TY - THES A1 - Arya, Pooja T1 - Light controlled active and passive motion of colloidal particles N2 - In this dissertation we introduce a concept of light driven active and passive manipulation of colloids trapped at solid/liquid interface. The motion is induced due to generation of light driven diffusioosmotic flow (LDDO) upon irradiation with light of appropriate wavelength. The origin of the flow is due to osmotic pressure gradient resulting from a concentration gradient at the solid/liquid interface of the photosensitive surfactant present in colloidal dispersion. The photosensitive surfactant consists of a cationic head group and a hydrophobic tail in which azobenzene group is integrated in. The azobenzene is known to undergo reversible photo-isomerization from a stable trans to a meta stable cis state under irradiation with UV light. Exposure to light of larger wavelength results in back photo-isomerization from cis to trans state. The two isomers have different molecular properties, for instance, trans isomer has a rod like structure and low polarity (0 dipole moment), whereas cis one is bent and has a dipole moment of ~3 Debye. Being integrated in the hydrophobic tail of the surfactant molecule, the azobenzene state determines the hydrophobicity of the whole molecule: in the trans state the surfactant is more hydrophobic than in the cis-state. In this way many properties of the surfactant such as the CMC, solubility and the interaction potential with a solid surface can be altered by light. When the solution containing such a surfactant is irradiated with focused light, a concentration gradient of different isomers is formed near the boundary of the irradiated area near the solid surface resulting in osmotic pressure gradient. The generated diffusioosmotic (DO) flow carries the particles passively along. The local-LDDO flow can be generated around and by each particle when mesoporous silica colloids are dispersed in the surfactant solution. This is because porous particles act as a sink/source which absorbs azobenzene molecule in trans state and expels it when it is in the cis state. The DO flows generated at each particle interact resulting in aggregation or separation depending upon the initial state of surfactant molecules. The kinetic of aggregation and separation can be controlled and manipulated by altering the parameters such as the wavelength and intensity of the applied light, as well as surfactant and particle concentration. Using two wavelengths simultaneously allows for dynamic gathering and separation creating fascinating patterns such as 2D disk of well separated particles or establishing collective complex behaviour of particle ensemble as described in this thesis. The mechanism of l-LDDO is also used to generate self-propelled motion. This is possible when half of the porous particle is covered by metal layer, basically blocking the pores on one side. The LDDO flow generated on uncapped side pushes the particle forward resulting in a super diffusive motion. The system of porous particle and azobenzene containing surfactant molecule can be utilized for various application such as drug delivery, cargo transportation, self-assembling, micro motors/ machines or micro patterning. N2 - In dieser Doktorarbeit führen wir das Konzept der lichtinduzierten Diffusioosmose (LDDO) zur licht-kontrollierten passiven und aktiven Bewegung von Kolloiden an der fest-flüssig Grenzfläche ein. Bei diesem neuartigen Phänomen wird ein Grenzflächenfluss mittels Lichtes bestimmter Wellenlänge erzeugt. Ein lichtempfindliches Tensid wirkt hierbei als Quelle der Diffusioosmose: Durch Einstrahlung von Licht wird ein Konzentrationsgradient an der Oberfläche erzeugt, der wiederum ein Ungleichgewicht im lateralen osmotischen Druck verursacht. Dieser Druckunterschied führt dann zu einem grenzflächennahen diffusioosmotischen Fluss. Das lichtaktive Molekül besteht aus einer kationischen Kopfgruppe und einer hydrophoben Kohlenstoffkette, in die die Azobenzolgruppe eingebettet ist. Azobenzol fungiert hier als Lichtschalter, da es mit Licht zwischen einem stabilen trans und einem metastabilen cis Zustand hin- und hergeschaltet werden kann. Nahes UV Licht führt hier zur trans-cis und sichtbares Licht zur cis-trans Isomerisation. Das trans Isomer unterscheidet sich in einigen Eigenschaften vom cis Isomer. So ist z.B. das trans-Isomer langgestreckt und besitzt eine geringe Polarität (verschwindendes Dipolmoment), währenddessen das cis Isomer gebogen ist und ein deutliches Dipolmoment von ca. 3 Debye besitzt. Durch die Integration der Azobenzolgruppe in die hydrophobe Kette des Tensids, bestimmt der Isomerisationszustand des Tensids die Hydrophobizität des gesamten Moleküls: Der trans Zustand ist deutlich hydrophober als der cis Zustand. Dieser Unterschied zeigt sich in den Löslichkeitseigenschaften des Moleküls, der kritischen Mizellenkonzentration sowie des Wechselwirkungspotentials zwischen Molekül und Grenzfläche. Dies kann genutzt werden, um diese Eigenschaften mittels Lichtbestrahlung zu ändern. Wird das Molekül in Wasser gelöst und mit fokussiertem Licht bestrahlt, kann ein isomerer Konzentrationsgradient im Bestrahlungsbereich an der fest-flüssig Grenzfläche erzeugt werden, der wiederum in einem osmotischen Druckgunterschied resultiert. Die daraus resultierende Diffusioosmose (DO), welche an der Grenzfläche erzeugt wird, ist in der Lage Kolloide, die sich an der Grenzfläche befinden, transportieren (passiv). Im Unterschied dazu kann ein sogenannter lokaler diffusioosmotischer Fluss (l-LDDO) um jedes einzelne Kolloid erzeugt werden, sobald es sich um meso-poröse Kolloide handelt. Hierbei agiert jedes Kolloid selbst als Konzentrationsquelle- bzw. –senke (ähnlich dem fokussiertem Licht im oberen Fall). Je nach Isomerisationszustand lagert sich das Molekül eher im Kolloid an oder bevorzugt die Umgebung des Wassers. Befindet sich das Molekül im trans Zustand lagert es sich im Kolloid an, während es im cis Zustand eher die Umgebung des Wassers sucht. Der diffusioosmotische Fluss wird um jedes einzelne Kolloid erzeugt, wodurch eine Wechselwirkung zwischen allen Kolloiden zustande kommt, die entweder anziehend oder abstoßend sein kann. Das hängt vom Isomerisationszustand der Tensidmoleküle vor der Bestrahlung ab. Durch die Änderung der folgenden Parameter kann die Bewegung der Kolloide kontrolliert werden: Lichtwellenlänge, Lichtintensität, Tensidkonzentration, Kolloidkonzentration. Durch die gleichzeitige Verwendung zweier verschiedener Lichtquellen (mit unterschiedlichen Wellenlängen), ist es möglich eine interessante Dynamik in der Anziehung und Abstoßung der Kolloide zu erzeugen, die faszinierende Kolloidformationen entstehen lassen wie sie in dieser Arbeit näher beschrieben werden. Das Phänomen der lokalen Diffusioosmose kann auch zu selbst-getriebener Bewegung führen, nämlich wenn eine Hälfte des Kolloids bedeckt wird (z.B. mit einer Metallschicht) und somit für Tensidmoleküle undurchlässig macht. Der diffusioosmotische Fluss, der auf der unbedeckten Seite des Kolloids erzeugt wird, bewegt das Kolloid in eine Richtung fort und führt so durch Überlagerung zur thermischen Bewegung zu super-diffusivem Verhalten. Das System, bestehend aus porösen Kolloiden und azobenzolhaltigem Tensidmolekül kann sinnführend genutzt werden, z.B. für folgende Anwendungen: gezieltem Medikamententransport, Mikrofrachttransport, Selbstassemblierung, Mikromotoren/-maschinen oder Mikrostrukturierung. T2 - Licht-kontrollierten passive und aktive Bewegung kolloidaler Partikel KW - Azobenzene containing surfactant KW - Photochemistry KW - Porous silica particles KW - Janus colloids KW - LDDO KW - diffusioosmotic flow KW - Hydrophobic and hydrophillic interactions KW - Kinetics of photoisomerization KW - Azobenzol enthaltendes Tensid KW - hydrophoben und hydrophile Wechselwirkungen KW - Janus-Kolloid KW - LDDO KW - Photochemie KW - poröse Siliciumdioxidpartikel KW - diffusioosmotischer Fluss KW - Isomerisierung Kinetik Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-483880 ER - TY - JOUR A1 - Arya, Pooja A1 - Jelken, Joachim A1 - Feldmann, David A1 - Lomadze, Nino A1 - Santer, Svetlana T1 - Light driven diffusioosmotic repulsion and attraction of colloidal particles JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - 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. Y1 - 2020 U6 - https://doi.org/10.1063/5.0007556 SN - 0021-9606 SN - 1089-7690 VL - 152 IS - 19 PB - American Institute of Physics CY - Melville, NY ER - TY - JOUR A1 - Feldmann, David A1 - Arya, Pooja A1 - Lomadze, Nino A1 - Kopyshev, Alexey A1 - Santer, Svetlana T1 - Light-driven motion of self-propelled porous Janus particles JF - Applied physics letters N2 - We introduce a versatile mechanism of light-driven self-propelled motion applied to porous Janus-type particles. The mechanism is based on the generation of local light-driven diffusio-osmotic (l-LDDO) flow around each single porous particle subjected to suitable irradiation conditions. The photosensitivity is introduced by a cationic azobenzene containing surfactant, which undergoes a photoisomerization reaction from a more hydrophobic trans-state to a rather hydrophilic cis-state under illumination with light. The negatively charged porous silica particles are dispersed in a corresponding aqueous solution and absorb molecules in their trans-state but expel them in their cis-state. During illumination with blue light triggering both trans-cis and cis-trans isomerization at the same time, the colloids start to move due to the generation of a steady-state diffusive flow of cis-isomers out of and trans-isomers into the particle. This is because a hemi-spherical metal cap partially sealing the colloid breaks the symmetry of the otherwise radially directed local flow around the particle, leading to self-propelled motion. Janus particles exhibit superdiffusive motion with a velocity of similar to 0.5 mu m/s and a persistence length of ca. 50 mu m, confined to microchannels the direction can be maintained up to 300 mu m before rotational diffusion reverts it. Particles forming dimers of different shapes can be made to travel along circular trajectories. The unique feature of this mechanism is that the strength of self-propulsion can be tuned by convenient external optical stimuli (intensity and irradiation wavelength) such that a broad variety of experimental situations can be realized in a spatiotemporal way and in situ. Y1 - 2019 U6 - https://doi.org/10.1063/1.5129238 SN - 0003-6951 SN - 1077-3118 VL - 115 IS - 26 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Arya, Pooja A1 - Jelken, Joachim A1 - Lomadze, Nino A1 - Santer, Svetlana A1 - Bekir, Marek T1 - Kinetics of photo-isomerization of azobenzene containing surfactants JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistry N2 - 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. KW - genomic DNA conformation KW - water-interface KW - light photocontrol KW - driven KW - manipulation KW - photoisomerization KW - molecules Y1 - 2020 U6 - https://doi.org/10.1063/1.5135913 SN - 0021-9606 SN - 1089-7690 VL - 152 IS - 2 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Frenkel, Mark A1 - Arya, Pooja A1 - Bormashenḳo, Edṿard A1 - Santer, Svetlana T1 - Quantification of ordering in active light driven colloids JF - Journal of colloid and interface science N2 - Hypothesis: Light driven diffusioosmosis allows for the controlled self-assembly of colloidal particles. Illuminating of colloidal suspensions built of nanoporous silica microspheres dispersed in aqueous solution containing photosensitive azobenzene cationic surfactant enables manufacturing self-assembled well-ordered 2D colloidal patterns. We conjectured that ordering in this patterns may be quantified with the Voronoi entropy. Experiments: Depending on the isomerization state the surfactant either tends to absorb (trans-state) into negatively charged pores or diffuse out (cis-isomer) of the particles generating an excess concentration near the colloids outer surface and thus resulting in the initiation of diffusioosmotic flow. The direction of the flow can be controlled by the wavelength and intensity of irradiation. Under irradiations with blue light the colloids separate within a few seconds forming equidistant particle ensemble where long range diffusioosmotic repulsion acts over distances exceeding several times the particle diameter. Hierarchy of ordering in the studied colloidal systems is distinguished, namely: i) ordering of individual separated colloidal particles; ii) ordering of clusters built of colloidal particles; iii) ordering within clusters of individual colloidal particles. Findings: The study of the temporal change in the Voronoi entropy for the light illuminated colloidal dispersions allowed quantification of ordering evolution on different lateral scales and under different irradiation conditions. Fourier analysis of the time evolution of the Voronoi entropy is presented. Fourier spectrum of the "small-area" (100 x 100 mu m) reveals the pronounced peak at f = 1.125 Hz reflecting the oscillations of individual particles at this frequency. Ordering in hierarchical colloidal system emerging on different lateral scales is addressed. The minimal Voronoi entropy is intrinsic for the close packed 2D clusters. (C) 2020 Published by Elsevier Inc. KW - Azobenzene containing cationic surfactants KW - Light induced diffusioosmotic flow KW - 2D colloid ordering KW - Voronoi entropy Y1 - 2021 U6 - https://doi.org/10.1016/j.jcis.2020.10.053 SN - 0021-9797 SN - 1095-7103 VL - 586 SP - 866 EP - 875 PB - Elsevier CY - San Diego ER - TY - JOUR A1 - Arya, Pooja A1 - Feldmann, David A1 - Kopyshev, Alexey A1 - Lomadze, Nino A1 - Santer, Svetlana T1 - Light driven guided and self-organized motion of mesoporous colloidal particles JF - Soft matter N2 - 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. Y1 - 2019 U6 - https://doi.org/10.1039/c9sm02068c SN - 1744-683X SN - 1744-6848 VL - 16 IS - 5 SP - 1148 EP - 1155 PB - Royal Society of Chemistry CY - Cambridge ER -