@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{UmlandtFeldmannSchnecketal.2020,
  author    = {Umlandt, Maren and Feldmann, David and Schneck, Emanuel and Santer, Svetlana and Bekir, Marek},
  title     = {Adsorption of photoresponsive surfactants at solid-liquid interfaces},
  series = {Langmuir},
  volume    = {36},
  journal   = {Langmuir},
  number    = {46},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {0743-7463},
  doi       = {10.1021/acs.langmuir.0c02545},
  pages     = {14009 -- 14018},
  year      = {2020},
  abstract  = {We report on the adsorption kinetics of azoben-zene-containing surfactants on solid surfaces of different hydrophobicity. The understanding of this processes is of great importance for many interfacial phenomena that can be actuated and triggered by light, since the surfactant molecules contain a photoresponsive azobenzene group in their hydrophobic tail. Three surfactant types are studied, differing in the spacer connecting the headgroup and the azobenzene unit by between 6 and 10 CH2 groups. Under irradiation with light of a suitable wavelength, the azobenzene undergoes reversible photoisomerization between two states, a nonpolar trans-state and a highly polar cis-state. Consequently, the surfactant molecule changes its hydrophobicity and thus affinity to a surface depending on the photoisomerization state of the azobenzene. The adsorption behavior on hydrophilic (glass) and hydrophobic (TeflonAF) surfaces is analyzed using quartz crystal microbalance with dissipation (QCM-D) and zeta-potential measurements. At equilibrium, the adsorbed surfactant amount is almost twice as large on glass compared to TeflonAF for both isomers. The adsorption rate for the trans-isomers on both surfaces is similar, but the desorption rate of the trans-isomers is faster at the glass-water interface than at the Teflon-water interface. This result demonstrates that the trans-isomers have higher affinity for the glass surface, so the trans-to-cis ratios on glass and TeflonAF are 80/1 and 2/1, respectively, with similar trends for all three surfactant types.},
  language  = {en}
}