Abstract: Fluid flow is ubiquitous in nature, occurring at very different scales and in diverse environments, from the macroscale to the nanoscale, from rivers to tissues. It regulates biological and ecological processes, such as nutrient uptake, reproduction, and embryogenesis, and is involved in many applications, including filtration and energy production. My research focuses on the role of fluid flow in transport phenomena at the micro- and nanoscale, through the development of techniques combining microfluidics technology and real-time visualization. 

In the first part of my talk, I will describe recent experimental work on transport through carbon nanotubes [1, 2]. In the past decade, measurements and simulations have suggested that water moves through carbon nanotubes at exceptionally high rates owing to nearly frictionless interfaces, yet without providing a satisfactory explanation of the exact mechanism of transport at the water-carbon interface. The setup developed in the present work met the considerable technical challenge of unambiguously measuring the permeability of a single nanotube. The pressure-driven flow rate through individual nanotubes was measured with unprecedented sensitivity based on the hydrodynamics of water jets as they emerge from single nanotubes into a surrounding fluid, revealing an unexpectedly large and radius-dependent surface slippage in carbon nanotubes. 

In the second part of my talk, I will discuss the flow of bacterial suspensions in microfluidic circuits. Although bacteria are ubiquitously exposed to liquid flow in natural environments and artificial systems, the influence of hydrodynamics on the transport of pathogenic bacteria and the formation of surface-attached communities (biofilms) remains poorly investigated and understood. I will first present experiments in which a dense suspension of bacteria was flowed through microchannels and the velocity statistics of the flowing suspension were quantified using a recently developed velocimetry technique, revealing an unexpected intermittency phenomenon [3, 4]. I will conclude by describing ongoing experiments on the role of hydrodynamics in the transport of bacteria around isolated micropillars and the subsequent formation of filamentous biofilm structures known as streamers. These phenomena highlight the role of flow in bacterial behavior, and how their understanding offers an opportunity to contribute to the design of industrial and medical systems to minimize bacterial colonization.

References:

[1]  E. Secchi, S. Marbach, A. Niguès, D. Stein, A. Siria and L. Bocquet, Nature, 537, 210-213 (2016) 

[2] E. Secchi, S. Marbach, A. Niguès, A. Siria and L. Bocquet, submitted to J. Fluid Mech. 

[3] E. Secchi, R. Rusconi, S. Buzzaccaro, M. Salek, S. Smriga, R. Piazza and R. Stocker, J. R. Soc. Interface, 13, 119 (2016) 

[4] S. Buzzaccaro, E. Secchi and R. Piazza, PRL, 111, 048101 (2013)