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 minuscule fluorescent material losses. The difficulty of locating a droplet on this surface is addressed here using wells in which a depth of 0.51 mm with 7 ml analyte volume was found to accommodate a comfortable tilt working range of up to 19 . Experiments revealed preconcentration rates to be faster with lower humidity but maximal concentrations were 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 subzero 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.

The prevailing exposition of the particle deposition mechanism in an evaporating droplet for self-assembly does not lead one to expect any deviation in deposition behavior to occur when two different particle types (of differing hydrophobicity) are used while keeping all other key parameters (i.e., particle concentration, particle size, and liquid volume) comparable. It is well known that if the contact angle that the droplet makes with the surface is sufficiently low a “coffee stain” pattern results, and that if the angle is sufficiently high few deposits are left behind except a dot at the center of the dried out droplet. We explore the behavior in the large intermediary region, and find that it depends strongly on particle properties. Droplet evaporation experiments conducted with micron sized polystyrene and silica particles on glass coated with silane suggest that the basic deposition mechanism outlined can be strongly influenced by a balance between capillary forces and the drag arising from flow patterns in the droplet, and interparticle capillary force factors.

No abstract

No abstract

Capillary force mechanisms have the advantages of providing the motive force to move groups of particles to locations of interest while holding them in place, offering delicateness, and obviating the use external energy sources. We report a capillary force method that permits particles to remain hydrated, while assembled and harvested in batches using a single setup, furthermore assembly of different sizes/types is possible. The physics behind the process is described and the technique demonstrated with the formation of an ensemble of 6 μm particles.