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.

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

Modern developments in shear-stress measurement ☆

I .. Rates of ionic mass transfer at nickel electrodes rotating about their axes In the center of stationary electrodes were studied using the ferri-ferrocyanide oouple In alkaline solutions. A general mass transfer correlation was found to apply equally well to the dissolution rates of rotating solids and to the rates of ionlo mass transfer at rotating electrodes. This correlation takes into account the physioal properties of the system as well as geometric and hydro dynamic factors. The correlation enables the prediction of limiting currents and concentration polarisation at rotating electrodes under a wide range of conditions. The nature of the polarisation involved in the reduotion of Fe(CNfe~» and the oxidation of Fe(CN#* was also Investigated. The polarisation was found to depend strongly on the presence of "electrode poisons." With freshly prepared solutions, under exclusion of light and with eathodlcally treated nickel electrodes relatively small ehesdcal polarisations were determined. For rotational speeds not exceeding Reynolds number 11,000, the chemical polarisation was found to be negligicle in comparison with the concentration polarisation* Under such conditions the ferro-ferrioyanide oouple can be conveniently used to obtain mass transfer rates for various hydrodynamic conditions, or, conversely, to verify the validity of mass transfer equations by a comparison of experimental .and calculated values of limiting currents and concentration polarisation. The rotating electrode model was found to be most suitable for studying the nature of eleotrolytio polarisation phenomena on aocount of the uniformity of the current distribution as well as of the hydrodynamio diffusion laver at the electrode surfsee.

Mass transfer in electrode processes. 3. ionic mass transfer and concentration polarization at rotating electrodes

Atherosclerosis is a disease of the large arteries that involves a characteristic accumulation of high-molecular-weight lipoprotein in the arterial wall. This review focuses on the mass transport processes that mediate the focal accumulation of lipid in arteries and places particular emphasis on the role of fluid mechanical forces in modulating mass transport phenomena. In the final analysis, four mass transport mechanisms emerge that may be important in the localization of atherosclerosis: blood phase controlled hypoxia, leaky endothelial junctions, transient intercellular junction remodeling, and convective clearance of the subendothelial intima and media. Further study of these mechanisms may contribute to the development of therapeutic strategies for atherosclerotic diseases.

Mass transport in arteries and the localization of atherosclerosis.

Knowledge of the wall shear stress is of both fundamental and practical importance The mean stress is indicative of the overall state of the flow over a given surface while the fluctuating stress is a “footprint” of the individual processes that transfer momentum to the wall The measurement techniques can be divided into two main categories: direct and indirect The indirect techniques can be further divided into momentum balance methods and correlation methods

The Measurement of Wall Shear Stress

The problem of heat transfer (or mass transfer at low transfer rates) to a strip of finite length in a uniform shear flow is considered. For small values of the Peclet number (based on wall shear rate and strip length), diffusion in the flow direction cannot be neglected as in the classical Leveque solution. The mathematical problem is solved by the method of matched asymptotic expansions and expressions for the local and overall dimensionless heat-transfer rate from the strip are found. Experimental data on wall mass-transfer rates in a tube at small Peclet numbers have been obtained by the well-known limiting-current method using potassium ferrocyanide and potassium ferricyanide in sodium hydroxide solution. The Schmidt number is large, so that a uniform shear flow can be assumed near the wall. Experimental results are compared with our theoretical predictions and the work of others, and the agreement is found to be excellent.

S. K. Gupta

Papers

The heat/mass transfer to a finite strip at small Peclet numbers