A new way of sticking

22 October 2019

Interview with 

Orlin Velev, North Carolina State University

Dendritic_microparticle.png

Soft dendritic microparticle

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A team from North Carolina State University have created a new type of super-sticky particle called a soft dendritic microparticle. They look like hairy microscopic meatballs, and they’re made by whipping a polymer solution at very high speeds; this creates chaotic turbulence that pulls out the fine filaments that form the hairs. And those hairs give the particles their stickiness because they generate attractive forces - called van der Waals attractions - with the surfaces they sit on. Phil Sansom got stuck into the paper, published in Nature Materials, with its author, Orlin Velev...

Orlin - This is a new type of particle, big hairy ball of fibres that will stick very strongly to any surface.

Phil - How strongly?

Orlin - Comparable to the form of double sided Scotch tape.

Phil - That sounds so bizarre! How do you make them.

They're made in the highly turbulent flow. Highly turbulent flow is considered to be very chaotic. We inject into this highly chaotic flow polymer solution so we'll use a very chaotic medium in order to make something which is organized in a way that will be called dendritic or hierarchical, because it has different size scales.

Phil - Okay what is this stuff actually made of?

Orlin - It is made of polymer such as polystyrene or cellulose acetate. You have a polymer which is dissolved in a solvent, so the moment you mix the solution of the polymer with the medium, you have the polymer begin precipitating because the polymer does not like the medium into which its solution is being mixed.

Phil - How do you actually make that turbulent flow? Do you stir it really hard?

Orlin - Stirring it really hard is the way to describe it in simple words! The device that they use is called a colloidal mill. You have a static encasing around a rota that rotates as very high speed. Think of them as having highly serrated surfaces.

So when this rotating cylinder in this case moves very fast is that the liquid is going to be sheered very strongly.

Phil - These particles that you get out. What's so strange about them?

Orlin - So there will be a material which is commonly called nanofibers, similar to the nanofibers that you would find on the legs of gecko lizards. Nanofibers have very interesting properties because they're very fine. They have very strong adhesion to any material because of the so-called Van der Waals forces. You can think of this as natural stickiness of any material on the nanoscale.

The reason gecko legs are sticky is because they have this very fine mat of nanofibres and those nanofibres have the ability to stick to any surface  because of the Van der Waals forces.

Phil - So you're saying these particles are sticky just like gecko feet are.

Orlin - Exactly so when you have the particles approach a surface the nanofibers would like to stick to that surface and because you have so many nanofibers sticking together you are going to have a high adhesion force, that is you are going to have high stickiness.

Phil - What kind of things can you use this for?

Orlin - We believe that there will be many potential applications in different technologies. One of the properties that we describe is how easy it is to make gels. So in order to make a gel you put particles in liquid until the particles are sticky. The liquid is going to stop flowing and it’s going to become a gel.

We only need less than 1 percent of our particles in any liquid in order to turn the liquid into a gel, so you can use them in products where do you need the small volume of particles in order to make a large volume of gel. For food, cosmetics or similar type of applications.

If you take those particles and dry them on a surface. If you take a hydrophobic polymer, the surface that you dry them on top is going to become super hydrophobic, water repellent. If you take hydrophilic particles it is going to become superhydrophilic.

You can make a sheet which is going to be super hydrophobic on one side, super hydrophilic on the other side, which can be used in different applications such as biomedical applications for example.

Phil - And presumably also we can make gloves that help us climate walls like Spider-Man!

Orlin - I would not say this is a very feasible application! In our case our particles stick strongly once we dry them on the surface. But then prying them off and then attaching them to the surface again is typically not possible.

Phil - So you'd have to chisel Spider-Man off the wall?

Orlin - If you tried to do that with this type of particles probably.

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