Surface Wettability (Science Box Experiment 3)
As part of CoCoMAD, Royal Society of Chemistry have funded science boxes which are being distributed to local school children - however - the experiments are available online for anyone to take part!
Below you’ll find the experiment introductions, equipment list, method, questions to explore and - very important - health and safety information.
Introduction
When a droplet sits on a solid surface it makes an angle at the point where liquid, solid and gas meet, this is known as the contact angle.
The contact angle shows how the liquid wets the solid surface and this depends on how much the liquid ‘likes’ the solid surface (think about how water doesn’t spread on a Teflon-coated frying pan).
θ is the contact angle
There are three descriptions of how a water droplet sits on a solid surface (‘Wetting state’):
Hydrophilic (water-liking) surfaces have a contact angle less than 90°.
Hydrophobic (water- hating) surfaces have a contact angle between 90° and 150°.
Superhydrophobic (water-repellent) surfaces have a contact angle greater than 150° and the droplet rolls off a surface very easily.
Equipment List
Lotus leaf (LINK) or a cabbage, lupin leaves or broccoli (other water repellent plants are available!)
Duck feather (can be found at fishing shops, e.g. LINK)
Dropper (such as a syringe, pipette, dropper bottle or even a squeezy ketchup bottle)
Magic sand (LINK)
Magnifying glass (LINK)
Other surfaces to test (Warning: Make sure you chose surfaces where the water will not come in contact with electricity)
Water
What to do
IT IS IMPORTANT THAT YOU HAVE READ THE HEALTH AND SAFETY SECTION BEFORE CARRYING OUT THE ACTIVITY – THE ROYAL SOCIETY OF CHEMISTRY, FRIENDS OF COTTERIDGE PARK AND THE DEVELOPER DOES NOT ACCEPT ANY LIABILITY
Experiment A – Water droplets
Draw a table with two columns saying “Material”’ and “Wetting state”
Fill up your dropper bottle with water then place a droplet of water onto different surfaces and note down whether it is it hydrophilic, hydrophobic or superhydrophobic.
Use a magnifying glass to look at the surface of hydrophobic and hydrophilic surfaces and describe the differences (a good example is a superhydrophobic surface such as cabbage and a holly leaf)
Experiment B – Submerged surfaces
Put some water in a centrifuge tube (or glass) and put in some magic sand. Describe what happens.
Things to investigate
Compare the surface of a hydrophobic leaf and a superhydrophobic surface and think about why water easily rolls off one and not the other.
Try to balance a water droplet on the Lotus leaf (or other superhydrophobic surface) and see whether or not you can see surface structure through the drop.
Try to balance a water droplet on the duck feather. Then try to balance a water droplet on the other side of the feather.
On the most water repellent side see how long you can balance a water droplet on the feather for.
Health and Safety (remember to always use common sense)
Risk(s) & Precaution(s)
1) Slipping on water
Spills
When using the water droplet do it over a towel, sink or absorbent surface (e.g lawn, soil, absorbent paper)
2) Electrocution
· Do not carry out this experiment near electrical devices
Where can I learn more?
Nature’s Raincoats – a website about bio-inspired superhydrophobic surfaces: https://naturesraincoats.com/
‘Nature’s Inspirations’ information sheet at Henry Royce Institute website: https://www.royce.ac.uk/outreach/outreach-learning-resources-for-teachers-and-pupils/
Water repellent surfaces video - https://www.youtube.com/watch?v=6-bbmCRqwGQ
Feedback
We really hope you enjoy the experiments and would love your feedback on the activities and on the science activities and talks at Virtual CoCoMAD (4th July).
To send feedback please use the QR code or the link below to get to the feedback form which will probably take about five minutes to complete. If you do you will be invited to leave your email address and you will be entered into a draw to win a pack of RSC Periodic Table Top Trumps.
https://bham.qualtrics.com/jfe/form/SV_3eIGeizxfkWcVx3
Please post your photos and comments on Twitter, Facebook and Instagram: #time4chem, #CoCoMADSciBox #CoCoMAD
