@article{AryaFeldmannKopyshevetal.2020, author = {Arya, Pooja and Feldmann, David and Kopyshev, Alexey and Lomadze, Nino and Santer, Svetlana}, title = {Light driven guided and self-organized motion of mesoporous colloidal particles}, series = {Soft matter}, volume = {16}, journal = {Soft matter}, number = {5}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1744-683X}, doi = {10.1039/c9sm02068c}, pages = {1148 -- 1155}, year = {2020}, abstract = {We report on guided and self-organized motion of ensembles of mesoporous colloidal particles that can undergo dynamic aggregation or separation upon exposure to light. The forces on particles involve the phenomenon of light-driven diffusioosmosis (LDDO) and are hydrodynamic in nature. They can be made to act passively on the ensemble as a whole but also used to establish a mutual interaction between particles. The latter scenario requires a porous colloid morphology such that the particle can act as a source or sink of a photosensitive surfactant, which drives the LDDO process. The interplay between the two modes of operation leads to fascinating possibilities of dynamical organization and manipulation of colloidal ensembles adsorbed at solid-liquid interfaces. While the passive mode can be thought of to allow for a coarse structuring of a cloud of colloids, the inter-particle mode may be used to impose a fine structure on a 2D particle grid. Local flow is used to impose and tailor interparticle interactions allowing for much larger interaction distances that can be achieved with, e.g., DLVO type of forces, and is much more versatile.}, language = {en} } @article{AryaJelkenFeldmannetal.2020, author = {Arya, Pooja and Jelken, Joachim and Feldmann, David and Lomadze, Nino and Santer, Svetlana}, title = {Light driven diffusioosmotic repulsion and attraction of colloidal particles}, series = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, volume = {152}, journal = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, number = {19}, publisher = {American Institute of Physics}, address = {Melville, NY}, issn = {0021-9606}, doi = {10.1063/5.0007556}, pages = {10}, year = {2020}, abstract = {In this paper, we introduce the phenomenon of light driven diffusioosmotic long-range attraction and repulsion of porous particles under irradiation with UV light. The change in the inter-particle interaction potential is governed by flow patterns generated around single colloids and results in reversible aggregation or separation of the mesoporous silica particles that are trapped at a solid surface. The range of the interaction potential extends to several times the diameter of the particle and can be adjusted by varying the light intensity. The "fuel" of the process is a photosensitive surfactant undergoing photo-isomerization from a more hydrophobic trans-state to a rather hydrophilic cis-state. The surfactant has different adsorption affinities to the particles depending on the isomerization state. The trans-isomer, for example, tends to accumulate in the negatively charged pores of the particles, while the cis-isomer prefers to remain in the solution. This implies that when under UV irradiation cis-isomers are being formed within the pores, they tend to diffuse out readily and generate an excess concentration near the colloid's outer surface, ultimately resulting in the initiation of diffusioosmotic flow. The direction of the flow depends strongly on the dynamic redistribution of the fraction of trans- and cis-isomers near the colloids due to different kinetics of photo-isomerization within the pores as compared to the bulk. The unique feature of the mechanism discussed in the paper is that the long-range mutual repulsion but also the attraction can be tuned by convenient external optical stimuli such as intensity so that a broad variety of experimental situations for manipulation of a particle ensemble can be realized.}, language = {en} } @article{AryaJelkenLomadzeetal.2020, author = {Arya, Pooja and Jelken, Joachim and Lomadze, Nino and Santer, Svetlana and Bekir, Marek}, title = {Kinetics of photo-isomerization of azobenzene containing surfactants}, series = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistry}, volume = {152}, journal = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistry}, number = {2}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-9606}, doi = {10.1063/1.5135913}, pages = {10}, year = {2020}, abstract = {We report on photoisomerization kinetics of azobenzene containing surfactants in aqueous solution. The surfactant molecule consists of a positively charged trimethylammonium bromide head group, a hydrophobic spacer connecting via 6 to 10 CH2 groups to the azobenzene unit, and the hydrophobic tail of 1 and 3CH(2) groups. Under exposure to light, the azobenzene photoisomerizes from more stable trans- to metastable cis-state, which can be switched back either thermally in dark or by illumination with light of a longer wavelength. The surfactant isomerization is described by a kinetic model of a pseudo first order reaction approaching equilibrium, where the intensity controls the rate of isomerization until the equilibrated state. The rate constants of the trans-cis and cis-trans photoisomerization are calculated as a function of several parameters such as wavelength and intensity of light, the surfactant concentration, and the length of the hydrophobic tail. The thermal relaxation rate from cis- to trans-state is studied as well. The surfactant isomerization shows a different kinetic below and above the critical micellar concentration of the trans isomer due to steric hindrance within the densely packed micelle but does not depend on the spacer length.}, language = {en} } @article{BekirJelkenJungetal.2021, author = {Bekir, Marek and Jelken, Joachim and Jung, Se-Hyeong and Pich, Andrij and Pacholski, Claudia and Kopyshev, Alexey and Santer, Svetlana}, title = {Dual responsiveness of microgels induced by single light stimulus}, series = {Applied physics letters}, volume = {118}, journal = {Applied physics letters}, number = {9}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0003-6951}, doi = {10.1063/5.0036376}, pages = {6}, year = {2021}, abstract = {We report on the multiple response of microgels triggered by a single optical stimulus. Under irradiation, the volume of the microgels is reversibly switched by more than 20 times. The irradiation initiates two different processes: photo-isomerization of the photo-sensitive surfactant, which forms a complex with the anionic microgel, rendering it photo-responsive; and local heating due to a thermo-plasmonic effect within the structured gold layer on which the microgel is deposited. The photo-responsivity is related to the reversible accommodation/release of the photo-sensitive surfactant depending on its photo-isomerization state, while the thermo-sensitivity is intrinsically built in. We show that under exposure to green light, the thermo-plasmonic effect generates a local hot spot in the gold layer, resulting in the shrinkage of the microgel. This process competes with the simultaneous photo-induced swelling. Depending on the position of the laser spot, the spatiotemporal control of reversible particle shrinking/swelling with a predefined extent on a per-second base can be implemented.}, language = {en} } @article{BekirSharmaUmlandtetal.2022, author = {Bekir, Marek and Sharma, Anjali and Umlandt, Maren and Lomadze, Nino and Santer, Svetlana}, title = {How to make a surface act as a micropump}, series = {Advanced materials interfaces}, volume = {9}, journal = {Advanced materials interfaces}, number = {12}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {2196-7350}, doi = {10.1002/admi.202102395}, pages = {11}, year = {2022}, abstract = {In this paper, the phenomenon of light-driven diffusioosmotic (DO) long-range attractive and repulsive interactions between micro-sized objects trapped near a solid wall is investigated. The range of the DO flow extends several times the size of microparticles and can be adjusted to point towards or away from the particle by varying irradiation parameters such as intensity or wavelength of light. The "fuel" of the light-driven DO flow is a photosensitive surfactant which can be photo-isomerized between trans and cis-states. The trans-isomer tends to accumulate at the interface, while the cis-isomer prefers to stay in solution. In combination with a dissimilar photo-isomerization rate at the interface and in bulk, this yields a concentration gradient of the isomers around single particles resulting in local light-driven diffusioosmotic (l-LDDO) flow. Here, the extended analysis of the l-LDDO flow as a function of irradiation parameters by introducing time-dependent development of the concentration excess of isomers near the particle surface is presented. It is also demonstrated that the l-LDDO can be generated at any solid/liquid interface being more pronounced in the case of strongly absorbing material. This phenomenon has plenty of potential applications since it makes any type of surface act as a micropump.}, language = {en} } @article{DiFlorioBruendermannYadavallietal.2014, author = {Di Florio, G. and Bruendermann, E. and Yadavalli, Nataraja Sekhar and Santer, Svetlana and Havenith, Martina}, title = {Graphene multilayer as nanosized optical strain gauge for polymer surface relief gratings}, series = {Nano letters : a journal dedicated to nanoscience and nanotechnology}, volume = {14}, journal = {Nano letters : a journal dedicated to nanoscience and nanotechnology}, number = {10}, publisher = {American Chemical Society}, address = {Washington}, issn = {1530-6984}, doi = {10.1021/nl502631s}, pages = {5754 -- 5760}, year = {2014}, abstract = {In this paper, we show how graphene can be utilized as a nanoscopic probe in order to characterize local opto-mechanical forces generated within photosensitive azobenzene containing polymer films. Upon irradiation with light interference patterns, photosensitive films deform according to the spatial intensity variation, leading to the formation of periodic topographies such as surface relief gratings (SRG). The mechanical driving forces inscribing a pattern into the films are supposedly fairly large, because the deformation takes place without photofluidization; the polymer is in a glassy state throughout. However, until now there has been no attempt to characterize these forces by any means. The challenge here is that the forces vary locally on a nanometer scale. Here, we propose to use Raman analysis of the stretching of the graphene layer adsorbed on top of polymer film under deformation in order to probe the strength of the material transport spatially resolved. With the well-known mechanical properties of graphene, we can obtain lower bounds on the forces acting within the film. Upon the basis of our experimental results, we can deduce that the internal pressure in the film due to grating formation can exceed 1 GPa. The graphene-based nanoscopic gauge opens new possibilities to characterize opto-mechanical forces generated within photosensitive polymer films.}, language = {en} } @article{DiFlorioBruendermannYadavallietal.2014, author = {Di Florio, G. and Bruendermann, E. and Yadavalli, Nataraja Sekhar and Santer, Svetlana and Havenith, Martina}, title = {Confocal raman microscopy and AFM study of the interface between the photosensitive polymer layer and multilayer graphene}, series = {Soft materials}, volume = {12}, journal = {Soft materials}, publisher = {Taylor \& Francis Group}, address = {Philadelphia}, issn = {1539-445X}, doi = {10.1080/1539445X.2014.945040}, pages = {S98 -- S105}, year = {2014}, abstract = {In this paper we report on the interaction between photosensitive azobenzene-containing polymer films and on top adsorbed graphene multilayers. The photosensitive polymer film changes its topography under irradiation with light interference patterns according to their polarization distribution. The multilayer graphene follows the deformation of the polymer film and stretches accordingly. Using confocal Raman microspectroscopy we can detect the appearance of additional peaks in the Raman spectrum of the photosensitive polymer film upon irradiation indicating a molecular interaction at the interface between the graphene multilayer and the polymer matrix. Multi-component analysis of the specific Raman bands shows that the interaction involves the graphene rings and the aromatic rings of the azobenzenes causing the strong adhesion between the two materials.}, language = {en} } @article{DiFlorioBruendermannYadavallietal.2014, author = {Di Florio, Giuseppe and Bruendermann, Erik and Yadavalli, Nataraja Sekhar and Santer, Svetlana and Havenith, Martina}, title = {Polarized 3D Raman and nanoscale near-field optical microscopy of optically inscribed surface relief gratings: chromophore orientation in azo-doped polymer films}, series = {Soft matter}, volume = {10}, journal = {Soft matter}, number = {10}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1744-683X}, doi = {10.1039/c3sm51787j}, pages = {1544 -- 1554}, year = {2014}, abstract = {We have used polarized confocal Raman microspectroscopy and scanning near-field optical microscopy with a resolution of 60 nm to characterize photoinscribed grating structures of azobenzene doped polymer films on a glass support. Polarized Raman microscopy allowed determining the reorientation of the chromophores as a function of the grating phase and penetration depth of the inscribing laser in three dimensions. We found periodic patterns, which are not restricted to the surface alone, but appear also well below the surface in the bulk of the material. Near-field optical microscopy with nanoscale resolution revealed lateral two-dimensional optical contrast, which is not observable by atomic force and Raman microscopy.}, language = {en} } @misc{DiFlorioBruendermannYadavallietal.2013, author = {Di Florio, Giuseppe and Br{\"u}ndermann, Erik and Yadavalli, Nataraja Sekhar and Santer, Svetlana and Havenith, Martina}, title = {Polarized 3D Raman and nanoscale near-field optical microscopy of optically inscribed surface relief gratings}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-95233}, pages = {1544 -- 1554}, year = {2013}, abstract = {We have used polarized confocal Raman microspectroscopy and scanning near-field optical microscopy with a resolution of 60 nm to characterize photoinscribed grating structures of azobenzene doped polymer films on a glass support. Polarized Raman microscopy allowed determining the reorientation of the chromophores as a function of the grating phase and penetration depth of the inscribing laser in three dimensions. We found periodic patterns, which are not restricted to the surface alone, but appear also well below the surface in the bulk of the material. Near-field optical microscopy with nanoscale resolution revealed lateral two-dimensional optical contrast, which is not observable by atomic force and Raman microscopy.}, language = {en} } @article{DiFlorioBruendermannYadavalletal.2014, author = {DiFlorio, Giuseppe and Br{\"u}ndermann, Erik and Yadavall, Nataraja Sekhar and Santer, Svetlana and Havenith, Martina}, title = {Polarized 3D Raman and nanoscale near-field optical microscopy of optically inscribed surface relief gratings: chromophore orientation in azo-doped polymer films}, issn = {1744-683x}, doi = {10.1039/c3sm51787j}, year = {2014}, abstract = {We have used polarized confocal Raman microspectroscopy and scanning near-field optical microscopy with a resolution of 60 nm to characterize photoinscribed grating structures of azobenzene doped polymer films on a glass support. Polarized Raman microscopy allowed determining the reorientation of the chromophores as a function of the grating phase and penetration depth of the inscribing laser in three dimensions. We found periodic patterns, which are not restricted to the surface alone, but appear also well below the surface in the bulk of the material. Near-field optical microscopy with nanoscale resolution revealed lateral two-dimensional optical contrast, which is not observable by atomic force and Raman microscopy.}, language = {en} }