ABSTRACT
Foam-based solid coatings appear to be a simple solution for giving new properties to solid surfaces. An efficient method is presented for producing open-cell foam coatings having a tunable pore radius distribution (i.e. monodisperse within the range 100-1000 µm, bidisperse or fully polydisperse), tunable thickness, and tunable bulk and surface porosities. This is achieved by mixing a precursor aqueous foam and particle suspension (here a micrometer-sized polyurethane dispersion), and by spreading with a nozzle the resulting particle-loaded foam on the solid surface to be coated. It is shown that the bubble size distribution of the precursor foam can be preserved in the final solid coating. This is highlighted by using a monodisperse aqueous foam, for which coatings showed a polycrystalline structure, as well as bidisperse or fully polydisperse foams. As a major advantage of our method, the bubble size and solid volume fraction are shown to be independent parameters, allowing the size of the microstructural elements to be tuned easily, and so the expected functional properties of the coating. Results obtained with the studied polyurethane dispersion are expected to be reproduced with other dispersions.
ABSTRACT
Foaming of particulate suspensions, followed by foam drying, is developed as an efficient method for production of highly porous materials with various applications. A key factor for success is the appropriate choice of surfactants which both modify the particle surface and stabilize the foam. Here we compare the efficiency of this method for silica suspensions containing two surfactants which lead to very different types of foam stabilization. Cationic TTAB leads to particle-stabilized foams (Pickering stabilization) whereas zwitterionic CAPB - to surfactant-stabilized foams. Thus we determined the general (common) features shared between the various surfactant systems: (1) The foaminess is controlled exclusively by the suspension viscosity under shearing conditions which mimic precisely the foaming process; (2) The foam stability to drainage and coarsening is controlled exclusively by the suspension yield stress; (3) The surfactant adsorption on the particle surface should occur in the time scale of seconds to minutes, thus ensuring appropriate rheological properties of the foaming suspension. Similar kinetic effects could be of high interest to other colloid systems and processes, e.g. for kinetic control of the internal structure and properties of aerogels produced from sheared suspensions, and for control of the transient rheological properties and non-Newtonian flow of particulate gels.
ABSTRACT
Oligodeoxynucleotides have been covalently linked to a 9-aminoacridine derivative via their 3'-phosphate group. Specific complexes are formed with the complementary sequence of the oligonucleotide. The stability is strongly increased due to intercalation of the acridine derivative. Absorption, fluorescence, nuclear magnetic resonance and circular dichroism have been used to characterize complex formation. The stability of the complexes depends on the length of the linker between the acridine derivative and the 3'-phosphate group of the oligonucleotide. Oligonucleotides covalently linked to an intercalating agent can be used to selectively control gene expression. Transcription initiation can be blocked when such an oligonucleotide binds to the transcribed strand in the open complex formed by E. coli RNA polymerase with the bla promoter. With some oligonucleotides, non-specific effects on transcription can be detected, most probably due to binding of the modified oligonucleotide to RNA polymerase. Translation of the messenger RNA from gene 32 of phage T4 can be prevented by using an oligonucleotide complementary to the sequence upstream from the Shine-Dalgarno sequence. Inhibition of translation does not occur in the absence of the intercalating agent covalently linked to the oligonucleotide nor with oligonucleotides which do not have a target sequence on the mRNA.
