Biochemical Analysis Techniques -> Discrete fluidics
- Superhydrophobic preconcentration and cryopreservation (NEW)
The preconcentration of analytes is important in biochemical analysis as it offers the ability to detect trace species, and increase signal-to-noise ratios when using optical sensing. A strong advantage of the evaporation technique lies in its ability to operate without any external energy source. We have found that the evaporation of droplets on typical surfaces is susceptible to the coffee staining effect that leaves remnant fluorescence on the surface. This is less pronounced with hydrophobic surfaces but shown here to be still significant. The use of a superhydrophobic surface is demonstrated to engender miniscule fluorescent material losses. The difficulty of locating a droplet on this surface is addressed using wells that accommodated a comfortable tilt working range. Experiments revealed preconcentration rates to be faster with lower humidity but maximal concentrations was achieved using 50% relative humidity. We have also shown the ability to attain solid-liquid state changes of the spherical shaped droplets of analytes using sub-zero temperatures. This allows for tandem preconcentration and cryopreservation without any analyte transfer, leading to the advantages of minimal sample handling loss and contamination. With EGFP used as a model protein, we found no deterioration in fluorescence for up to 8 freeze-thaw cycles.
F. Shao, T.W. Ng, O.W. Liew, J. Fu, T. Sridhar Evaporative preconcentration and cryopreservation of fluorescent analytes using superhydrophobic surfaces. Soft Matter. Accepted 13/01/2012.
- Discrete transfer using liquid bridges
Discrete microfluidics offers distinct advantages over continuous microfluidics as the need for flow presents significant problems. We demonstrate here a method of achieving gentle transfer of liquid samples between two capillaries with the use of air actuation which limits flow and is amenable to automation. As the stability of liquid bridges is in operation, there is a relationship established between the gap distance and the liquid volume, thereby resulting in three physical response types that were identified. Only one of these allows for efficient liquid transfer. We advanced a model for the optimal gap distance and show it to match well with the experimental data. During the process of liquid transfer, favorable mixing is also achieved. The method is highly amenable for use in capillary well microplates .
J.K.K. Lye, T.W. Ng, W.Y.L. Ling Discrete microfluidics transfer across capillaries using liquid bridge stability. Journal of Applied Physics. Accepted 30/10/2011.
- Putting on a bubble brake on droplets
The use of droplets in isolation is a logical solution that is, not surprisingly, finding increasing use in assay applications. The obvious challenge in designing a device for such a purpose lies in moving discrete liquid drops on a surface. In addition, there is a challenge then to deliver the motive energy source only to specific droplets while leaving the others unmoved. We have developed a novel solution of momentarily pinning specific droplets to the surface while allowing the rest to be moved. We demonstrate this concept via the injection of a sizeable bubble which is attached to a PTFE surface within a droplet. This then affects the contact line of the droplet, pinning it despite the introduction of an incline that will normally result in sliding. The use of bubbles offers easy release of pinning at will by simple rupture using mechanical means.
W.Y.L. Ling, T.W. Ng, A. Neild, Effect of an encapsulated bubble in inhibiting droplet sliding. Langmuir. 26 (2010) 17695.
- Small bubbles to lubricate droplets
Smaller bubbles cause the droplet to behave in a different way. By taking advantage of the spontaneous and copious formation of visible air bubbles within water droplets on a polytetrafluoroethylene (PTFE) surface, we uncovered a direct correlation between their presence and the ability of droplets to slide down an incline. We forward two possible mechanisms to account for this behavior. The first is attributed to the air bubbles creating regions where additional solid-liquid-vapor phase interfaces are present; wherein due to the buoyancy force acting upwards, the orientation of the contact angles of each bubble (which should also be in hysteresis but in the opposite direction of the hysteresis at the droplet rim contact lines) dictate that the net force of the bubbles in the droplet act down an incline. We found that this mechanism cannot fully account for the bubble enhanced sliding behavior. The second mechanism is based on the occurrence of the droplet front advancing first, causing the droplet to elongate and thus allowing the receding contact line to partially sweep inwards over the bubbles. This causes a series of point-wise disruptions on the contact line that permits the droplet to slide down more readily. To watch bubbles in droplet disappearing over time click here.
W.Y.L. Ling, T.W Ng, A. Neild, Q. Zheng, Sliding variability of droplets on a hydrophobic incline due to surface entrained air bubbles. Journal of Colloid and Interface Science. 354 (2010) 832-842.
- Bubbles behaving different on superhydrophobic surfaces
For inverted superhydrophobic surfaces, it has been reported that the tendency of an air bubble to collapse as buoyancy brings it to the surface. This behavior is attributed to the violation of the wetting film – the thin layer of liquid that separates the solid surface from the air bubble – when interacting with air pockets entrained within the nano-structured surface. The case with non-inverted superhydrophobic surfaces yields an interesting result. We compared computational and theoretical models of millimeter sized bubbles placed on upright hydrophobic and superhydrophobic surfaces with experimental data. Whilst the experimental data for a hydrophobic surface corroborated with computational and theoretical data, the case of a superhydrophobic surface showed the bubbles to be able to contain significantly larger volumes than predicted. This is attributed to the greater ability of the bubble contact line to advance compared with its tendency to detach from the surface due to buoyancy.
W.Y.L. Ling, G. Lu, T.W. Ng, Increased stability and size of a bubble on a superhydrophobic surface. Langmuir. 27 (2011) 3233–3237.
- Pulling discrete volumes from capillaries
Pressure differentials are routinely used to actuate flow in capillaries, even with discrete volumes. We advance here an alternative means of flow generation that capitalizes on the extension of a liquid bridge achieved by the drawing of a rod through the action of surface tension. This meets the exigencies of creating controllable flow using simpler and more compact means. We found the ability to generate controllable flow to be strongly affected by the liquid bridge sustaining features, and that the use of rod diameters larger than the capillary was more conducive. The extensional flow resulting from the rupture of the liquid bridge was also found to have a strong circulation component which facilitated mixing. The approach here is highly amenable for use in capillary well microplates which have significant advantages over standard microplates. The features of this approach offer usage possibilities in biochemical applications in the field, such as in the leukocyte cell adhesion and hemagglutination tests of blood samples. Click here to see video of flow generated.
W. Schwalb, T.W. Ng, J.K.K. Lye, O.W. Liew, B.H.-P. Cheong Surface tension drawing of liquid from microplate capillary wells. Journal of Colloid and Interface Science. Accepted 15/09/2011.
- Rivulets show circulation flow for mixing
The dispensation of low fluid flow rates from a syringe results in a series of droplets sliding down the inclined surface. As flow rate increases, the distance between successive droplets decreases to ultimately form a single straight laminar rivulet. These rivulets are narrow streams of liquid flowing along a solid surface and sharing its interface with the surrounding fluid (air). They typically manifest in different regimes depending on the flow rate and properties of the solid-liquid surface system. We have examined the transient regime in which the conditions are such that a linear rivulet is formed. We tackle this by considering two regimes in the progression. The first is that of an initially forming rivulet in which a drop descends the slope with a trail of fluid behind. The second is a transient formed rivulet which starts when the droplet head of the forming rivulet meets the end of the solid surface incline. Within these regimes we experimentally examine the nature of the flow and demonstrate their peculiar upstream flow characteristics. The findings here strongly challenge common assumptions made regarding cleaning, in that fluid will exclusively transport particulate matter downhill. To watch the peculiar behavior of particles being able to transport upstream in a formed rivulet via this mechanism click here.
J.K.K. Lye, A. Neild, T.W. Ng, Strong upstream flow characteristics in the formation of rivulets. Physical Review E. 83 (2011) 026304.
Major Research Thrusts
Biochemical Analysis TechniquesWe seek to develop better methods, assays, and procedures that enable analysis of substances found in living organisms and the chemical reactions underlying life processes.
Particle/Cell ManipulationOur quest to find novel methods to move particles and cells around to accomplish important functions.
Instrumentation DevelopmentThe natural outcome of experimentation where we produce more advanced sensors, actuators, processors, and software.