Biomedical subjects

Rossen Sedev

Publications and source records attributed to Rossen Sedev.

4 recordsLinked to original sources

PubMed2006

Directed crystallisation of zinc oxide on patterned surfaces.

Patterned self-assembled monolayers of functionalised alkane thiols were prepared on gold substrates, using UV-photolithography. Two alkane thiols, 11-mercaptoundecanoic acid (MUA) and a fluorinated decane thiol (FDT, CF3(CF2)7CH2CH2SH) were used to fabricate chemically structured surfaces which served as templates for zinc oxide (ZnO) crystallisation. When these patterns, containing high (MUA) and low (FDT) surface energy regions were exposed to a 10 mM zinc nitrate crystallising solution, nucleation occurred selectively on the low energy regions. After 90 min, hexagonal prisms had grown upright on these areas. The crystal growth is uniform with a crystal length of about 1 mum and a diameter between 50 and 100 nm. We attribute the selective growth to a combination of crystallographic frustration of the zinc ions on the high energy regions and an accumulation of hydroxide ions on the low energy regions.

Journal Article↗

PubMed2006

Electrowetting of ionic liquids.

We have successfully demonstrated that imidazolium- and pyrrolidinium-based commercial room-temperature ionic liquids can electrowet (with a dc voltage) a smooth fluoropolymer (Teflon AF1600) surface. Qualitatively, the process is analogous to the electrowetting of aqueous electrolyte solutions: the contact angle versus voltage curve has a parabolic shape which saturates at larger voltages (positive or negative). On the other hand we observed several peculiarities: (i) the efficiency is significantly lower (by about an order of magnitude); (ii) the influence of the bulky cation is larger and the importance of the smaller anion is lesser, especially with respect to electrowetting saturation; (iii) there is an asymmetry in the saturation contact angles found for positive and negative voltages. The asymmetry may be correlated with the cation-anion asymmetry of the ionic liquids. The low efficiency may be caused by the presence of water and other impurities in these commercial materials.

Electrochemistry↗

PubMed2005

Marangoni effects in aqueous polypropylene glycol foams.

The foam behavior of three polypropylene glycols covering the molecular weight range between 192 and 725 g/mol has been examined. Static and dynamic surface tension data, as well as bubble size distribution and retention time in the foam, were incorporated into a simple model of foam stability. The latter clearly indicates that surface tension differences between the plateau border and lamellar region adjacent to the bubble surface are the dominant factor in controlling foamability, causing liquid flow in the direction opposite to liquid drainage, a process termed the Marangoni effect.

Journal Article↗

PubMed2005

Contact angle saturation in electrowetting.

Electrowetting is the phenomenon of contact angle decrease under the influence of an external voltage applied across the solid/liquid interface. Electrowetting offers an interesting possibility to enhance the wettability of hydrophobic materials without altering the chemical composition of the system and thus could be incorporated in various microfluidic devices. Electrowetting is fundamentally an electrocapillary effect occurring on an insulated solid electrode (hence the change of the solid/liquid interfacial tension with voltage follows Lippmann's equation). A limiting contact angle value larger than zero is achieved even at very large external voltages. Saturation precludes full wetting of the substrate and restricts the magnitude of the capillary force variation. Contact angle saturation has been given various interpretations (e.g., charge trapping, air ionization) but appears to reflect a natural thermodynamic limit rather than being simply a defective property. The limiting value of the contact angle is given by the Young equation when the value of the solid/liquid interfacial tension reaches zero. The model is in excellent agreement with our own results and often gives an adequate description of published data. It also suggests that the saturation limit is determined by the material properties of the system and electrowetting at voltages exceeding this threshold is essentially a nonequilibrium process.

Electrochemistry↗