Durability of Superhydrophobic Coatings - Sand Abrasion Test

The surfaces were abraded for 30 seconds at a time and the static, receding, and advancing contact angles along with the roll-of angle was measured.. Measurements showed that the roll-of

TVE 16 064 juni Examensarbete 15 hp

Juli 2016

Durability of Superhydrophobic

Coatings - Sand Abrasion Test

Hugo Harlin

Max Holmberg

Teknisk- naturvetenskaplig fakultet

UTH-enheten

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Ångströmlaboratoriet

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Hus 4, Plan 0

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Box 536

751 21 Uppsala

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018 – 471 30 03

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Abstract

Durability of Superhydrophobic Coatings - Sand Abrasion Test

Hugo Harlin & Max Holmberg

The interest in superhydrophobic coatings have increased exponentially in the recent decades due to their potential and versatility in their applications The use for superhydrophobic surfaces range from water repellent fabric, to self cleaning surfaces and numerous applications in industry In this project the durability of 6 different superhydrophobic coatings have been examined The durability was tested by dropping sand on the surfaces from a set hight of 10 cm and a flow of 40 g/min The surfaces were mounted on a 45° angle The surfaces were abraded for 30 seconds at

a time and the static, receding, and advancing contact angles along with the roll-of angle was measured Five of the surfaces were built up with nano particles and one was sand blasted and anodized to create a superhydrophobic structure The surfaces that withstood the most abrasion was the surface that had been calcined to improve adhesion and the surface that had been sand blasted and anodized Measurements showed that the roll-off angle and the receding contact angle were the two best indicators of the deterioration of a surface, while the static contact angle and the advancing contact angle varied little with abrasion The project was done at the company Technical Research Institute of Sweden (SP) at their chemistry, surfaces and materials department in Stockholm All coatings and equipment was supplied by SP

ISSN: 1401-5757, TVE 16 064 juni Examinator: Martin Sjödin Ämnesgranskare: Viviana Lopes Handledare: Mikko Tuominen & Mikael Järn

2.1 Basics of Superhydrophobicity 2

3.1 Materials 4 3.2 Sand abrasion test 4

1 Popul¨ arvetenskaplig sammanfattning

Ytor med vattenavst¨otande egenskaper ¨ar vanligt f¨orekommande i naturen Ta till exempel en vattenskr¨addare som kan st˚a p˚a en vattenyta med hj¨alp av sina l˚anga vattenavst¨otande ben I praktiken ¨ar vattenavst¨otande ytor v¨aldigt anv¨andbara med m˚anga till¨ampningsomr˚aden Om husfasader och tak behandlas s˚a skulle vatten rulla av ist¨allet f¨or att bl¨ota ytorna, vilket f¨orhindrar tillv¨axten av m¨ogel och v¨axtlighet En smutsig vattenavst¨otande yta kan enkelt reng¨oras genom att den spolas av med vatten, smutsen plockas upp av vattendropparna som i sin tur rullar av ytan En vattenavst¨otande yta m˚aste vara best¨andig mot slitage, och i detta projekt unders¨oks slitt˚aligheten av olika h¨ogpresterande vattenavst¨otande ytor Slitaget bestod av sand som f¨oll mot ytan, vartefter

en droppe placerades p˚a ytan och dess form studerades Provet vinklades sedan och vinkeln d˚a droppen rullar av, den s˚a kallade roll-off vinkeln, m¨attes Roll-off vinkeln var den b¨asta indikatorn p˚a hur h˚allfasta ytorna var F¨or att tillverka en h¨ogpresterande vattenavst¨otande yta beh¨ovs tv˚a faktorer; en oj¨amn ytstruktur (p˚a v¨aldigt liten skala), och en vattenavst¨otande ytkemi F¨or att en yta skall var slitt˚alig m˚aste ytstrukturen och ytkemin t˚ala slitage Eftersom det finns flera metoder att skapa ytstrukturen samt applicera ytkemin, var projektet ett s¨att att j¨amf¨ora kombinationer av ytstrukturer och appliceringsmetoder f¨or att uppn˚a maximal h˚allbarhet

The interest in superhydrophobic coatings have increased exponentially in the recent decades due to their potential and versatility in their applications The use for superhydrophobic surfaces range from water repellent fabric, to self cleaning surfaces and numerous applications in industry Surfaces with superhydrophobic properties are common

in nature Take for example the legs of the water strider, which enables it to sit comfortably on the water’s surface due to their water repellency [1] Another example of a superhydrophobic surface is the leaves of the lotus plant Water droplets simply roll off the leaves, hindering the growth of harmful pathogens and the build up of dirt [2] Hydrophobic means water repellent, and is a very useful property since dirt, pollutants, chemicals, bacteria, and other unwanted substances are often water soluble and can easily be washed off a hydrophobic surface The ability

to shed water efficiently means a surface stays clean and lessens the threat that contact with unwanted substances imply

Superhydrophobic coatings can be used in many different applications, for example self-cleaning optical surfaces, energy conversion and conservation, environment-friendly self-cleaning underwater surfaces etc [3] The coatings that are examined in this report are intended to be used on heat sinks designed for heat pumps Hydrophobic coatings are widely applicable in industry, but for a coating to be viable it has to be durable enough for the given application In some cases this can be a demanding criteria to fulfil, and increasing the durability of hydrophobic coatings is thus of great interest

2.1 Basics of Superhydrophobicity

The definition of a hydrophobic surface is a contact angle larger than 90 degrees A superhydrophobic surface is defined as having a contact angle larger than 150 degrees [4] The contact angle is defined as the angle between the liquid fluid interface and the tangent to the solid interface at the contact line between the three faces (see figure 1) [5] Measurements of static contact angles is not as simple as it may seem, on a non ideal surface a water droplet will have more than one stable contact angle (local energy minima) and thus it may be difficult to draw any conclusion just from the static contact angle measurement A better way of describing the contact angle is

by measuring the Advancing Contact Angle (ADCA) and the Receding Contact Angle (RCA) As can be seen

in figure 2 ADCA is the highest contact angle for which there exists a local energy minimum and RCA is the lowest angle for which there exist a local energy minimum The ideal static CA will be between the RCA and the ADCA [5] Some hydrophobic surfaces may have a high contact angle but droplets will stick to the surface, this

is in many cases an undesirable effect and can be measured with the role-off angle (RA) The role-off angle is the smallest angle between the sample surface and the horizontal plane, where a drop of water will roll off the surface

No real surface is truly flat, and therefore one must consider the surface roughness when evaluating the hy-drophobicity of a surface There are in general three different cases of how a droplet sits on top of a rough surface, the Wenzel state [6], the Cassie state, or a combination of the two [7] In the Wenzel state the droplet sits on top of the roughness and in the Cassie state the droplet is merged into the roughness of the surface (see figure 3).These states are called wetting states, and the Cassie state corresponds to a fully wetted droplet while the Wenzel state corresponds to a non wetted droplet

In order to acquire a superhydrophobic surface there are two main properties a surface needs; Surface roughness (on the micro and nano scale) and a coating of some water repellent substance The roughness of a surface is an important factor when creating a superhydrophobic coating [8] To acquire a good combination of the nano-scale and micro-scale structure one can utilize either a bottom up process, building up the surface with new material (e.g nano particles), or a top down process, wearing down the surface in such a way so that the right kind of structure emerges (e.g sand blasting)

Figure 1: The contact angle is the angle between the liquid fluid interface and the tangent to the solid interface

at the contact line between the three faces

Figure 2: Illustration of contact angle versus Gibbs energy RCA is the lowest contact angle for which there exists

a local energy minimum ADCA is the highest contact angle for which there exists a local minimum

Another vital aspect of creating superhydrophobic coatings is the surface chemistry The surface chemistry dictates the value of the surface energy, the lower the value the more hydrophobic the surface is However, if the adhesion of the top chemical coating to the underlying structure is poor, then the surface will deteriorate quickly when subject to abrasion [9] In some cases the surface properties can even be dynamic, for example the chemistry can be changed by light irradiation [10], an electric field [11], or thermal treatments [12]

The goal of the project is to compare different techniques for creating surface structure and applying surface chemistry, and determining which combinations lead to the most durable superhydrophobic surfaces

Figure 3: A) Cassie state B) Wenzel state C) A combination of Wenzel and Cassie states

3.1 Materials

The different coatings that were tested are tabulated in table 1 All of the coatings were applied on a small aluminium plate, see figure 4 One of the samples (No 1) has a top down approach to creating the hydrophobic structure, the rest are bottom up Observe that sample No 3 and No 4 have the same hydrophobic material but

No 4 has been calcined to improve adhesion

Table 1: Superhydrophobic coatings materials These were supplied to us by our supervisors Mikko Tuominen and Mikael J¨arn at SP Stockholm

No Superhydrophobic material Application method/structure creation

with fluorosilane

2 Neverwet, commercial nanoparticle-based coating Spray coating a bottom coat followed by a topcoat

3 0.5 wt% Aerosil r972 (Nanoparticle-based coating) Dipcoated and calcined to improve adhesion and

then hydrophobized through self assembly of a fluorosilane

4 0.5 wt% Aerosil r972 (Nanoparticle-based coating) Dipcoated (No calcination)

6 Fumed SiO2 + fluorosilane nFog (aerosol wet coating) 60 s

3.2 Sand abrasion test

To study the durability of superhydrophobic coatings we used the sand abrasion test The test involves applying sand to a coated surface from a certain hight and for a certain time (figure 5) Sand was dropped onto the samples from a height of 10 cm, with a particle size of 50-70 µm The samples were mounted at a 45◦ angle Above the sample a sieve was mounted The sieve used in the sand abrasion test was used in order to increase the affected area

The downside of the sieve is that the velocity of the sand hitting the samples varies more, without the sieve every grain of sand hitting the surface should in theory have the exact same kinetic energy This means that the

Figure 4: An example of what the samples looked like The area of the plate is roughly 7x3 cm.

average kinetic energy of a grain of sand hitting the surface is harder to calculate

The kinetic energy of a single grain of sand hitting the surface of a sample was in the range 0.3 nJ The flow rate of sand onto the sample was 40 g/minute 20 grams of sand were poured Each sample was subjected to around 30 seconds of sand abrasion before performing a measurement Static, receding, advancing contact angle and the roll-off angle was measured after each abrasion This procedure was repeated on each of the surfaces until they were completely worn and the roll-off angle was consistently around 90◦

The contact angles were measured with an OCA contact angle measuring system from Dataphysics producing images like figure 1 The ADCA, RCA and RA were measured by tilting the surface until the drop fell of, this process was filmed and analysed (see figure 10 The RCA is the contact angle on the left side of the drop (if seen from the side and the surface is tilted clockwise, see figure 10) and the ADCA is the contact angle on the right side of the drop just before the drop falls of The drops that were used were of size 5-6 µl, and 5 drops were used per sample per abrasion

Figure 5: The sand apparatus that was used to abrade the samples The red cable was used together with copper foil to ground the tip of the plastic tube to prohibit the build-up of static electricity, which would otherwise cause the tube to clog

In figure 6 to figure 9 the different contact angles and roll-off angles for all the surfaces in the sand test are displayed For all the tested coatings it is the RCA and the RA that are affected the most, and thus are the best indicators of how well a coating performs in this kind of test Observe that sample 3 was significantly better at withstanding the abrasion than the rest, this surface was abraded for a total of 360 seconds before it consistently had a roll-of angle of 90◦ Sample 1 also performed well, withstanding abrasion for a total of 250 seconds Figure

10 displays 3 snapshots of one of the films where receding, advancing and roll-of angle were measured

Figure 6: Measurements of roll-of angle (RA) during 140-400 seconds of abrasion A) Sample 1 and 2 B) Sample

3 and 4 C) Sample 5 and 6

Figure 7: Measurements of advancing contact angle (ADCA) during 140-400 seconds of abrasion A) Sample 1 and 2 B) Sample 3 and 4 C) Sample 5 and 6

Figure 8: Measurements of receding contact angle (RCA) during 140-400 seconds of abrasion A) Sample 1 and

2 B) Sample 3 and 4 C) Sample 5 and 6

Figure 9: Measurements of static contact angle during 140-400 seconds of abrasion A) Sample 1 and 2 B) Sample

3 and 4 C) Sample 5 and 6

Figure 10: A) The surface tilted 0 B) The surface tilted 10 C) The surface tilted 23.3 (just before the drop roles of)

When comparing the RA,RCA,ADCA, and static contact angle of the different surfaces, the RA and RCA is affected much more by abrasion than the ADCA and static contact angle This is the reason why we chose to use the RA as an indicator of abrasion and stopped abrading the surfaces when the RA reached 90 degrees consistently Sample 3 withstood the highest amount of abrasion, 360 seconds Recall that sample 3 was calcined, and when compared to sample 4 which was the same coating except it was not calcined, it is evident that the adhesion of the particles (that make up the surface roughness) to the underlying substrate is critical to the coatings durability The second best surface was sample 1, which withstood 250 seconds of abrasion (see figure 6) Sample 1 was made with a top down method, the surface was sand blasted and anodized, removing material from the surface and thus creating the necessary surface structure We think that sample 1 performed well compared to the other samples because of the top down technique, the surface roughness needed was originally part of the bulk of the substrate and thus bonded well with surrounding material

The standard deviation of the data tends to increase as the roll off angle reaches 90 degrees, due to a phenomenon called pinning When a droplet is pinned it is in a (partial) Cassie state and its adhesion to the surface is greater than that of other droplets This is a probabilistic phenomenon and depends on where exactly the droplet was placed The RA can vary as much as 85+ degrees, depending on if the droplet is pinning or not When measuring the RA of sample 5 after 90 seconds of abrasion, the RA varied between 1.1 and 90 degrees, with several values

in between We found that this was common when the surfaces started to show wear, sample 5 being the most extreme in its range of angles

When comparing the RA, ADCA, RCA, and static contact angle of sample 5 and 6, the durability of the two samples are very similar This implies that there is no clear advantage using an aerosol wet coating technique (nFog) over using a dip coating technique, for this particular coating

One of the largest problems with the sand test apparatus was the build up of static electricity, causing the sand

to flow erratically or not at all through the nozzle This problem was partially solved by grounding the outside of the nozzle, however to fully solve the issue one could use a nozzle made with glass instead of plastic

The method of using sand as abrasion material was chosen to imitate the realistic condition that small particles (carried by the wind for example) hit the surface For many applications this kind of abrasion is very relevant and similar sand tests have also been performed by others [9]

However it could be interesting to utilize other types of abrasion methods to further study superhydrophobic coatings, to better mimic the different types of abrasions a surface could be exposed to One such method could be

an icing-melting cycle where a drop of water freezes and melts on top of the coating The icing-melting abrasion was examined in this project, but due to difficulties with freezing the droplets we decided to focus on the sand