Versatile and Remotely Controllable Light-Induced Coagulation of Particles Under Flow in a 2D Channel.

On-demand switch on/off blood clogging is of paramount importance for the survival of mammals, for example as a quick response to seal damage wounds to minimize their bleeding rate. This mechanism is a complex chain process from initiated red blood cell aggregation at the target location (open wound) that quickly seals on a macroscopic scale the damaged flash. Inspired by nature an on-demand switchable particle clogging mechanism is developed with high spatial resolution down to micrometer size using light as an external non-invasive stimulation. Particle clogging can be adjusted on demand strong enough to even withstand pressure-driven fluid flow, additionally building up walls of aggregated particles, which stop the momentum of big particles under shear. The principle relies on a photosensitive surfactant, which induces under light illumination a long-ranged lateral attractive phoretic-osmotic activity of silica microparticles forcing them to aggregate. The strength of aggregation and therefore motion reduction or even stop of the particles against the fluid flow depends on the ratio between the aggregation strength and the velocity of the particles. The aggregation strength can be precisely controlled by the applied light intensity and adjusted particle concentration. Increasing both parameters results in a stronger aggregation tendency.

Complexes of photosensitive surfactant and fluorescent dye for light-induced manipulation of colloids.

Light-driven diffusioosmosis is a membrane-free method for manipulating colloidal ensembles at solid-liquid interfaces based on photo-sensitive molecules inducing fluid flows along solid surfaces. In this study, we present our findings on porous colloids settled at a solid wall in an aqueous solution comprising a photo-sensitive azobenzene-containing cationic surfactant and a cyanine-based dye, capable of ionically binding to each other. The surfactant acts as an activation agent for diffusioosmotic flow. When exposed to modulated light, it undergoes photo-isomerization from a hydrophobic trans-state to a more hydrophilic cis-state, creating a concentration gradient near the irradiated area of the wall. The resulting osmotic pressure gradient sets the flow in motion. Porous colloids actively participate in flow generation by readily incorporating the surfactant molecules in the trans-state and releasing them in the cis-state, creating a constant source of diffusioosmotic flow. Under UV illumination, an excess of cis-isomers near the porous colloids elicits long-range repulsive interactions, tenfold the diameter of a particle. The dye acts as a sensor for the surfactant filling or emptying the pores of the colloids. It forms a complex with the trans-isomer and diffuses into the pores, where photoisomerization to cis-state destroys the complex and causes both the dye and the surfactant to leave the pores, altering the luminescence brightness within the colloids. We demonstrated that the presence of the dye affects cis-trans isomer ratios of the surfactant at photo-stationary states, thereby influencing the process of diffusioosmosis. This process enables the manipulation of colloidal particles and remote control of the interaction potential between them, facilitating the formation of well-ordered surface aggregates.

Artifact Correction of Light Induced Detuning in QCM-D Experiments.

The quartz crystal microbalance with dissipation (QCM-D) has become an efficient and versatile measurement technique for investigating in situ the external stimuli responsiveness such as pH, temperature, or chemical gradients of surface-active substances at solid-liquid interfaces. However, light responsive adsorption investigation is more challenging presumably since the quartz crystal itself reacts to optical stimulation, showing frequency and dissipation shifts known as light induced detuning (LID). This yields an effective measurement artifact and makes data interpretation with respect to dynamic interactions of light responsive materials rather challenging. Here we introduce a simple guideline for correcting the artifacts of the QCM sensor response on irradiation to ensure quantitative analysis for light responsive materials via OCM-D. We also show that the LID depends on the adsorption properties of the sensor and the solvent properties (ionic concentration or viscosity), providing a guideline to minimize impact of the LID.

Making microgels photo-responsive by complexation with a spiropyran surfactant.

We report on triggering of p(NIPAM-AA) microgels' photo-responsiveness by making complexes with a spiropyran (SP) containing surfactant. Being dissolved in water, the SP surfactant in its merocyanine state bears three charges, while irradiation with UV and vis light leads to the partial or complete reversal of the SP state. The complexation of the photo-responsive amphiphile with swollen anionic microgels results in charge compensation within the gel interior and as a consequence its size reduces and the volume phase transition temperature (VPTT) decreases down to 32 °C. Under irradiation the MC form photo-isomerizes to a ring closed SP state generating a more hydrophobic surfactant with one positive charge at the head. The increase in the hydrophobicity of the surfactant and thus of the interior of the gel results in the reversible size change of the microgel. We investigate the photo-responsivity of the microgel as a function of wavelength and irradiation intensity, as well as of surfactant concentration and charge density of the microgel. We show that the change in the size and VPTT of the microgels during irradiation occurs through a combination of two processes: heating of the solution during light absorption by the surfactant (more pronounced in the case of UV irradiation) and the change in the hydrophobicity of the surfactant.

Electrostatic Interactions in the Formation of DNA Complexes with Cis- and Trans-Isomers of Azobenzene-Containing Surfactants in Solutions with Di- and Trivalent Metal Ions.

The effect of the presence of divalent and trivalent metal ions in solutions upon DNA packaging induced by the photosensitive azobenzene-containing surfactant is considered. It has been shown that the addition of divalent and trivalent metal ions does not affect the DNA-surfactant interaction for both the cis- and the trans-isomers of the surfactant. At the same time, the ionic strength of the solution, which is provided by a certain concentration of the salt, has a huge impact. It affects the association of surfactant molecules with each other and their binding to DNA. It has been shown by computer simulation that cobalt hexamine is attracted to the N7 atom of guanine in the major groove of DNA and does not penetrate into grooves near the AT base pairs.

Versatile Microfluidics Separation of Colloids by Combining External Flow with Light-Induced Chemical Activity.

Separation of particles by size, morphology, or material identity is of paramount importance in fields such as filtration or bioanalytics. Up to now separation of particles distinguished solely by surface properties or bulk/surface morphology remains a very challenging process. Here a combination of pressure-driven microfluidic flow and local self-phoresis/osmosis are proposed via the light-induced chemical activity of a photoactive azobenzene-surfactant solution. This process induces a vertical displacement of the sedimented particles, which depends on their size and surface properties . Consequently, different colloidal components experience different regions of the ambient microfluidic shear flow. Accordingly, a simple, versatile method for the separation of such can be achieved by elution times in a sense of particle chromatography. The concepts are illustrated via experimental studies, complemented by theoretical analysis, which include the separation of bulk-porous from bulk-compact colloidal particles and the separation of particles distinguished solely by slight differences in their surface physico-chemical properties.

Viscoplastic Modeling of Surface Relief Grating Growth on Isotropic and Preoriented Azopolymer Films.

We report on solving of two intriguing issues concerning the inscription of surface relief gratings within azopolymer thin films under irradiation with SS, PP and RL interference patterns. For this, we utilize the orientation approach and viscoplastic modeling in combination with experimental results, where the change in surface topography is acquired in situ during irradiation with modulated light. First, the initial orientation state of polymer backbones is proved to be responsible for the contradictory experimental reports on the efficiency of the SS interference pattern. Different orientation states can influence not only the phase of SS grating but also its height, which is experimentally confirmed by using special pretreatments. Second, the faster growth of gratings inscribed by the RL interference pattern is shown to be promoted by a weak photosoftening effect. Overall, the modeled results are in good agreement with the order of relative growth efficiency: RL-PP-SS.

Spiropyran/Merocyanine Amphiphile in Various Solvents: A Joint Experimental-Theoretical Approach to Photophysical Properties and Self-Assembly.

This joint experimental-theoretical work focuses on molecular and photophysical properties of the spiropyran-containing amphiphilic molecule in organic and aqueous solutions. Being dissolved in tested organic solvents, the system demonstrates positive photochromism, i.e., upon UV stimulus the colorless spiropyran form is transformed into colorful merocyanine isomer. However, the aqueous solution of the amphiphile possesses a negative photochromism: the orange-red merocyanine form becomes thermodynamically more stable in water, and both UV and vis stimuli lead to the partial or complete photobleaching of the solution. The explanation of this phenomenon is given on the basis of density functional theory calculations and classical modeling including thermodynamic integration. The simulations reveal that stabilization of merocyanine in water proceeds with the energy of ca. 70 kJ mol-1, and that the Helmholtz free energy of hydration of merocyanine form is 100 kJ mol-1 lower as compared to the behavior of SP isomer in water. The explanation of such a difference lies in the molecular properties of the merocyanine: after ring-opening reaction this molecule transforms into a zwitterionic form, as evidenced by the electrostatic potential plotted around the opened form. The presence of three charged groups on the periphery of a flat conjugated backbone stimulates the self-assembly of merocyanine molecules in water, ending up with the formation of elongated associates with stack-like building blocks, as shown in molecular dynamics simulations of the aqueous solution with the concentration above critical micelle concentration. Our quantitative evaluation of the hydrophilicity switching in spiropyran/merocyanine containing surfactants may prompt the search for new systems, including colloidal and polymeric ones, aiming at remote tuning of their morphology, which could give new promising shapes and patterns for the needs of modern nanotechnology.

Light-Triggered Manipulations of Droplets All in One: Reversible Wetting, Transport, Splitting, and Merging.

Here, we establish different ways of light-triggered droplet manipulation such as reversible wetting, splitting, merging, and transport. The unique features of our approach are that the changes in the wetting properties of microscopic droplets of isotropic (oil) or anisotropic (liquid crystalline) liquids adsorbed on photoswitchable films can be triggered just by application of soft optical stimuli, which lead to dynamical, reversible changes in the local morphology of the structured surfaces. The adaptive films consist of an azobenzene-containing surfactant ionically attached to oppositely charged polymer chains. Under exposure to irradiation with light, the azobenzene photoisomerizes between two states, nonpolar trans-isomer and polar cis-isomer, resulting in the corresponding changes in the surface energy and orientation of the surfactant tails at the interface. Additionally, the local increase in the surface temperature due to absorption of light by the azobenzene groups enables diverse processes of manipulation of the adsorbed small droplets, such as the reversible increase of the droplet basal area up to 5 times, anisotropic wetting during irradiation with modulated light, and precise partition of the droplet into many small pieces, which can then be merged on demand to the desired number of larger droplets. Moreover, using a moving focused light spot, we experimentally demonstrate and theoretically explain the locomotion of the droplet over macroscopic distances with a velocity of up to 150 µm·s-1. Our findings could lead to the ultimate application of a programmable workbench for manipulating and operating an ensemble of droplets, just using simple and gentle optical stimuli.

Generation of Local Diffusioosmotic Flow by Light Responsive Microgels.

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.

A Dual pH- and Light-Responsive Spiropyran-Based Surfactant: Investigations on Its Switching Behavior and Remote Control over Emulsion Stability.

A cationic surfactant containing a spiropyran unit is prepared exhibiting a dual-responsive adjustability of its surface-active characteristics. The switching mechanism of the system relies on the reversible conversion of the non-ionic spiropyran (SP) to a zwitterionic merocyanine (MC) and can be controlled by adjusting the pH value and via light, resulting in a pH-dependent photoactivity: While the compound possesses a pronounced difference in surface activity between both forms under acidic conditions, this behavior is suppressed at a neutral pH level. The underlying switching processes are investigated in detail, and a thermodynamic explanation based on a combination of theoretical and experimental results is provided. This complex stimuli-responsive behavior enables remote-control of colloidal systems. To demonstrate its applicability, the surfactant is utilized for the pH-dependent manipulation of oil-in-water emulsions.

Cis-Isomers of Photosensitive Cationic Azobenzene Surfactants in DNA Solutions at Different NaCl Concentrations: Experiment and Modeling.

The DNA interaction with cis-isomers of photosensitive azobenzene-containing surfactants was studied by both experimental methods and computer simulation. It was shown that before the organization of micelles, such surfactants in the cis-conformation form associates of only a single type with a disordered orientation of molecules. In contrast, for trans-isomers, there exist two types of associates with head-to-head or head-to-tail orientations of molecules in dependence on salt concentration in a solution. The comparison of cis- and trans-isomer binding to DNA and the influence of salt concentration on the formation of their complexes with DNA were studied. It was shown that cis-isomers interact with phosphate groups of DNA and that their molecules were also located along the minor groove of DNA.

Light-induced manipulation of passive and active microparticles.

We consider sedimented at a solid wall particles that are immersed in water containing small additives of photosensitive ionic surfactants. It is shown that illumination with an appropriate wavelength, a beam intensity profile, shape and size could lead to a variety of dynamic, both unsteady and steady state, configurations of particles. These dynamic, well-controlled and switchable particle patterns at the wall are due to an emerging diffusio-osmotic flow that takes its origin in the adjacent to the wall electrostatic diffuse layer, where the concentration gradients of surfactant are induced by light. The conventional nonporous particles are passive and can move only with already generated flow. However, porous colloids actively participate themselves in the flow generation mechanism at the wall, which also sets their interactions that can be very long ranged. This light-induced diffusio-osmosis opens novel avenues to manipulate colloidal particles and assemble them to various patterns. We show in particular how to create and split optically the confined regions of particles of tunable size and shape, where well-controlled flow-induced forces on the colloids could result in their crystalline packing, formation of dilute lattices of well-separated particles, and other states.

Quantification of ordering in active light driven colloids.

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 × 100 µm) 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.

Photo-Isomerization Kinetics of Azobenzene Containing Surfactant Conjugated with Polyelectrolyte.

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.

Self-Assembly of Molecular Brushes with Polyimide Backbone and Amphiphilic Block Copolymer Side Chains in Selective Solvents.

Three-component molecular brushes with a polyimide backbone and amphiphilic block copolymer side chains with different contents of the "inner" hydrophilic (poly(methacrylic acid)) and "outer" hydrophobic (poly(methyl methacrylate)) blocks were synthesized and characterized by molecular hydrodynamics and optics methods in solutions of chloroform, dimethylformamide, tetrahydrofuran and ethanol. The peculiarity of the studied polymers is the amphiphilic structure of the grafted chains. The molar masses of the molecular brushes were determined by static and dynamic light scattering in chloroform in which polymers form molecularly disperse solutions. Spontaneous self-assembly of macromolecules was detected in dimethylformamide, tetrahydrofuran and ethanol. The aggregates size depended on the thermodynamic quality of the solvent as well as on the macromolecular architectural parameters. In dimethylformamide and tetrahydrofuran, the distribution of hydrodynamic radii of aggregates was bimodal, while in ethanol, it was unimodal. Moreover, in ethanol, an increase in the poly(methyl methacrylate) content caused a decrease in the hydrodynamic radius of aggregates. A significant difference in the nature of the blocks included in the brushes determines the selectivity of the used solvents, since their thermodynamic quality with respect to the blocks is different. The macromolecules of the studied graft copolymers tend to self-organization in selective solvents with formation of a core-shell structure with an insoluble solvophobic core surrounded by the solvophilic shell of side chains.

Adsorption of Photoresponsive Surfactants at Solid-Liquid Interfaces.

We report on the adsorption kinetics of azobenzene-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 ζ-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.

Some Features of Surfactant Organization in DNA Solutions at Various NaCl Concentrations.

The photosensitive azobenzene-containing surfactant C4-Azo-OC6TMAB is a promising agent for reversible DNA packaging in a solution. The simulation of the trans-isomer surfactant organization into associates in a solution with and without salt as well as its binding to DNA at different NaCl concentrations was carried out by molecular dynamics. Experimental data obtained by spectral and hydrodynamic methods were used to verify the results of simulation. It was shown that head-to-tail aggregates with close to antiparallel orientation of surfactant molecules were formed at certain NaCl and surfactant concentrations (below critical micelle concentration). Such aggregates have two positively charged ends, and therefore, they can be attracted to negatively charged DNA phosphates far located along the chain, as well as those that belong to different molecules. This contributes to the formation of intermolecular DNA-DNA contacts, and this way, the experimentally observed precipitation of DNA can be explained.

Extremely Long-Range Light-Driven Repulsion of Porous Microparticles.

The repulsive surface forces, such as electrostatic or steric, acting between particles explain why they remain well separated in aqueous electrolyte solutions and are responsible for the stability of colloidal dispersions. However, the effective range of these interactions is always well below hundreds of nanometers and typically can be controlled by advanced manipulations such as tuning the electrolyte concentration or modifying the particle surface or, in some more specific cases, via subjecting the suspension to an external electric or magnetic field. Here we employ solutions with small additives of a photosensitive ionic surfactant to investigate if a repulsive interaction of microsized particles sedimented at the solid surface can be remotely controlled simply by illuminating it with an appropriate wavelength. We show that interactions of conventional impermeable particles remain practically unaffected by light, but, in contrast, for porous particles, we observe a long-range repulsion, several orders of magnitude longer than any conceivable equilibrium surface force. This repulsion emerges due to the diffusio-osmotic flow generated near the porous particles that in this scenario are playing a role of micropumps. The diffusio-osmotic repulsion of porous particles can be used for a remote control of their two-dimensional assemblies at the solid wall, and in particular, we demonstrate that by simply using two different illumination wavelengths it is possible to reversibly switch the state of porous particle dispersion from densely packed surface aggregates to a periodic lattice of particles separated by distances on the order of tens of micrometers.

Kinetics of photo-isomerization of azobenzene containing surfactants.

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 3CH2 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.