The Si will become hydrophobic if you strip the native oxide in HF and leave the surface F-terminated. Oxide will make the Si hydrophillic. You can also create a hydrophobic surface by depositing a thin FC layer by e.g. a CF4 plasma. If the surface is roughened e.g. by a CF4/O2 plasma with ion bombardment (RIE), then deposit the FC layer you can create a super-hydrophobic surface (contact angle >170 deg).

Low power sputtering can help you as films deposited by sputtering are nonhomogeneous and their thickness decrease from centre position to edge of wafer. Use bigger size wafer.

One option would be to use a UV/Ozone treatment which would remove any organic contaminants, as well as activate the surface of your substrates, therefore, changing the contact angle and wettability.

Hi Dmitry,

If you want to reach high contact angle (around 160) you may use silver nitrate (see Larmour et al.). I tried this method several times and it works very well. However, I have never tried it at high temperature but I think it should be OK.

Otherwise, if you use a soft textured elastic substrate, you can change the contact angle by changing the stretching of the substrate (see Coux et al.). I have no experienced with this technique but it sounds quite useful. I don't know if you can use it or not and if it supports heating...

I hope it will help,

All the best

Dear Albert Queraltó,

Thanks for your reply.

How stable is the contact angle after such treatment if the substrate is open to the atmosphere?

Dear Dmitry,

As far as I know, the wettability of the specimens is controlled by two approaches, which are basically related to the morphology and chemical bonding of them. For the time being, my suggestion is controlling the morphology of your case. The main idea behind the experiments related to the morphological variations is found to be surface roughness. As a matter of fact, upon increasing the surface roughness the wettability of samples is strongly variable.

In this regard, I profoundly suggest you to apply "glancing angle deposition" (GLAD) technique. Actually, in this technique different deposition angles of incident flux with respect to the source normal are applied to change the morphology. As a result, using this technique, the structural properties especially the surface roughness are alterable. And finally, I am confident that this pervasive technique can be impressively efficient in the direction of decreasing the wettability and in turn increment of the hydrophobicity of your desired sample.

With best wishes,

Naser

Hi Dmitry,

have you considered the possibility to apply Electrowetting to your systems? I am sure a thery thin dielectric coating can be safely applied to your substrates ( if not already present, e.g. on Si) and then , by imposing an ad-hoc electrical potential the contact angle can sometimes be made to vary from high wetting to nearly complete non-wetting. May I suggest you to have a look to specialized papers. This is one:

Journal of Physics: Condensed Matter, Volume 17 (2005) Number 28

Best wishes!

Alberto Passerone

Hi Dmitry,

just a few minutes ago I have seen the following paper:

Inorganic Surface Coating with Fast Wetting–Dewetting Transitions for Liquid Manipulations

Yajie Yang, Jianlin Zhao, et al.

DOI:10.1021/acsami.8b02537

appeared on

ACS Appl. Mater. & Interf. Vol 10, n.22 (2018) on line

Could it be useful to you?

Regards,

Alberto

The Si will become hydrophobic if you strip the native oxide in HF and leave the surface F-terminated. Oxide will make the Si hydrophillic. You can also create a hydrophobic surface by depositing a thin FC layer by e.g. a CF4 plasma. If the surface is roughened e.g. by a CF4/O2 plasma with ion bombardment (RIE), then deposit the FC layer you can create a super-hydrophobic surface (contact angle >170 deg).

You can use self-assembled monolayers to make the surface hydrophobic. By controlling carbon-chain length, you can control contact angle. Normally, the longer the chain is, the more hydrophobic. For contact angle less than 90C. You may try some high-carbon alcohol, such as 1-octanol.

Try the nano copper and nano silicon.

Dear All,

Thank you very much for your valuable answers.

As I can see, most existing solutions are to make the surface hydrophobic.

Are there any methods to make the surface hydrophilic, with the (advancing) contact angle around ~10° for water?

If you can apply a sublayer to the substrates, then you need to apply the material with the required surface tension. Say, apply bismuth, it has a small surface tension or smeared with indium gallium eutectic, it has a large surface tension. These metals easily work up to 130 degrees and are easily applied to your substrates even by rubbing.

Попробуйте ультразвук

To get a hydrophilic surface you can use a plasma treatment or you can use commercially available anti-fog products.

Станислав, ультразвук?

To dial it in use 2 coatings. 1 coating giving you the lowest contact angle and the other coating the highest value. Blend them as need to dial it in. To get to 10, you will probably need to add corona treatment as well.

Hi Dmitry,

I quickly went through all the answers and found them very valuable. I also have suggestions for you.

If you want to make your silicon substrate superhydrophobic, the easiest method that I have seen is to use a gold or silver coating and then immersion into a HF:H2O2:H2O (1:1:1) solution for 60s. Check my paper here . With this method you do not need any other fluoro-coating on your nanostructures. If you use evaporation for your gold or metal, remember to use an adhesion Ti layer (2nm). You can also use inks with silver nanoparticles. It does the same.

If you wan to make your silicon surface superhydrophilic, high temperature annealing to 350C will turn your previous surface superhydrophilic. It means by one sample you can choose which way to go. This process is reversible meaning that, after the thermal annealing, you can recover the sample to superhydrophobic just simply by immersion of the sample into the same solution HF:H2O2:H2O (1:1:1).

Article Robust Superhydrophobic Silicon without a Low Surface-Energy...

Article micromachines Non-Lithographic Silicon Micromachining Using ...

Surface properties can be changed by following methods

1) Plasma treatment with O2, N2 or Ar gas

2) Removal of native oxide on Si substrates by buffered HF-DIP

3) Ozone treatment - UV exposure.

4) Surface treatment with Silanization solution.

Article Silane modification of glass and silica surfaces to obtain e...

https://www.researchgate.net/post/What_is_the_most_effective_cleaning_activation_method_of_the_glass_slide_for_silanization

All the best!

Dear Dmitry,

A way of treating substrates can be using plasma treatment. The following paper from our group exhibits a gradient in the surface energy of the substrate resulting from varied amounts of exposure of each region of the substrate to the plasma as a consequence of inclined orientation of the sample to be treated. May be it may help answering your question.

All The Best!

Be cautious with using synthetic coatings as they degrade over time or with prolonged use. Same goes for plasma/UV treatments - they are not permanent.

I have found that copper(II) oxide (CuO) is a way to make a copper surface hydrophilic. Moreover, higher roughness corresponded to lower water contact angle. CuO can be prepared on copper from many routes such as heating in air, solution-based, electrochemical, hydrothermal, and solid-state methods. I do not have the same experience with silicon, but I assume the concept is similar.

@ James:

For silicon the roughness of the substrate depends on the kind of etchant used in the cleaning or etching process. KOH and IPA are the most prefered alkaline solutions. Where IPA acts as the wetting agent.

Dear All,

Thank you so much for your answers.

Most of your recommendations are to change morphology or chemistry of the surface.

And how about application of coatings? For example, spin coatings or

synthesis of nano-coatings (forest of carbon nano-tubes, graphene, ...)?

If you want to apply a coating on your surface, again you should consider both roughness and chemistry (hydrophilic or hydrophobic) to obtain those extreme wetting or non-wetting states (superhydrophilic or superhydrophobic). The issue with most of these coating is that they are not durable enough. There are many examples in literature:

Article Highly transparent superhydrophobic organic–inorganic nanoco...

Article Hydrophobic and super-hydrophobic coatings based on nanopart...

http://iopscience.iop.org/article/10.1088/1748-3190/aaa1c1/meta

https://www.nature.com/articles/s41598-018-21051-3#author-information

One common way to modify the wettability of substrates is to apply a thin layer of sol-gel coating via sparying or dip-coating. Depending on the composition of the coating you can achieve a wide range of hydrophobic or hydrophilic surfaces. Presence of alkyl groups can increase the hydrophobicity of the coating depending on their chain size and their populating in the coating. There are various sources available online on sol-gel coatings but I would suggest a non-hydrolytic sol-gel route with a combination of both tetra-alkoxysiloxane and alkyltri-alkoxysilanes . This combination gels relatively fast and gives you a more flexible coating that can withstand heating process with zero to minimal cracking.

In the case of dewetting during coating process, appropriate surfactant can be used.

I suggest electric discharge treatments

On silicon substrate, you can try surface treatments with silane compounds (e.g. DMOAP, different exposures will give you different wettabilities). See this (https://www.nature.com/articles/nmat4348) as a referrence (that was on colloidal particles, but the principle is the same).

Dear Dmitry,

In response to your above question, I recommend you to use physical vapor deposition (PVD) technique. Actually, according to my previous answer that was applying GLAD technique, more uniform coatings are achievable by this technique.

With Regards,

Naser

Dear All,

Thanks a lot for this very useful information.

Depending upon the nature and chemistry of surface there are two main considerations to make it hydrophobic 1.Hierarchical nanoroughness 2. Lower surface energy.

Nanoroghness can be achieved by plasma treatment or nanoparticles adhesion.

Lower Surface energy can b obtained by chemical treatment like long chain aliphatic hydrocarbons/alkoxysilanes or coating with some lower surface energy materials like polyurethane and Teflon etc.

Hydrophilic surface can be made by plasma trearment, UV trearment, Enzymatic treatment, acid and NaOH treatment that might be responsible for alteration of surface chemistry by bond breakage and formation of new hydrophilic groups.

Dear Shagufta Riaz,

Thank you for your reply.

Most of hydrophobic surfaces are known to be quite stable and durable. However, this is not the case for typical hydrophilic surfaces.

What method of treatment can you reccomend that would provide most stable and durable hydrophilic surfaces?

Hi,

We have investigated such area but on an aluminium surface/ substrate.

changing the liquid pH, liquid temperature, surface temperature, and surface coating can all effect the surface wettability nature.

check our article, which I think it will be useful.

Article The Effect of Aluminium Nanocoating and Water pH Value on Th...

Regards,

Naser

We have investigated metal-plating substrate for varying wettability. Gold plating make wettability better.

Dear Naser Ali,

Thank you for your reply. Your paper seems very interesting. Could you pls provide a pdf?

Dear Dmitry,

Kindly find attached a copy of the paper for your information.

Best regards,

Naser

You can get the hydrophobic surface easily by coating the organic polysilazan.

I made microarray using this method.

https://www.linkedin.com/feed/update/urn:li:activity:6394943709061976064

OK, thank you!

Dmitry,

Maybe, this info would be relevant:

https://www.nanowerk.com/nanotechnology-news2/newsid=50636.php

Posted: Jul 10, 2018

Wetting of surfaces is surprisingly difficult to measure reliably

* * *

Published: 09 July 2018

Surface-wetting characterization using contact-angle measurements

Tommi Huhtamäki, Xuelin Tian, Juuso T. Korhonen & Robin H. A. Ras

Nature Protocols (2018)

Dear Kamil,

Thanks for pointing out on this interesting paper.

Dear Dmitry and other colleagues,

Please excuse me for the four-fold response to Dmitry’s question – it happened due to my faulty mouse…

Kamil