TL: How did the idea of particle transport in liquid lead to sorting?
AK: In 2010, we already had this nanofluidic project running, and since we have the capability to write very high-precision patterns, we were looking for applications. A paper from Madhavi Krishnan appeared more or less at the same time, where she showed that the geometrical recess of a surface can produce an energy landscape, at least a potential minimum. So, we thought it would be a nice fit to combine single-digit nanometer precision and the ability to generate potential energy landscapes. Energy always controls the behaviour of particles, and if you are really able to manipulate the energy landscape you can manipulate the behaviour of the particle.
A nice thing about lithography is that you really control all the energy barriers because you write them. When you know the energy barriers, you know the timing in the system, you can tune it from the top down. For Brownian motors, in our implementation at least, the particle transport strongly depends on the size of the particle because a bigger particle feels an amplified energy landscape. So, a larger particle rests closer to the interfaces of the nanofluidic channel, and therefore, the interaction energy amplifies exponentially. The energy landscape needs to be stronger than the thermal fluctuations, which is at least 3–4 kBT. But you don't want it to be at 20-30 kBT, because then you don't have sufficient force to tilt the energy landscape anymore. At around 5-10 kBT, the transport works nicely. And now, if there are different-sized particles, the smaller ones might not see enough of the energy landscape, so they will not be transported, and the ones that are too big will get stuck. But there is a size range in which the particles will be nicely transported. When we built our motor it was clear that this would happen. Then, we designed a slightly more complicated device optimized to achieve very high efficiency in separating particles.