No abstract

We report an elastocapillary flow—driven lab on a membrane device based on differential wetting and sedimentation effect for the separation of plasma from whole blood. Interaction between a thin polydimethylsiloxane (PDMS) membrane (thickness ~35 μm) bonded to the edge of a PDMS substrate and a sample blood drop (of volume ~70 μl) gives rise to deformation of the soft membrane due to the capillary forces providing a conduit and consequent elastocapillary flow of blood. The surface of the PDMS membrane is hydrophilic up to a certain length along the flow direction to support the elastocapillary flow and hydrophobic thereafter to impede the flow. In the hydrophobic region, owing to a much smaller sedimentation time scale (~100 s) as compared to the capillary flow time scale (~1000 s), sedimentation of blood cells occurs thus facilitating separation of plasma from the blood cells in the hydrophobic region. The role of differential wetting and sedimentation effects on the blood plasma separation is studied. The effects of membrane width and thickness, length of the hydrophilic region, erythrocyte sedimentation rate (ESR) on the separation of plasma were investigated. Using a membrane of width 3 mm, thickness 35 μm, total length 25 mm and hydrophilic length of 4 mm and 70 μl of whole blood with ESR varying in the range 4 mm h−1 to 40 mm h−1, the volume of plasma was in the range of 7.5 μl to 20 μl, respectively, which corresponds to a plasma recovery of 22%–49%, respectively. Purity of the plasma from the proposed device was compared with that obtained from centrifugation which showed a good match. The device was integrated with a commercially available detection strip to detect the level of glucose present in the plasma from blood samples of healthy and diabetic patients which are in qualitative agreement with that obtained from conventional tests.

We report bio-inspired (from a hummingbird's tongue) liquid transport via elastocapillary interaction of a thin membrane with a liquid meniscus. A soft wedge-thin rectangular membrane forming a wedge with a rigid substrate and a flat thin rectangular membrane undergo large deformation while interacting with liquid menisci. The membrane deformation leads to the formation of confinement which in turn results in elastocapillary flow along the membrane length. A simple theoretical model based on the Euler Bernoulli law is used to predict the membrane deformation profiles, which compare well with that obtained from experiments. In the wedge case, the membrane surface and liquid are selected such that the Concus-Finn criterion is not satisfied to contrast the present case of elastocapillary flow from the typical corner flow reported in the literature. The meniscus location versus time studies indicated that the flow exhibits the typical Washburn regime with , except for a sudden increase in velocity at the end of the membrane length. The effects of membrane thickness and width, liquids and substrates were studied to determine the expression for the modified Washburn constant W in both the wedge and flat membranes. It was found that gravity plays a role for Bo > 0.94 and for Bo = 1.9, the effect of inclination angle on the flow was studied. The elastocapillary flow with thin membranes could open up an opportunity for a new area, namely "membrane microfluidics" or "lab on a membrane", for diagnostics and other applications.

We report the behavior of a thin and flat rectangular polymeric membrane, fixed at both ends, in contact with a solvent droplet. Depending on the solvent type and volume, and membrane thickness, three different regimes —no buckling, buckling, and snapping— are observed. Our study reveals that the behavior depends on the solubility parameter of the solvent and the ratio of the sum of swelling-induced force and capillary force to the elastic restoring force, i.e., the force ratio. We attempt to explain the phenomenon using the poroelastic and elastic recoil timescales. Using correlations from simple scaling, our model can estimate the maximum force ratio in terms of known quantities and predict the operating regime and maximum deflection. We expect our study will find significance in the design and analysis of systems involving flexible microbeams.

We report the interaction of counter elastocapillary flows in parallel microchannels across a thin membrane. At the crossing point, the interaction between the capillary flows via the thin membrane leads to significant retardation of capillary flow. The drop in velocity at the crossing point and velocity variation after the crossing point are predicted using the analytical model and measured from experiments. A non-dimensional parameter J, which is the ratio of the capillary force to the mechanical restoring force, governs the drop in velocity at the crossing point with the maximum drop of about 60% for J = 1. The meniscus velocity after the crossing point decreases (J < 0.5), remains constant (0.5 < J < 0.6), or increases (J > 0.6) depending on the value of J. The proposed technique can be applied for the manipulation of capillary flows in microchannels.