) can measure the height and density of the nanocoating. XPS (Photo-electron spectroscopy) can measure the chemical surface elements and concentration. As a last resort, IST has collaborated with NIST to use their synchrotron-based near edge x-ray absorption fine structure (NEXAFS) spectroscopy tool to measure the surface orientation of the deposited molecules. (Contact IST for more discussions on NEXAFS.)
Chemical resistance depends on number of factors. One class of hydrophobic coatings are perfluorinated which is like a “Teflon”. These files are very chemically inert to organic solvents, alcohols and general household cleaners. These are also hydrophobic in nature which can provide water resistance.
Once a substrate is hydrophobic nano-coated, soldering can be troublesome since the flux may not wet the surfaces to be joined. With some practice, the nano-coating can be disrupted, and normal soldering can be performed.
Repellix is IST patented nano-composite film which is an aluminum oxide super-hydrophobic coating. Repellix offers superior protection against liquids and is oleophobic against substances like Cerumen (ear wax).
Please contact us at IST. We have years of experience and will be glad to investigate collaboration with you.
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