@article{LoebnerYadavLomadzeetal.2022, author = {Loebner, Sarah and Yadav, Bharti and Lomadze, Nino and Tverdokhleb, Nina and Donner, Hendrik and Saphiannikova, Marina and Santer, Svetlana}, title = {Local direction of optomechanical stress in azobenzene containing polymers during surface relief grating formation}, series = {Macromolecular materials and engineering}, volume = {307}, journal = {Macromolecular materials and engineering}, number = {8}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1438-7492}, doi = {10.1002/mame.202100990}, pages = {10}, year = {2022}, abstract = {In this work, it is revealed how the photoinduced deformation of azobenzene containing polymers relates to the local direction of optomechanical stresses generated during irradiation with interference patterns (IPs). It can be substantiated by the modeling approach proposed by Saphiannikova et al., which describes the directional photodeformations in glassy side-chain azobenzene polymers, and proves that these deformations arise from the reorientation of rigid backbone segments along the light polarization direction. In experiments and modeling, surface relief gratings in pre-elongated photosensitive colloids of few micrometers length are inscribed using different IPs such as SS, PP, +/- 45, SP, RL, and LR. The deformation of colloidal particles is studied in situ, whereby the local variation of polymer topography is assigned to the local distribution of the electrical field vector for all IPs. Experimentally observed shapes are reproduced exactly with modeling azopolymer samples as visco-plastic bodies in the finite element software ANSYS. Orientation approach correctly predicts local variations of the main axis of light-induced stress in each interference pattern for both initially isotropic and highly oriented materials. With this work, it is suggested that the orientation approach implements a self-sufficient and convincing mechanism to describe photoinduced deformation in azopolymer films that in principle does not require auxiliary assumptions.}, 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{MuravevaBekirLomadzeetal.2022, author = {Muraveva, Valeriia and Bekir, Marek and Lomadze, Nino and Großmann, Robert and Beta, Carsten and Santer, Svetlana}, title = {Interplay of diffusio- and thermo-osmotic flows generated by single light stimulus}, series = {Applied physics letters}, volume = {120}, journal = {Applied physics letters}, number = {23}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0003-6951}, doi = {10.1063/5.0090229}, pages = {5}, year = {2022}, abstract = {Flow control is a highly relevant topic for micromanipulation of colloidal particles in microfluidic applications. Here, we report on a system that combines two-surface bound flows emanating from thermo-osmotic and diffusio-osmotic mechanisms. These opposing flows are generated at a gold surface immersed into an aqueous solution containing a photo-sensitive surfactant, which is irradiated by a focused UV laser beam. At low power of incoming light, diffusio-osmotic flow due to local photo-isomerization of the surfactant dominates, resulting in a flow pattern oriented away from the irradiated area. In contrast, thermo-osmotic flow takes over due to local heating of the gold surface at larger power, consequently inducing a flow pointing toward the hotspot. In this way, this system allows one to reversibly switch from outward to inward liquid flow with an intermittent range of zero flow at which tracer particles undergo thermal motion by just tuning the laser intensity only. Our work, thus, demonstrates an optofluidic system for flow generation with a high degree of controllability that is necessary to transport particles precisely to desired locations, thereby opening innovative possibilities to generate advanced microfluidic applications.}, language = {en} } @article{SharmaBekirLomadzeetal.2022, author = {Sharma, Anjali and Bekir, Marek and Lomadze, Nino and Jung, Se-Hyeong and Pich, Andrij and Santer, Svetlana}, title = {Generation of local diffusioosmotic flow by light responsive microgels}, series = {Langmuir}, volume = {38}, journal = {Langmuir}, number = {20}, publisher = {American Chemical Society}, address = {Washington}, issn = {0743-7463}, doi = {10.1021/acs.langmuir.2c00259}, pages = {6343 -- 6351}, year = {2022}, abstract = {Here we show that microgels trapped at a solid wall can issue liquid flow and transport over distances several times larger than the particle size. The microgel consists of cross-linked poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-AA) polymer chains loaded with cationic azobenzene-containing surfactant, which can assume either a trans-or a cis-state depending on the wavelength of the applied irradiation. The microgel, being a selective absorber of trans-isomers, responds by changing its volume under irradiation with light of appropriate wavelength at which the cis-isomers of the surfactant molecules diffuse out of the particle interior. Together with the change in particle size, the expelled cis-isomers form an excess of the concentration and subsequent gradient in osmotic pressure generating a halo of local light-driven diffusioosmotic (l-LDDO) flow. The direction and the strength of the l-LDDO depends on the intensity and irradiation wavelength, as well as on the amount of surfactant absorbed by the microgel. The flow pattern around a microgel is directed radially outward and can be maintained quasi-indefinitely under exposure to blue light when the trans-/cis-ratio is 2/1, establishing a photostationary state. Irradiation with UV light, on the other hand, generates a radially transient flow pattern, which inverts from inward to outward over time at low intensities. By measuring the displacement of tracer particles around neutral microgels during a temperature-induced collapse, we can exclude that a change in particle shape itself causes the flow, i.e., just by expulsion or uptake of water. Ultimately, it is its ability to selectively absorb two isomers of photosensitive surfactant under different irradiation conditions that leads to an effective pumping caused by a self-induced diffusioosmotic flow.}, language = {en} } @article{TitovSharmaLomadzeetal.2021, author = {Titov, Evgenii and Sharma, Anjali and Lomadze, Nino and Saalfrank, Peter and Santer, Svetlana and Bekir, Marek}, title = {Photoisomerization of an azobenzene-containing surfactant within a micelle}, series = {ChemPhotoChem}, volume = {5}, journal = {ChemPhotoChem}, number = {10}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {2367-0932}, doi = {10.1002/cptc.202100103}, pages = {926 -- 932}, year = {2021}, abstract = {Photosensitive azobenzene-containing surfactants have attracted great attention in past years because they offer a means to control soft-matter transformations with light. At concentrations higher than the critical micelle concentration (CMC), the surfactant molecules aggregate and form micelles, which leads to a slowdown of the photoinduced trans -> cis azobenzene isomerization. Here, we combine nonadiabatic dynamics simulations for the surfactant molecules embedded in the micelles with absorption spectroscopy measurements of micellar solutions to uncover the reasons responsible for the reaction slowdown. Our simulations reveal a decrease of isomerization quantum yields for molecules inside the micelles. We also observe a reduction of extinction coefficients upon micellization. These findings explain the deceleration of the trans -> cis switching in micelles of the azobenzene-containing surfactants.}, 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{SharmaBekirLomadzeetal.2020, author = {Sharma, Anjali and Bekir, Marek and Lomadze, Nino and Santer, Svetlana}, title = {Photo-Isomerization Kinetics of Azobenzene Containing Surfactant Conjugated with Polyelectrolyte}, series = {Molecules}, volume = {29}, journal = {Molecules}, number = {1}, publisher = {MDPI}, address = {Basel}, issn = {1420-3049}, doi = {10.3390/molecules26010019}, pages = {12}, year = {2020}, abstract = {Ionic complexation of azobenzene-containing surfactants with any type of oppositely charged soft objects allows for making them photo-responsive in terms of their size, shape and surface energy. Investigation of the photo-isomerization kinetic and isomer composition at a photo-stationary state of the photo-sensitive surfactant conjugated with charged objects is a necessary prerequisite for understanding the structural response of photo-sensitive complexes. Here, we report on photo-isomerization kinetics of a photo-sensitive surfactant in the presence of poly(acrylic acid, sodium salt). We show that the photo-isomerization of the azobenzene-containing cationic surfactant is slower in a polymer complex compared to being purely dissolved in aqueous solution. In a photo-stationary state, the ratio between the trans and cis isomers is shifted to a higher trans-isomer concentration for all irradiation wavelengths. This is explained by the formation of surfactant aggregates near the polyelectrolyte chains at concentrations much lower than the bulk critical micelle concentration and inhibition of the photo-isomerization kinetics due to steric hindrance within the densely packed aggregates.}, language = {en} } @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{KopyshevKanevcheLomadzeetal.2019, author = {Kopyshev, Alexey and Kanevche, Katerina and Lomadze, Nino and Pfitzner, Emanuel and Loebner, Sarah and Patil, Rohan R. and Genzer, Jan and Heberle, Joachim and Santer, Svetlana}, title = {Light-Induced Structuring of Photosensitive Polymer Brushes}, series = {ACS Applied polymer materials}, volume = {1}, journal = {ACS Applied polymer materials}, number = {11}, publisher = {American Chemical Society}, address = {Washington}, issn = {2637-6105}, doi = {10.1021/acsapm.9b00705}, pages = {301 -- 3026}, year = {2019}, abstract = {We investigate light-induced irreversible structuring of surface topographies in poly(3-sulfopropyl methacrylate/potassium salt) (PSPMK) brushes on flat solid substrates prepared by surface-initiated atom transfer radical polymerization. The brushes have been loaded with azobenzene-based surfactant comprised of positively charged headgroups and hydrophobic tail. The surfactant exhibits photoresponsive properties through photoisomerization from the trans to cis states leading to significant changes in physicochemical properties of grafted polymer chains. The azobenzene surfactant enables photoresponsive behavior without introducing irreversible changes to chemical composition of the parent polymer brush. Exposing these photosensitive brushes to irradiation with UV interference beams causes the polymer brush to form surface relief grating (SRG) patterns. The cationic surfactant penetrates only similar to 25\% of the upper portion of the PSPMK brush, resulting in the formation of two sections within the brush: a photoresponsive upper layer and nonfunctional buried layer, which is not affected by the UV irradiation. Using nano-FTIR spectroscopy, we characterize locally the chemical composition of the polymer brush and confirm partial penetration of the surfactant within the film. Strong optomechanical stresses take place only within the upper layer of the brush that is impregnated with the surfactants and causes surface topography alternation due to a local rupture of grafted polymer chains. The cleaved polymer chains are then removed from the surface by using a good solvent, leaving behind topographical grating on top of the nonfunctional brush layer. We demonstrate that photostructured polymer brush can be used for reversible switching of brush topography by varying external humidity.}, language = {en} }