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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.
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.
Cavitation at the solid surface normally begins with nucleation, in which defects or assembled molecules located at a liquid-solid interface act as nucleation centers and are actively involved in the evolution of cavitation bubbles. Here, we propose a simple approach to evaluate the behavior of cavitation bubbles formed under high intensity ultrasound (20 kHz, 51.3 W cm (2)) at solid surfaces, based on sonication of patterned substrates with a small roughness (less than 3 nm) and controllable surface energy. A mixture of octadecylphosphonic acid (ODTA) and octadecanethiol (ODT) was stamped on the Si wafer coated with different thicknesses of an aluminium layer (20-500 nm). We investigated the growth mechanism of cavitation bubble nuclei and the evolution of individual pits (defects) formed under sonication on the modified surface. A new activation behavior as a function of Al thickness, sonication time, ultrasonic power and temperature is reported. In this process cooperativity is introduced, as initially formed pits further reduce the energy to form bubbles. Furthermore, cavitation on the patterns is a controllable process, where up to 40-50 min of sonication time only the hydrophobic areas are active nucleation sites. This study provides a convincing proof of our theoretical approach on nucleation.
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.
In this paper we report on an opto-mechanical scission of polymer chains within photosensitive diblock-copolymer brushes grafted to flat solid substrates. We employ surface-initiated polymerization of methylmethacrylate (MMA) and t-butyl methacrylate (tBMA) to grow diblock-copolymer brushes of poly(methylmethacrylate-b-t-butyl methacrylate) following the atom transfer polymerization (ATRP) scheme. After the synthesis, deprotection of the PtBMA block yields poly(methacrylic acid) (PMAA). To render PMMA-b-PMAA copolymers photosensitive, cationic azobenzene containing surfactants are attached to the negatively charged outer PMAA block. During irradiation with an ultraviolet (UV) interference pattern, the extent of photoisomerization of the azobenzene groups varies spatially and results in a topography change of the brush, i.e., formation of surface relief gratings (SRG). The SRG formation is accompanied by local rupturing of the polymer chains in areas from which the polymer material recedes. This opto-mechanically induced scission of the polymer chains takes place at the interfaces of the two blocks and depends strongly on the UV irradiation intensity. Our results indicate that this process may be explained by employing classical continuum fracture mechanics, which might be important for tailoring the phenomenon for applying it to poststructuring of polymer brushes.
This paper describes a strategy for preparing photosensitive polymeric grafts on flat solid surfaces by loading diblock-copolymer or homopolymer brushes with cationic azobenzene-containing surfactants. In contrast to previous work, we utilize photosensitive surfactants that bear positively-charged polyamine head groups whose charge varies between 1(+) and 3(+). Poly(methylmethacrylate-b-methacrylic acid) (PMMA-b-PMAA) brushes were prepared by employing atom transfer radical polymerization, where the bottom poly(methyl methacrylate) block was grown first followed by the synthesis of t-butyl methacrylate block that after de-protection yielded poly(methacrylic acid). We used PMMA-b-PMAA brushes with constant grafting density and length of the PMMA block, and three different lengths of the PMAA block. The azobenzene-based surfactants attached only to the PMAA block. The degree of binding (i.e., the number of surfactant molecules per binding site on the brush backbone) of the surfactants to the brush depends strongly on the valence of the surfactant head-group; within the brushes the concentration of the surfactant carrying unit charge is larger than that of multivalent surfactants. We detect pronounced response of the brush topography on irradiation with UV interference pattern even at very low degree of binding (as small as 0.08) of multi-valence surfactant. Areas on the sample that receive the highest UV dose exhibit chain scission. By removing the ruptured chains from the substrate via good solvent, one uncovers a surface topographical relief grating, whose spatial arrangement follows the intensity distribution of the UV light on the sample during irradiation. Due to strong coupling of the multi-valence surfactants to the polymer brush, it was possible in some cases to completely remove the polyelectrolyte block from the PMMA layer. The application of multi-valence azobenzene surfactants for triggering brush photosensitive has important advantage over usage of surfactant with unit charge because relative to single-valence surfactants much lower concentrations of the multivalent surfactant are needed to achieve comparable response upon UV irradiation. (C) 2016 Elsevier Ltd. All rights reserved.
We investigate light-induced changes in thickness and roughness of photosensitive polymer brushes containing azobenzene cationic surfactants by atomic force microscopy (AFM) in real time during light irradiation. Because the cis-state of azobenzene unit requires more free volume than its trans counterpart, the UV light-induced expansion of polymer thin films associated with the trans-to-cis isomerism of azobenzene groups is expected to occur. This phenomenon is well documented in physisorbed polymer films containing azobenzene groups. In contrast, photosensitive polymer brushes show a decrease in thickness under UV irradiation. We have found that the azobenzene surfactants in their trans-state form aggregates within the brush. Under irradiation, the surfactants undergo photoisomerization to the cis-state, which is more hydrophilic. As a consequence, the aggregates within the brush are disrupted, and the polymer brush contracts. When subsequently irradiated with blue light the polymer brush thickness returns back to its initial value. This behavior is related to isomerization of the surfactant to the more hydrophobic trans-state and subsequent formation of surfactant aggregates within the polymer brush. The photomechanical function of the dry polymer brush, i.e., contraction and expansion, was found to be reversible with repeated irradiation cycles and requires only a few seconds for switching. In addition to the thickness change, the roughness of the brush also changes reversibly between a few Angstroms (blue light) and several nanometers (UV light). Photosensitive polymer brushes represent smart films with light responsive thickness and roughness that could be used for generating dynamic fluctuating surfaces, the function of which can be turned on and off in a controllable manner on a nanometer length scale.
We report on rendering polyelectrolyte brushes photosensitive by loading them with azobenzene-containing cationic surfactants. Planar poly(methacrylic acid) (PMAA) brushes are synthesized using the “grafting from” free-radical polymerization scheme followed by exposure to a solution of photosensitive surfactants consisting of positively-charged head groups and hydrophobic tails into which azobenzene moieties are inserted. In this study the length of the hydrophobic methylene spacer connecting the azobenzene and the charged head group ranges from 4 to 10 CH2 groups. Under irradiation with UV light, the photo-isomerization of azobenzene integrated into a surfactant results in a change in size, geometry, dipole moment and free volume of the whole molecule. When the brush loaded with photosensitive surfactants is exposed to irradiation with UV interference patterns, the topography of the brush deforms following the distribution of the light intensity, exhibiting surface relief gratings (SRG). Since SRG formation is accompanied by a local rupturing of polymer chains in areas from which the polymer material is receding, most of the polymer material is removed from the surface during treatment with good solvent, leaving behind characteristic patterns of lines or dots. The azobenzene molecules still integrated within the polymer film can be removed by washing the brush with water. The remaining nano-structured brush can then be re-used for further functionalization. Although the opto-mechanically induced rupturing occurs for all surfactants, larger species do not penetrate deep enough into the brush such that after rupturing a leftover layer of polymer material remains on the substrate. This indicates that rupturing occurs predominantly in regions of high surfactant density.
In this paper, we report on the properties of nano-slits created in metal thin films using atomic force microscope (AFM) nanolithography (AFM-NL). We demonstrate that instead of expensive diamond AFM tips, it is also possible to use low cost silicon nitride tips. It is shown that depending on the direction of scratching, nano-slits of different widths and depths can be fabricated at constant load force. We elucidate the reasons for this behavior and identify an optimal direction and load force for scratching a gold layer.
We report on light-induced deformation of colloidal spheres consisting of azobenzene-containing polymers. The colloids of the size between 60 nm and 2 mu m in diameter were drop casted on a glass surface and irradiated with linearly polarized light. It was found that colloidal particles can be deformed up to ca. 6 times of their initial diameter. The maximum degree of deformation depends on the irradiation wavelength and intensity, as well as on colloidal particles size. On the basis of recently proposed theory by Toshchevikov et al. [J. Phys. Chem. Lett. 2017, 8, 1094], we calculated the optomechanical stresses (ca. 100 MPa) needed for such giant deformations and compared them with the experimental results.