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Scientific Research of the Union of Scientists in Bulgaria - Plovdiv, series G. Medicine, Pharmacy and Dental medicine, Vol. XVII, ISSN 1311-9427, International Conference of Young Scientists, 11 - 13 June 2015, Plovdiv
ИЗСЛЕДВАНЕ НА ВЪЗМОЖНОСТИТЕ ЗА ПРИЛОЖЕНИЕ НА
суперхидрофобността при разработването на сензори
ЗА МЪГЛА
Данаил Владимиров Вълков, Кирил Николов Ангелов Институт по Физика на Твърдото тяло, БАН
EXAMINING THE POSSIBILITIES FOR USING SUPERHYDROPHO-BICITY IN DEVELOPING FOG SENSORS Danail Vladimirov Valkov, Kiril Nikolov Angelov Institute of Solid State Physics, BAS
Abstract
For the purpose of developing sensors for measuring specific parameters of liquid layers, based on the Surface photo charge effect, the application of superhydrophobicity is examined. The goal is to obtain a superhydrophobic layer, which will prevent the accumulation of condensate and contamination on the active surface, which could lead to errors and the inability to take measurements. In this report, specific characteristics of hydrophobicity, experiments and ideas for solving issues are reviewed.
Key words: superhydrophobic layer, micro and nanostructures, SPCE
Main
Physical properties and chemical composition of liquid layers are interesting for a number of processes with high impact on society and industry.
A possible solution for determining these characteristics is using the Surface photo charge effect (SPCE) presented in Figure 1 [1]. This effect is observed when there is an interaction between solid and electromagnetic field, which induces an alternating potential difference with the same frequency as the one of the incident field. Experimental data confirmed that SPCE is observed in frequency range of visible spectrum and its adjacent infrared and near-UV Not only this but the effect is present in the range from 1 Hz to 1 GHz, and is expected to be present in the entire electromagnetic range [2].
An important feature of this method is that a measured signal is strongly dependent on the irradiated solid's properties. That gives the possibility for many practical applications - sensors for gases, liquids and vapours [3-5]; contactless characterization of semiconductors [6]; measuring the deposition of components of a liquid solution on a surface (limestone from water) [7] and many others.
The usage of this technique allows to determine fog features such as presence of fog, the size distribution of droplets and to observe changes in its chemical composition.
Light
I I i
3 v
X. T T T
Signal
Fig. 1 SPCE at liquid-sold interface: 1-solid, 2-solid-liquid interface, generating the signal,
3-fluid under study, 4-electrode
With the case of examining fog characteristics, there is a solid-liquid interface, which is one of the most significant features of a SPCE based method. This might introduce substances and dust that could change or prevent the interaction between the irradiated sample and the source of electromagnetic radiation. To prevent this from happening, we have examined the possibility to use materials and products with superhydrophobic properties. For this reason, more in-depth research was made to examine when and why this effect is observed.
After close examination, in turns out three are the factors playing main role in what kind of surface properties a structure will have:
1. Surface energy
2. Contact angle
3. Surface roughness
1. Surface energy - It is a contractile force and it is caused by the cohesive force between the molecules of a liquid. The distribution of the energy state at which the molecules are not equal and for this reason a counter force is observed. Its direction is focused on minimizing the surface area of a liquid and by doing so, to reduce the number of molecules at higher energy state [8]. A droplet could be almost entirely spherical or, on the other hand, completely spread on a surface like a thin film. Surface energy is not a property only of liquids, but for of any surfaces (interface between two phases).
The surface energy of a solid must be taken into account as well. Solids are divided in two groups: high-energy solids and low-energy solids. High energy solids are characterized with strong bonds between the atoms of the solid itself. On the other hand, low-energy solids are held together by weak forces, such as van der Waals forces and hydrogen bonds. This is directly related to the wetting property of a solid. It determines whether the surface has high or low surface energy, which corresponds to hydrophobic or hydrophilic properties. Surfaces with low surface energy are hydrophobic and the surfaces with high surface energy are hydrophilic.
2. Contact angle - Taken all interactions together, liquid and solid, solid and air or air and liquid, gives Young's relation [9]
LA cos 0 = SA - SL
where 0D is the equilibrium contact angle , □ LA - surface energy -liquid/air , □ SA - surface
energy - solid/air , DSL - surface energy - solid/liquid
LIQUID
1/ v 7
4
A
LA
— V 'SL Y SA
SOLID
Fig 2. Water droplet on solid surface
AIR
If a contact angle is exceeding 90°, then the surface is hydrophobic and if a contact angle is below 90°, the surface in is hydrophilic (Fig.2). But these effects could go to extremes when they exceed certain values. For superhydrophobicity this is 150 and for superhydrophilic effect below 5°.
3 Surface roughness - This feature could increase the property of a substrate to be more hydrophobic or hydrophilic by increasing the contact area of solid-liquid phase. There are two models that describe a water droplet in contact with a rough surface - Wenzel and Cassie-Baxter state.
0D = equilibrium contact angle (contact angle for an ideal flat surface) 0* = apparent contact angle (contact angle for a rough surface) Wenzel state
There are no air bubbles between the liquid and the surface (Fig. 3-a) [10] r=real surface area / projected surface area >1 (since every surface has some roughness at a molecular level)
For this model - cos0A*=r*cos0
_e
Since r > 1, cos 0* > cos 0D (a very important statement) Cassie Baxter state
Where there is air between the surface and the liquid (Fig. 3-b). This occurs at very rough surfaces. [11]
®s = % of solid (percentage of contact surface between solid and liquid)
Fig. 3 a-Wenzel state b- Cassie Baxter state
Experiments
Several experiments were carried out in order to test hydrophobic effects and observing surfaces after spraying them with water droplets corresponding to the size rage of naturally occurring fog. All this was tested with graphene covered substrates, a commercially available product for water repelling and silicon substrates covered with a layer of a superhydrophobic substance.
An array of high quality graphene covered substrates were purchased and tested. Even though the graphene film showed good hydrophobic properties, small droplets remain on the surface of the samples. (Fig. 4)
Fig. 4 Substrate covered with graphene More or less the same happened with the substrates covered with a commercially available water repelling product KY-805. The smaller the droplet size was, the higher inability for the droplets to slide from the surface was observed. (Fig. 5)
Fig. 5 a- an untreated substrate, b,c- substrates covered with KY-805
The same effect was present on superhydrophobic coating on silicon substrates. Big droplets tend to easily roll off and usually entrain smaller ones on their path. Small droplets remained on the surface until they have reached a certain size (Fig. 6).
Fig. 6 Substrates with superhydrophobic layer
Conclusion
Basic principles of hydrophobicity were reviewed. Despite expected results, experiments indicate a dependence of the water repelling properties of the film on droplet size. A possible reason for this is that the mass of certain droplets, which remain on top of a surface, is not enough to roll off. This might be due to both forces, gravitational pull and adhesive strength, having relatively close values. Another factor is that in misty conditions the surface energy of water droplets is decreased due to the higher air humidity [12]. For this reason, water droplets are turning into more spread like shape, which makes them more stable on surfaces. A possible solution for this obstacle is to install a device that releases air under pressure in direction, parallel to the surface of a substrate with superhydrophobic coating, in order to remove such small droplets. In the same time new techniques are tested for their compatibility with SPCE.
This work has been funded by EU FP7 Security program under contract 312804.
References
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