Augmented longitudinal acoustic trap for scalable microparticle enrichment.

We introduce an acoustic microfluidic device architecture that locally augments the pressure field for separation and enrichment of targeted microparticles in a longitudinal acoustic trap. Pairs of pillar arrays comprise "pseudo walls" that are oriented perpendicular to the inflow direction. Though sample flow is unimpeded, pillar arrays support half-wave resonances that correspond to the array gap width. Positive acoustic contrast particles of supracritical diameter focus to nodal locations of the acoustic field and are held against drag from the bulk fluid motion. Thus, the longitudinal standing bulk acoustic wave (LSBAW) device achieves size-selective and material-specific separation and enrichment of microparticles from a continuous sample flow. A finite element analysis model is used to predict eigenfrequencies of LSBAW architectures with two pillar geometries, slanted and lamellar. Corresponding pressure fields are used to identify longitudinal resonances that are suitable for microparticle enrichment. Optimal operating conditions exhibit maxima in the ratio of acoustic energy density in the LSBAW trap to that in inlet and outlet regions of the microchannel. Model results guide fabrication and experimental evaluation of realized LSBAW assemblies regarding enrichment capability. We demonstrate separation and isolation of 20 µm polystyrene and ∼10 µm antibody-decorated glass beads within both pillar geometries. The results also establish several practical attributes of our approach. The LSBAW device is inherently scalable and enables continuous enrichment at a prescribed location. These features benefit separations applications while also allowing concurrent observation and analysis of trap contents.

Thermal diffusivity measurements using insulating and isothermal boundary conditions.

Many methods exist to measure thermal diffusivity using either steady state or transient techniques. Steady state methods yield large experimental error and inaccuracies. Transient techniques, namely, the laser flash method, are expensive and require specialized equipment and advanced data analysis. In this paper, a novel experimental setup is devised to evaluate thermal diffusivity. In this experiment hot isothermal and insulating boundary conditions are imposed on a flat disk sample. The transient temperature profile of the insulated side of the sample is analytically similar to a classic time constant formulation. The thermal diffusivity is proportional to the inverse time constant. This method hosts a variety of advantages over other methods such as accuracy comparable to other methods, low cost, integrated modeling of interface effects, and small sample size. Several materials with low to medium thermal diffusivity (0.1 → 3 mm(2)/s) have been measured. The diameter of the sample is 32 mm and its thickness ranges from 2 to 6.5 mm. The thermal diffusivity measurements in this experiment have an accuracy of 5% or better in comparison to the literature values.