SURFACE ACOUSTIC WAVES FOR CONCENTRATING SOLUTES
UPON EVAPORATION OF SESSILE DROPLETS
Dileep Mampallil, Julien Reboud, Rab Wilson and Jonathan M. Cooper*
Division of Biomedical Engineering, University of Glasgow,
Oakfield Avenue, Glasgow, G12 8LT, UK
ABSTRACT
Surface acoustic waves (SAW) are mechanical waves often generated by actuating a piezoelectric
wafer electrically. When SAW is applied to sessile droplets, internal flows are generated by acoustic
streaming, together with oscillations of the droplets. We use this property to suppress the ubiquitous
formation of ring-like, spatially heterogeneous residues upon evaporation of sessile droplets. Our method
is novel in the fact that it is insensitive to the wetting properties of the substrate and is applicable to a
wide range of solutions, from colloidal suspensions to biological samples.
KEYWORDS: Surface acoustic waves, evaporation, droplet, coffee stain effect
INTRODUCTION
Evaporation of sessile droplets containing non-volatile solutes leaves behind ring-like, spatially heterogeneous residues. Deegan et al. explained that this phenomenon, called the coffee stain effect, originates from the pinning of the three-phase contact line of the droplet and the resulting evaporation-driven
convective flux of liquid towards the contact line [1]. The formation of the ring-like stains compromises
the performance of many processes, which involve drying of sessile droplets; for instance, DNA microarrays [2] or matrix-assisted laser desorption/ionization (MALDI) [3] spectroscopy.
Various techniques such as introducing Marangoni effect [4], electric field inside the droplet [5],
modification of relative thermal conductivity of the substrate [6], modifying the shape of the particles [7]
or electrowetting (EW) [8,9] have been introduced to overcome the coffee stain effect. However, these
methods are either invasive or require specific substrate properties, which on the other hand may enhance
unwanted surface adsorption especially when biological samples are used.
To enable the use of biological samples, we introduce a non-invasive technique that involves the use
of SAW [10]. SAW can generate strong circulating flows that prevent the solute deposition at the contact line. Additionally, on hydrophobic surfaces SAW shakes the contact line and on hydrophilic surfaces
it enables the contact line to retract without pinning. These properties result in the formation of concentrated spot-like residue upon evaporation as illustrated in the figure. 1.
EXPERIMENTAL
The SAW transducer contained patterned gold interdigitated electrodes, with a pitch of 130 µm, and
a width of 100 µm, on a 128o Y-cut X-propagating LiNbO3 wafer. We used three types of superstrates of
typical area 25 × 25 mm2: (i) <100> Si wafer coated with 100 nm thick Au layer with 10 nm Ti as
adhesion layer by evaporation, (ii) hydrophobized (vide infra) <100> Si wafer and (iii) Si wafer with an
array of holes (diameter of 160 μm, pitch 200 μm, depth 235 μm) called phononic array. The superstrate
was placed at the edge of the SAW-transducer substrate as illustrated in figure 1, with about 5 μl KY
jelly (Johnson & Johnson) as the coupling agent. The details about the fabrication of the SAW
transducer, phononic array, hydrophobization and the electrical connections can be found in ref. [11,12].
Solution containing polystyrene (PS) fluorescent beads of diameter 5 µm with volume fraction 0.02%
was prepared in deionized water. Bovine serum albumin (BSA) solution was prepared in deionized water
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18th International Conference on Miniaturized
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with concentrations 1.2 mg/ml containing 5 µm polystyrene fluorescent beads with a volume fraction of
0.01%. Screened blood samples were diluted 50 times in the above prepared BSA solution.
Figure 1: Illustration of the SAW setup. When SAW is applied asymmetrically (partially blocked by the phononic array), a spot-like residue is formed after evaporation. Red arrows indicate the flow direction.
RESULTS AND DISCUSSION
As shown in figure 2, drying with SAW on, resulted in the spot-like accumulation of the beads, while the
undisturbed drying resulted in a ring-like heterogeneous deposition. The method works with a broad
range of volume fractions of the beads as depicted in figure 3. Without SAW, at high volume fractions
(0.005% and 0.02%) the contact line pins strongly at the early stage of the evaporation, resulting in larger
rings relative to the case with low volume fraction (0.001%). With SAW, independent of the volume
fractions the deposition at the contact line is negligible due to the fast flows (several mm/s). It resulted in
small spot-like residues.
Figure 2: Intensity profile of the residues of PS fluorescent beads. When SAW is applied the intensity is more
homogenous at the center. Inset: the images of the residue prepared on the substrate with phononic array.
Figure 3: The diameter of the residues vs initial volume
fraction of the PS beads. It shows the suppression of the
coffee stain effect at wide range of volume fractions.
The strong flows induced by SAW can overcome the adsorption of proteins in biological samples onto
the surface. We demonstrate that the beads can get concentrated even when they are dispersed in a solution containing a protein, for example, BSA (figure 4 (a) & (b)). Without SAW the contact line gets
strongly pinned resulting in a ring-like residue. On the other hand, with SAW the strong flows prevent
the adsorption of the proteins at the contact line. This facilitates relatively smooth retraction of the contact line while drying. The increased BSA concentration at the end of the evaporation forms a slightly
distributed residue. Similar results were obtained using blood samples as shown in figure 4 (c) & (d).
Mainly, the red blood cells are found to be accumulated at the center. We assume that the white patch is
due to the salts adsorbed on to the surface. One of the applications of the suppression of the coffee stain
effect is sample preparation for MALDI analysis (figure 5).
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As long as the substrate is acoustically conducting, e.g. glass or metals, SAW can be used to suppress
the coffee stain effect. The droplets can be deposited directly on the steel MALDI-plate without any surface modification.
Figure 4: The residues formed on hydrophobized silicon
surface with (panels a and c) and without (panels b and
d) the coffee stain effect present. (a, b) A mixture of
polystyrene beads and BSA. (c, f) Diluted blood sample.
Figure 5: The residues formed on Au coated hydrophilic
surface. The solution contained 5 nM BSA in saturated
CHCA matrix solution. The concentrated spot-like
residue is expected to give enhanced signals from
MALDI analysis (like in ref [7] but with EW instead of
SAW).
CONCLUSION
In conclusion, we have presented a technique to prepare concentrated dried samples based on SAW.
This technique requires no prior surface modifications and can work directly on glass or metal surfaces.
REFERENCES
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CONTACT
* Phone: +44 (0)141-330 5231; [email protected]
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