Transport of ions and particles through confining channels is of fundamental importance for biological systems as well as for biotechnology like single molecule sensing. I will discuss three recent examples of our experimental approach to understand hydrodynamic and electro-kinetic phenomena using highly controlled model systems. 

The flow of ions and water molecules are intimately linked and lead to a wealth of unexpected behaviour in confined sys-tems especially at low Reynolds numbers. Using optical tweezers we are able to detect the electro-osmotic flows (EOF) conical glass nanopores. Reversal of the voltage polarity we find that the EOF velocity is not symmetric for perm-selective nanopores. This flow rectification can be quantitatively understood using finite element simulations [1]. 

Surprisingly, at large Debye length the flow pattern around the glass nanopore shows flow reversal: the apparent EOF is directed in the opposite direction than expected from surface charge and voltage. We have developed a simple analytical model which reveals that, as the salt concentration is reduced, the flow rates inside the pore are geometrically constrained, whereas there is no such limit for flows outside the pore [2]. 

In the third part, I will discuss particle-particle interactions in narrow channels. Many well-known interactions like electro-static, van der Waals or screened Coulomb, decay exponentially or with negative powers of the particle spacing r. Similarly, hydrodynamic interactions between particles undergoing Brownian motion decay as 1/r in bulk, and are assumed to decay quickly in small channels. Such interactions are ubiquitous in biological and technological systems. Here we confine two particles undergoing Brownian motion in narrow, microfluidic channels and study their coupling through hydrodynamic inter-actions. Our experiments show that, in contrast to expectations from current theoretical understanding, the hydrodynamic particle-particle interactions are long-range and non-decaying in these channels. This new effect is of fundamental im-portance for the interpretation of experiments where dense mixtures of particles or molecules diffuse through finite length, water-filled channels or pore networks [3]. 

[1] N. Laohakunakorn, B. Gollnick, F. Moreno-Herreo, D. G. A. L. Aart, R. P. A Dullens, S. Ghosal, and U. F. Keyser. A Landau-Squire nanojet. Nano Letters, 13(11):5141-5146, 2013. 

[2] N. Laohakunakorn, V. V. Thacker, M. Muthukumar, and U. F. Keyser. Electroosmotic flow reversal outside glass nano-pores. Nano Letters, 15(1):695-702, 2014. 

[3] K. Misiunas, S. Pagliara, E. Lauga, J. R. Lister, and U. F. Keyser. Non-decaying hydrodynamic interactions along narrow channels. submitted, 2015. http://arxiv.org/abs/1503.06048