Insulator-based dielectrophoresis (iDEP) has emerged as a powerful tool for multiple biomicrofluidic operations, such as cell separation and concentration. The key feature for iDEP systems is the alteration of insulating microchannel geometries to create strong electric field gradients. Under AC electric fields, this strong electric field gradient can affect fluid flow by (at least) two nonlinear electrokinetic phenomena; (a) electrothermal flow due to Joule heating and (b) induced charge electroosmosis (ICEO) near the microchannel constrictions of small (but finite) permittivity and conductivity. This paper presents an experimental and theoretical study on the interplay of electrothermal and ICEO flows near microchannel constrictions with various geometries and fluid ionic strengths, which are crucial design factors for iDEP systems. Temperature rise and fluid velocities in 2D Gaussian-shaped constrictions were studied experimentally with supporting analytical estimations and numerical simulations. Additionally, we show qualitatively distinct recirculating flow patterns in 2D and 3D microchannel constrictions used for iDEP systems. Approximate analytical expressions for electrothermal and ICEO velocity scales are provided as a function of constriction geometry, bulk electrolyte concentration, and the applied electric field. Insights from this study will be useful in designing microfluidic systems for electrokinetic particle manipulation.

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We present a theoretical model to investigate the influence of soft polyelectrolyte layers on bacteria polarizability. We resolve soft-layer electrokinetics by considering the pH-dependent dissociation of ionogenic groups and specific interactions of ionogenic groups with the bulk electrolyte to go beyond approximating soft-layer electrokinetics as surface conduction. We model the electrokinetics around a soft particle by modified Poisson-Nernst-Planck equations (PNP) to account for the effects of ion transport in the soft layer and electric double layer. Fluid flow is modeled by modified Stokes equations accounting for soft-layer permeability. Two test cases are presented to demonstrate our model: fibrillated and unfibrillated Streptococcus salivarius bacteria. We show that electrolytic and pH conditions significantly influence the extent of soft-particle polarizability in dc fields. Comparison with an approximate analytical model based on Dukhin-Shilov theory for soft particles shows the importance of resolving soft-layer electrokinetics. Insights from this study can be useful in understanding the parameters that influence soft-particle dielectrophoresis in lab-on-a-chip devices.

Objectives Skin is arguably one of the most important organs that plays an active role in our everyday biological functions after brain. Owing to the wide range of applications in medicine, cosmetics industry and more recently robotics, skin research has gained tremendous attention with respect to its mechanical behaviour. Various macro modelling approaches are available for modelling skin's mechanical behaviour. The objective of this paper is to study skin's mechanical property change with age and demonstrate anti‐ageing effects of cosmetic formulations from skin mechanical property change perspective. Methods In this study, skin's mechanical behaviour was modelled using a 1D linear viscoelastic phenomenological model and the model was validated using two sets of experimentally observed skin data (strain, stress relaxation and cyclical loading). The model was further modified to study the effect of the presence of a thin layer of cosmetic polymer and to demonstrate anti‐ageing effects of the cosmetic polymer from the perspective of change in the mechanical behaviour of skin with cosmetic layer. Results The estimated values of skin mechanical properties from the model agree with those in literature. The extracted model features show good correlation with skin age (viscosity and time constant). The results from our model indicate that the cosmetic polymers enhance the mechanical properties of skin significantly. Conclusions This work will find its applications in designing and testing anti‐ageing formulations. This model can be used to filter various combinations of cosmetic formulations by knowing the mechanical response of polymer on skin, thereby accelerating the product development.