Todd M. Squires

Making it stick: convection, reaction and diffusion in surface-based biosensors

The past decade has seen researchers develop and apply novel technologies for biomolecular detection, at times approaching hard limits imposed by physics and chemistry. In nearly all sensors, the transport of target molecules to the sensor can play as critical a role as the chemical reaction itself in governing binding kinetics, and ultimately performance. Yet rarely does an analysis of the interplay between diffusion, convection and reaction motivate experimental design or interpretation. Here we develop a physically intuitive and practical understanding of analyte transport for researchers who develop and employ biosensors based on surface capture. We explore the qualitatively distinct behaviors that result, develop rules of thumb to quickly determine how a given system will behave, and derive order-of-magnitude estimates for fundamental quantities of interest, such as fluxes, collection rates and equilibration times. We pay particular attention to collection limits for micro- and nanoscale sensors, and highlight unexplained discrepancies between reported values and theoretical limits.

Induced-charge electro-osmosis

We describe the general phenomenon of ‘induced-charge electro-osmosis’ (ICEO) – the nonlinear electro-osmotic slip that occurs when an applied field acts on the ionic charge it induces around a polarizable surface. Motivated by a simple physical picture, we calculate ICEO flows around conducting cylinders in steady (DC), oscillatory (AC), and suddenly applied electric fields. This picture, and these systems, represent perhaps the clearest example of nonlinear electrokinetic phenomena. We complement and verify this physically motivated approach using a matched asymptotic expansion to the electrokinetic equations in the thin-double-layer and low-potential limits. ICEO slip velocities vary as

Forces during Bacteriophage DNA Packaging and Ejection

u_s \,{\propto}\,E_0^2 L

Breaking symmetries in induced-charge electro-osmosis and electrophoresis

, where

A simple paradigm for active and nonlinear microrheology

E_0

Like-charge attraction and hydrodynamic interaction

is the field strength and

Platelet-like Nanoparticles: Mimicking Shape, Flexibility, and Surface Biology of Platelets To Target Vascular Injuries

L

Induced-charge electrokinetic phenomena

is a geometric length scale, and are set up on a time scale

Active microrheology and simultaneous visualization of sheared phospholipid monolayers.

\tau_c \,{=}\,\lambda_D L/D

A mathematical model for top-shelf vertigo: The role of sedimenting otoconia in BPPV

, where