@article{RajuKosmellaFribergetal.2017, author = {Raju, Rajarshi Roy and Kosmella, Sabine and Friberg, Stig E. and Koetz, Joachim}, title = {Pickering Janus emulsions and polyelectrolyte complex-stabilized Janus gels}, series = {Colloids and surfaces : an international journal devoted to the principles and applications of colloid and interface science ; A, Physicochemical and engineering aspects}, volume = {533}, journal = {Colloids and surfaces : an international journal devoted to the principles and applications of colloid and interface science ; A, Physicochemical and engineering aspects}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0927-7757}, doi = {10.1016/j.colsurfa.2017.08.022}, pages = {241 -- 248}, year = {2017}, abstract = {Janus emulsions, containing olive oil (OO) and silicone oil (SiO), were formed in presence of polyelectrolyte complex particles, i.e., gelatin-sodium polyacrylate (NaPAA) complexes. The diameter of completely engulfed Janus droplets can be tuned between 50 and 200 mu m by varying the gelatin/NaPAA complex particle size between 200 and 400 nm. The gelatin/NaPAA complex particles adsorbed at the olive oil interface decrease the interfacial tension and stabilize the resulting completely engulfed Pickering Janus emulsions. Long-term stable Janus gels can be synthesized in presence of gelatin/sodium carboxymethylcellulose (NaCMC) mixtures. In that case Coulombic forces are of relevance with regard to the stabilization of the Janus droplets embedded in a gelatin/NaCMC gel matrix. Janus gels show elastic reological behavior and thixotropic properties.}, language = {en} } @article{RajuLiebigKlemkeetal.2020, author = {Raju, Rajarshi Roy and Liebig, Ferenc and Klemke, Bastian and Koetz, Joachim}, title = {Ultralight magnetic aerogels from Janus emulsions}, series = {RSC Advances}, volume = {10}, journal = {RSC Advances}, number = {13}, publisher = {RSC Publishing}, address = {London}, issn = {2046-2069}, doi = {10.1039/c9ra10247g}, pages = {7492 -- 7499}, year = {2020}, abstract = {Magnetite containing aerogels were synthesized by freeze-drying olive oil/silicone oil-based Janus emulsion gels containing gelatin and sodium carboxymethylcellulose (NaCMC). The magnetite nanoparticles dispersed in olive oil are processed into the gel and remain in the macroporous aerogel after removing the oil components. The coexistence of macropores from the Janus droplets and mesopores from freeze-drying of the hydrogels in combination with the magnetic properties offer a special hierarchical pore structure, which is of relevance for smart supercapacitors, biosensors, and spilled oil sorption and separation. The morphology of the final structure was investigated in dependence on initial compositions. More hydrophobic aerogels with magnetic responsiveness were synthesized by bisacrylamide-crosslinking of the hydrogel. The crosslinked aerogels can be successfully used in magnetically responsive clean up experiments of the cationic dye methylene blue.}, language = {en} } @article{RajuLiebigKlemkeetal.2018, author = {Raju, Rajarshi Roy and Liebig, Ferenc and Klemke, Bastian and Koetz, Joachim}, title = {pH-responsive magnetic Pickering Janus emulsions}, series = {Colloid and polymer science : official journal of the Kolloid-Gesellschaft}, volume = {296}, journal = {Colloid and polymer science : official journal of the Kolloid-Gesellschaft}, number = {6}, publisher = {Springer}, address = {New York}, issn = {0303-402X}, doi = {10.1007/s00396-018-4321-z}, pages = {1039 -- 1046}, year = {2018}, abstract = {We report ultrasonically generated pH-responsive Pickering Janus emulsions of olive oil and silicone oil with controllable droplet size and engulfment. Chitosan was used as a pH-responsive emulsifier. The increase of pH from 2 to 6 leads to a transition from completely engulfed double emulsion droplets to dumbbell-shaped Janus droplets accompanied by a significant decrease of droplet diameter and a more homogeneous size distribution. The results can be elucidated by the conformational change of chitosan from a more extended form at pH 2 to a more flexible form at pH 4-5. Magnetic responsiveness to the emulsion was attributed by dispersing superparamagnetic nanoparticles (Fe3O4 with diameter of 13 +/- 2 nm) in the olive oil phase before preparing the Janus emulsion. Incorporation of magnetic nanoparticles leads to superior emulsion stability, drastically reduced droplet diameters, and opened the way to control movement and orientation of the Janus droplets according to an external magnetic field.}, language = {en} } @article{RajuKoetz2022, author = {Raju, Rajarshi Roy and Koetz, Joachim}, title = {Pickering Janus emulsions stabilized with gold nanoparticles}, series = {Langmuir : the ACS journal of surfaces and colloids / American Chemical Society}, volume = {38}, journal = {Langmuir : the ACS journal of surfaces and colloids / American Chemical Society}, number = {1}, publisher = {American Chemical Society}, address = {Washington}, issn = {0743-7463}, doi = {10.1021/acs.langmuir.1c02256}, pages = {147 -- 155}, year = {2022}, abstract = {We report a modified approach to the batch scale preparation of completely engulfed core-shell emulsions or partially engulfed Janus emulsions with colorful optical properties, containing water, olive oil, and silicone oil. The in situ reduction of gold chloride, forming gold nanoparticles (AuNPs) at the olive oil interface in the absence or presence of chitosan, leads to the formation of compartmentalized olive-silicone oil emulsion droplets in water. In the absence of additional reducing components, time-dependent morphological transformations from partial engulfment to complete engulfment were observed. Similar experiments in the presence of chitosan or presynthesized AuNPs show an opposite time-dependent trend of transformation of core-shell structures into partially engulfed ones. This behavior can be understood by a time-dependent rearrangement of the AuNPs at the interface and changes of the interfacial tension. The Pickering effect of AuNPs at oil-water and oil-oil interfaces brings not only color effects to individual microdroplets, which are of special relevance for the preparation of new optical elements, but also a surprising self-assembly of droplets.}, language = {en} } @article{RajuKoetz2021, author = {Raju, Rajarshi Roy and Koetz, Joachim}, title = {Inner rotation of Pickering Janus emulsions}, series = {Nanomaterials : open access journal}, volume = {11}, journal = {Nanomaterials : open access journal}, number = {12}, publisher = {MDPI}, address = {Basel}, issn = {2079-4991}, doi = {10.3390/nano11123312}, pages = {6}, year = {2021}, abstract = {Janus droplets were prepared by vortex mixing of three non-mixable liquids, i.e., olive oil, silicone oil and water, in the presence of gold nanoparticles (AuNPs) in the aqueous phase and magnetite nanoparticles (MNPs) in the olive oil. The resulting Pickering emulsions were stabilized by a red-colored AuNP layer at the olive oil/water interface and MNPs at the oil/oil interface. The core-shell droplets can be stimulated by an external magnetic field. Surprisingly, an inner rotation of the silicon droplet is observed when MNPs are fixed at the inner silicon droplet interface. This is the first example of a controlled movement of the inner parts of complex double emulsions by magnetic manipulation via interfacially confined magnetic nanoparticles.}, language = {en} } @phdthesis{Raju2021, author = {Raju, Rajarshi Roy}, title = {'Smart' Janus emulsions}, school = {Universit{\"a}t Potsdam}, year = {2021}, abstract = {Emulsions constitute one of the most prominent and continuously evolving research areas in Colloid Chemistry, which involves the preparation of mixtures or dispersions of immiscible components in a continuous medium. Besides conventional oil-in-water or water-in-oil emulsions, other emulsions of complex droplet morphologies have recently attracted significant research interests. Especially Janus emulsions, in which each droplet is comprised of two distinct sub-regions, have shown versatile potential applications. One of their advantages is the possibility of compartmentalization, which enables to play with two different chemistries in a single droplet. Though microfluidic methods are conventionally used to prepare Janus emulsions, their industrial applications are largely hindered by low throughput and extensive instrumentations. Recently, it has been discovered that simply one-pot moderate/high energy emulsification is also capable of developing Janus morphology, although their preparation and stabilization remain rather substantially challenging. This cumulative doctoral thesis focuses on the preparation and characterization of 'smart' Janus emulsions, i.e. Janus emulsions with special stimuli-responsive features. One-step moderate/high energy emulsification of olive and silicone oil in an aqueous medium was carried out. Special consideration was devoted to the interfacial tensions among the components to maintain the criteria of forming characteristic droplet architectures, in addition to avoiding multiple emulsion destabilization phenomena like imminent phase separation or even separated droplet formation. A series of investigations were conducted related to the formation of complexes of charged macromolecules and role of them as stabilizers to achieve stable Janus emulsions for a realistic timeframe (more than 3 months). The correlation between the size of the stabilizer particles and the droplet size of emulsion was established. Furthermore, it was observed that Janus emulsion gels with interesting rheological properties can be fabricated in the presence of suitable polyelectrolyte complexes. Janus emulsions that could be influenced by pH, temperature or magnetic field were successfully produced in presence of characteristic stimuli-responsive stabilizers. Afterwards, the effect of these changes was studied by different characterization techniques. The size and morphology could be tuned easily by changing the pH. The incorporation of iron oxide magnetic nanoparticles (synthesized separately by a co-precipitation method) to one component of the Janus emulsion was carried out so that the movement and orientation of the complex droplets in aqueous media could be controlled by an external magnetic field. Additionally, temperature-triggered instantaneous reversible breakdown of Janus droplets was also accomplished. The responses of the Janus droplets by the stimuli were well-documented and explained. Another goal of the present contribution was to exploit this special morphological feature of emulsions as a template for producing porous materials. This was demonstrated by the preparation of ultralight magnetic responsive aerogels, utilizing Janus emulsion gels. The produced aerogels also showed the capacity to separate toxic dye from water. To the best of our knowledge, this is the first example of investigation towards batch scale production of Janus emulsion with such special stimuli-responsive properties by a simple bulk emulsification method.}, language = {en} } @article{RajuLiebigHessetal.2019, author = {Raju, Rajarshi Roy and Liebig, Ferenc and Hess, Andreas and Schlaad, Helmut and Koetz, Joachim}, title = {Temperature-triggered reversible breakdown of polymer-stabilized olive}, series = {RSC Advances}, volume = {9}, journal = {RSC Advances}, number = {35}, publisher = {RSC Publishing}, address = {London}, issn = {2046-2069}, doi = {10.1039/c9ra03463c}, pages = {19271 -- 19277}, year = {2019}, abstract = {A one-step moderate energy vibrational emulsification method was successfully employed to produce thermo-responsive olive/silicone-based Janus emulsions stabilized by poly(N,N-diethylacrylamide) carrying 0.7 mol\% oleoyl side chains. Completely engulfed emulsion droplets remained stable at room temperature and could be destabilized on demand upon heating to the transition temperature of the polymeric stabilizer. Time-dependent light micrographs demonstrate the temperature-induced breakdown of the Janus droplets, which opens new aspects of application, for instance in biocatalysis.}, language = {en} }