# Seminars & Discussions

## Week beginning 23 October 2016

Monday 24 Oct 16 - 1:00pm

Microfluidic Pumping by Micromolar Salt Concentrations

Joost de Graff (University of Edinburgh)

Moving fluid around on the microscale is not as simple as taking a syringe off the shelf and pushing liquid through, because if you do this, things tend to break. In this talk, a colloidal ion-exchange-resin-based microfluidic pump is introduced and experimentally characterized. It operates in almost deionized water for periods exceeding 24h and effects fluid flows of um/s over hundreds of microns. This fluid flow displays a far-field, power-law decay which is characteristic of two-dimensional (2D) flow when the system is strongly confined in the vertical direction, and of three-dimensional (3D) flow when the system is less confined. Using theory and numerical calculations we show that our observations are consistent with electroosmotic pumping driven by umol/L ionic impurities in the sample cell, revealing the surprising impact of trace amounts of charge carriers. In addition, we explain how the power-law decay of the fluid flow can be understood on the basis of the confinement imposed by the sample cell. These two insights should benefit the design of a new class of microfluidic pumps that operate at low solute concentration.

Friday 28 Oct 16 - 11:30am - JCMB 2511

How Far from Equilibrium Is Active Matter?

Authors: Étienne odor, Cesare Nardini, Michael E. Cates, Julien Tailleur, Paolo Visco, Frédéric van Wijland

Speaker: Martin Evans

Active matter systems are driven out of thermal equilibrium by a lack of generalized Stokes-Einstein relation between injection and dissipation of energy at the microscopic scale. We consider such a system of interacting particles, propelled by persistent noises, and show that, at small but finite persistence time, their dynamics still satisfy a time-reversal symmetry. To do so, we compute perturbatively their steady-state measure and show that, for short persistent times, the entropy production rate vanishes. This endows such systems with an effective fluctuation-dissipation theorem akin to that of thermal equilibrium systems. Last, we show how interacting particle systems with viscous drags and correlated noises can be seen as in equilibrium with a viscoelastic bath but driven out of equilibrium by nonconservative forces, hence providing energetic insight into the departure of active systems from equilibrium.

PRL

pdf version

PRL

**117**article 038103 (2016)pdf version

## Upcoming meetings

Monday 31 Oct 16 - 1:00pm - Room 2511

Ana Fialho (University of Edinburgh)

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