Microfluidics and Nanofluidics

Modifications of fluid flow within microscale flow passages by internal surface roughness is investigated in the laminar, transitional, and turbulent regimes using pressure-drop measurements and instantaneous velocity fields acquired by microscopic particle-image velocimetry (micro-PIV). The microchannel under study is rectangular in cross-section with an aspect ratio of 1:2 (depth: width) and a hydraulic diameter of $$D_{\rm h} =600\,\upmu \hbox{m}.$$ Measurements are first performed under smooth-wall conditions to establish the baseline flow characteristics within the microchannel followed by measurements for two different rough-wall cases [with RMS roughness heights of $$7.51\,\upmu \hbox{m}$$ (0.0125D h) and $$15.1\,\upmu \hbox{m}$$ (0.025D h)]. The roughness patterns under consideration are unique in that they are reminiscent of surface irregularities one might encounter in practical microchannels due to imperfect fabrication methods. The pressure-drop results reveal the onset of transition above $$Re_{\rm cr}=1{,}800$$ for the smooth-wall case, consistent with the onset of transition at the macroscale, along with deviation from laminar behavior at progressively lower Re with increasing roughness. Mean velocity profiles computed from the micro-PIV ensembles at various Re for each surface condition confirm these trends, meaning $$Re_{\rm cr}$$ is a strong function of roughness. The ensembles of velocity fields at each Re and surface condition in the transitional regime are subdivided into fields embodying laminar behavior and fields containing disordered motions. This decomposition reveals a clear hastening of the flow toward a turbulent state due both to the roughness dependence of Re cr and an enhancement in the growth rate of the non-laminar fraction of the flow when the flow is in the early stages of transition. Nevertheless, the range of Re relative to Re cr over which the flow transitions from a laminar to a turbulent state is found to be essentially the same for all three surface conditions. From a structural viewpoint, instantaneous velocity fields embodying disordered behavior in the transitional regime are found to contain large-scale motions consistent with hairpin-vortex packets irrespective of surface condition. These observations are in accordance with the characteristics of transitional and turbulent flows at the macroscale and therefore indicate that the overall structural paradigm of the flow is relatively insensitive to roughness. From a quantitative viewpoint, however, the intensity of both the velocity fluctuations and structural activity appear to increase substantially with increasing roughness, particularly in the latter stages of transition. These differences are further supported by the trends of single-point statistics of the non-laminar ensembles and quadrant analysis in which an intensification of the velocity fluctuations by surface roughness is noted in the region close to the wall, particularly for the wall-normal fluctuations.

In this paper, we implement rotational flow control on a polymeric microfluidic “lab-on-a-disc” platform by combining serial siphoning and capillary valving for sequential release of a set of on-board stored liquid reagents into a common (assay) channel. The functionality of this integrated, multi-step, multi-reagent centrifugal assay platform critically depends on the capability to establish very reproducible, capillary-driven priming of the innately only weakly hydrophilic siphon microchannels made from common poly(methyl methacrylate) (PMMA) substrates. Due to the relatively high contact angle of the native PMMA substrate, it was practically impossible to ensure sequential release of on-board stored reagents using the capillary-driven serial siphon valves. In this work, we demonstrate that spin-coated hydrophilic films of poly(vinyl alcohol) (PVA) and (hydroxypropyl)methyl cellulose (HPMC) provide stable contact angles on PMMA substrates for more than 60 days. The deposited films were characterized using contact angle measurements, surface energy calculations and X-ray photoelectron spectroscopy spectra. The PVA and HPMC films reduced the water contact angle of the PMMA substrate from 68° to 22° and 27° while increasing their surface energies from 47 to 62 and 57 mN m−1, respectively. On the centrifugal microfluidic platform, the films were validated to enable the effective and reproducible priming of the serial siphon microchannels at low rotational frequencies while ensuring that the in-line capillary valves are not opened until their respective burst frequencies are passed. Furthermore, the biocompatibility of the proposed surface modification method was examined, and the platform was used to run a sandwich immunoassay for the detection of human immunoglobulin G, and its performance was proven to be comparable to dynamic coating using surfactants.

Despite the enormous scientific and technological importance of micro-channel gas flows, the understanding of these flows, by classical fluid mechanics, remains incomplete including the prediction of flow rates. In this paper, we revisit the problem of micro-channel compressible gas flows and show that the axial diffusion of mass engendered by the density (pressure) gradient becomes increasingly significant with increased Knudsen number compared to the pressure driven convection. The present theoretical treatment is based on a recently proposed modification (Durst et al. in Proceeding of the international symposium on turbulence, heat and mass transfer, Dubrovnik, 3–18 September, pp 25–29, 2006) of the Navier–Stokes equations that include the diffusion of mass caused by the density and temperature gradients. The theoretical predictions using the modified Navier–Stokes equations are found to be in good agreement with the available experimental data spanning the continuum, transition and free-molecular (Knudsen) flow regimes, without invoking the concept of Maxwellian wall-slip boundary condition. The simple theory also results in excellent agreement with the results of linearized Boltzmann equations and Direct Simulation Monte Carlo (DSMC) method. Finally, the theory explains the Knudsen minimum and suggests the design of future micro-channel flow experiments and their employment to complete the present days understanding of micro-channel flows.

The lubricant-infused surface (LIS) has emerged as a promising drag reduction surface for flow enhancement. At present, there are some applications of LISs in polymer-based microchannels for drag reduction. However, the use of the LIS in metal capillaries or microchannels for drag reduction or flow enhancement is lacking. The present work proposes a method for the fabrication of a LIS on the inner surface of copper (Cu) capillaries to grant them sustainable drag reduction properties. Cu capillaries with an inner superhydrophobic surface (SHS) (contact angle > 160°) are also prepared as a reference for comparison. To test the hydrodynamic performance under different working conditions, we fabricated LIS Cu capillaries with lubricants of varying viscosities, and their frictional factors were experimentally measured with a Reynolds number (Re) ranging from 0 to 1500. The drag reduction of the LIS Cu capillary (32%) is slightly lower than that of the SHS Cu capillary (36%), but the LIS Cu capillary has much better sustainability. The threshold Re for initiating the failure of LIS Cu capillaries is almost three times that of the SHS Cu capillary, and the durability of the LIS Cu capillary under high shear force conditions is also much higher. The superior sustainability of the LIS Cu capillaries is due to the enhanced capillary and Van der Waals (vdW) forces caused by the composite morphology and functional groups applied to the LIS. The present study will provide useful insights for designing robust and sustainable LISs for drag reduction or flow enhancement in Cu capillaries.

Mixing of biological species in microfluidic channels is challenging since the mixing process is limited by the small mass diffusion coefficient of the species and by the dominance of viscous effects, captured by the low value of Reynolds number characteristic of laminar liquid flow in microchannels. This paper investigates the use of pulsating flows to enhance mixing in microflows. The dependence of the degree of mixing on various dimensionless groups is investigated. These dimensionless numbers are Strouhal number, pulse amplitude divided by base velocity, Reynolds number, location along the mixing channel normalized by the channel width, channel cross section aspect ratio, and phase difference between the inlet streams. The degree of mixing, observed to experience both spatial fluctuations down the mixing channel and temporal fluctuations over a pulsation cycle at the quasi-stationary state, is shown to be most sensitive to changes in pulsation amplitude and frequency. For a fixed pulsation amplitude and Reynolds number, the degree of mixing has a peak value for a certain Strouhal number above and below which the degree of mixing decreases. Increasing the pulsation amplitude improves mixing with the behavior becoming asymptotic at large pulsation amplitudes. The temporal fluctuations in the degree of mixing over a cycle at the quasi-stationary state decrease and the average degree of mixing increases downstream the mixing channel. The fluctuations are also smaller at higher values of the Strouhal number and are generally larger for larger pulsation amplitudes. This study also takes into account the rate of work input required to overcome viscous effects. While this power input is independent of the pulsation frequency, it exhibits a parabolic dependence on the pulsation amplitude. Finally, considering the dependence of the degree of mixing (mean and standard deviation), mixing length, and energy consumption on these dimensionless groups, examples of the trade-off that has to be made in choosing the operating conditions based on different constraints are presented.

Microscopic hair-like structures, such as cilia, exist ubiquitously in nature and are used by various organisms for transportation purposes. Many efforts have been made to mimic the fluid pumping function of cilia, but most of the fabrication processes of these “artificial cilia” are tedious and expensive, hindering their practical applications. In this paper, an attractive and potentially cost-effective, magnetic fiber drawing fabrication technique of magnetic artificial cilia is demonstrated. Our artificial cilia are able to generate a substantial fluid net flow velocity of water of up to 70 µm/s (corresponding to a generated volumetric flow rate about 0.6 µL/min and a pressure difference of about 0.04 Pa) in a closed-loop microfluidic channel when actuated using an external magnetic field. A detailed analysis of the relationship between the experimentally observed cilia kinematics and corresponding induced flow is in line with a previously reported theoretical/numerical study.

A microfluidic device with ferromagnetic microstructures designed to locally induce strong magnetophoretic force for capturing infected blood cells has been employed for malaria diagnosis for a long time. In this study, new configurations of V- and W-shaped nickel microstructures fabricated by an electroplating process and magnet arrays were proposed to enhance the capture efficacy from the conventional square one. The simulation of magnetophoretic and hydrodynamic forces was conducted to reveal the capture mechanism of them. After that, the microfluidic device was built and tested. The experimental results with magnetic beads showed that capture efficiency increased by almost twice as much when compared to the conventional square microstructures. Additionally, malaria-infected blood cells at the level of parasitemia at 2%, 10%, and 80% were tested, and the capture efficiency was in agreement with the tests with magnetic beads.