July 2011

I had the opportunity to explain to my teenage nephew the ins and outs of contact angle, wetting, and what I do for a living recently at a family retreat. The next day we're eating some pizza and with a cup of soda in his hand, he said to me, "Uncle Carl, what if someone made a reusable superhydrophobic plastic cup? Think of the benefits: most guys in college don't wash dishes anyway. If the cup were liquid-repellant, then no drops of juice or soda or coffee would remain in the bottom to dry up?" It seemed like a compelling idea - and I have to admit, I'd never thought of it: a cup you could use through four years of college without a single wash. It could put the Dixie cup out of business.

Increasingly consumers are demanding products with surfaces that improve performance, lower cost, and reduce maintenance. The oleophobic touchscreen on the iPad and iPhone, the superhydrophobic toilet plunger that doesn't dribble a drop of germy water across your bathroom floor, and glass coatings for side view automobile mirrors which repel water to improve driving visibility in rain and are self-cleaning - are all products that come to mind.

Material scientists are looking for new ways to make their plastic products more hydrophobic. By increasing hydrophobicity, a surface becomes more liquid-repellant, a self-cleaning attribute is added and the product appeal improves. In our recent February 2011 newsletter, we looked at a series of products that are used to lower surface energy and increase the contact angle on Lexan which among other things can improve visibility for motorcyclists who use helmet face shields.

Researchers in Italy¹ are working a method that will replicate the hydrophobic Femtosecond laser-produced patterns on PMMA (Polymethyl methacrylate) and polystyrene to additional samples of the same material via a PFPE (perfluoropolyether) elastomer mold. This method may enhance manufacturing of products that benefit from improved hydrophobicity or that employ microfluidic design enhancements which control the flow of liquids on the surface.

In Singapore², scientists have developed a unique method for coating silicon surfaces with two thin-film coatings which result in water contact angles of 134°. The first layer is UHMWPE (ultra-high-molecular-weight polyethylene) followed by a top layer of PFPE. In addition to significantly increasing contact angle, the dual-film coating reduces the coefficient of friction in excess of 6 times and increases wear resistance by at least 1000 times. This technology can  lead to greatly improved MEMS (micro-electro-mechanical systems) devices.

Researchers at the Hong Kong Polytechnic University³ are developing methods for switching between hydrophobic and hydrophilic states on P3AT (poly 3-alkylthiophene) by using a low-voltage electrochemical process. The switching occurs through the oxidation and deoxidation processes on coated micro-scale patterned surfaces. In one case the surface could be made to change from 62° to 147° and back. In addition to microfluidic applications, this method could also be used to improve contact-printing technology.

No matter what type of polymer you are working with and what you are trying to change regarding the behavior of its surfaces, experts around the world will agree on one thing: You need a ramé-hart contact angle goniometer to capture and quantify contact angle, advancing and receding contact angle, or to measure surface energy. Contact us today for a quotation or for more information on any of our world-class instruments. I'm still working on selling a Model 190 to my nephew. He said he would buy one if I'd invest in his superhydrophobic cup venture.

¹ Langmuir, 2011, 27 (13), pp 8391–8395, DOI: 10.1021/la200724g
² Sensors and Actuators A: Physical Volume 128, Issue 1, 31 March 2006, Pages 98-108, doi:10.1016/j.sna.2005.12.042
³ Langmuir, 2009, 25 (13), pp 7465–7470, DOI: 10.1021/la900387m
 

The Adhesion Society has issued a Call for Papers for their next annual meeting which is scheduled to be held in New Orleans February 26-29, 2012. Topics will include contact angle and liquid interactions with surfaces. For more details on the meeting, please visit their website: http://www.adhesionsociety.org/