Alessio Caciagli - Academia.edu (original) (raw)
Papers by Alessio Caciagli
Polymers
Nano-sized particles functionalised with short single-stranded (ss)DNAs can act as detectors of c... more Nano-sized particles functionalised with short single-stranded (ss)DNAs can act as detectors of complementary DNA strands. Here we consider tri-block-copolymer-based, self-assembling DNA-coated nanoparticles. The copolymers are chemically linked to the DNA strands via azide (N3) groups. The micelles aggregate when they are linked with complementary ssDNA. The advantage of such block-copolymer-based systems is that they are easy to make. Here we show that DNA functionalisation results in inter-micellar attraction, but that N3-groups that have not reacted with the DNA detector strands also change the phase behaviour of the tri-block polymer solution. We studied the triblock copolymer, Pluronic® F108, which forms spherical micelles in aqueous solutions upon heating. We find that the triblock chains ending with either an N3 or N3-DNA complex show a dramatic change in phase behaviour. In particular, the N3-functionalisation causes the chain ends to cluster below the critical micelle temp...
arXiv (Cornell University), Nov 30, 2022
Soft' patchy surfactant micelles have become an additional building tool in selfassembling system... more Soft' patchy surfactant micelles have become an additional building tool in selfassembling systems. The triblock copolymer, Pluronic ® F108, forms spherical micelles in aqueous solutions upon heating leading to a simple phase diagram with a micellar crystalline solid at higher temperatures and concentrations. Here we report the strong influence of end-functionalising the chain ends either with an azide or azide-DNA complex on the systems phase behaviour. We find that the azide(N 3)functionalisation renders the chain ends weakly hydrophobic at lower temperatures, causing them to self-assemble into flower-micelles. This hydrophobicity increases with increasing temperature and poses a competing self-assembling mechanism to the solvent induces hydrophobic interactions between the middle-blocks of F108 at higher temperatures and leads to a macroscopic phase separation that is absent in the pure F108 system. However, when we attached short, hydrophilic single-stranded (ss)DNA to the azide groups via click chemistry the chain ends became 'sticky due to DNA hybridisation below the melting temperature of the complementary ssDNA ends while reverting to hydrophilic behaviour above. We characterise their structural and rheological properties via Dynamic Light Scattering (DLS) and DLS-based passive microrheology with an improved time-frequency domain inversion step. We present the structural behaviour of dilute and semi-dilute solutions of the original F108 system and compare the results with solutions containing either the F108azide (F108-N 3) or partially DNA-functionalised F108-azide chains. Our DLS and microrheology studies inform us on how the attachment of azide groups on F108 changes the mechanical and structural properties of micellar fluids pioneering further characterisation and design of these hybrid systems.
arXiv (Cornell University), Oct 25, 2022
We characterize the self-assembly of colloidal particles with surface mobile DNA linkers under ki... more We characterize the self-assembly of colloidal particles with surface mobile DNA linkers under kinetically limited valence conditions. For this, we put forward a computer simulation model that captures quantitatively the interplay between the main dynamic processes governing these systems and allows the simulation of the long time scales reached in experiments. The model is validated by direct comparison with available experimental results, showing an overall good agreement that includes measurements of the average effective valence and its probability distribution as a function of the density of DNA linkers on the particles surface. Finally, simulation results are used to evidence the opposite impact of particle density and characteristic DNA hybridization time on the effective valence.
arXiv (Cornell University), Jun 29, 2018
Today colloids are employed in various products from creams and coatings to electronics. The abil... more Today colloids are employed in various products from creams and coatings to electronics. The ability to control their chemical, optical, or electronic features by controlling their size and shape explains why these materials are so widely employed. Nevertheless, altering some of these properties may also lead to some undesired side effects, one of which is an increase in optical scattering upon concentration. Here, we address this strong scattering issue in films made of colloids with high surface roughness. We focus on raspberry type polymeric particles made of a spherical polystyrene core decorated by small hemispherical domains of acrylate. Owing to their surface charge and model 'roughness', aqueous dispersions of these particles display an unusual stability against aggregation. Under certain angles, their solid films display a brilliant red color due to Bragg scattering but otherwise appear completely white on account of`strong scattering. To suppress the scattering and induce transparency, we prepared films by hybridizing them either with oppositely-charged PSparticles that fit the length-scale of the raspberry roughness or with quantum dots. We report that the smaller PS-particles prevent raspberry particle aggregation in solid films and suppress scattering by decreasing the spatial variation of the refractive index. We believe that the results presented here provide a simple strategy to suppress strong scattering of rough particles and allow for their utilization in optical coatings, cosmetics, or photonics.
arXiv (Cornell University), Aug 19, 2018
The intensity and brilliance of such photonic crystals require high size-monodispersity of the co... more The intensity and brilliance of such photonic crystals require high size-monodispersity of the colloids, a low number of lattice defects and disorder, as well as a relatively large refractive index contrast between the scattering colloids and the continuous background. Here we present the unexpected photonic properties of aqueous suspensions of charge-stabilized, 186 nm large, fluorinated colloids with a refractive index of 1.37. Employing reflectivity, optical observation, small angle x-ray scattering measurements and reflectivity modeling, we demonstrate that these suspensions become partially transparent while showing strong, almost angle-independent color in reflection despite the very small refractive index difference. Under certain conditions additional sharp Bragg reflections are observed. We were able to tune the observed structural colors continuously across the entire visible range by simply changing the volume fraction of these colloidal suspensions, which show a white appearance when dilute, structural color and Bragg peaks when concentrated enough to form Wiegner crystal, and angle-independent color when very concentrated and in a glassy phase.
arXiv (Cornell University), Mar 23, 2017
We demonstrate the realization of (laterally) optically bounded colloidal structures on a liquidl... more We demonstrate the realization of (laterally) optically bounded colloidal structures on a liquidliquid interface of an emulsion droplet. We use DNA tethers to graft the particles to the droplet surface, effectively confining them to a quasi-2D plane with minimum constraints on the lateral movement even when optically trapped in a common single-beam configuration. We show that relatively weak interactions such as depletion can be measured in the optically bounded crystals by video-microscopy imaging and analysis. This illustrates the possible use of our system as template to study optically controlled quasi two-dimensional colloidal assembly on liquid-liquid interfaces. Optical tweezing is an established tool to examine interactions at the micro and nanometer level and between biomolecules, whose interactions are typically in the range from pico-to femtoNewtons [1-4]. Such forces are typically assessed by using an optically trapped particle as a probe [5]. Since opticaltrap sizes are usually on the same order of magnitude as the bead diameter, multiple particles can enter the same trap. This results in multiple scattering between the (dielectric) particles themselves, which display the tendency to reversibly form ordered structures: the phenomenon is dubbed optical binding [4]. The ordered structures are either chains of particles longitudinal to the direction of propagation of the light or two dimensional crystalline patterns perpendicular to the laser beam [6-8]. The latter, referred to as lateral optical binding, is of particular interest as the creation of ordered, quasi-2D patterns of many distinct colloids or nano-particles is one of the key challenges in complex self-assembly. However, its previous realization involved either complex multiple beam trapping setups [8, 9] or the use
![Research paper thumbnail of Microrheology of DNA hydrogels [Physics]](https://a.academia-assets.com/images/blank-paper.jpg)
](Proceedings of the National Academy of Sciences of the United States of America, Oct 3, 2019
Frontiers in Physics, 2022
Spatially ordered arrangements of spherical colloids are known to exhibit structural colours. The... more Spatially ordered arrangements of spherical colloids are known to exhibit structural colours. The intensity and brilliance of these structural colours typically improve with colloidal monodispersity, low concentrations of point and line defects and with increasing refractive index contrast between the colloids and the embedding medium. Here we show that suspensions of charge stabilised, fluorinated latex particles with low refractive-index contrast to their aqueous background form Wigner crystals with FCC symmetry for volume fractions between 13 and 40%. In reflection they exhibit both strong, almost angle-independent structural colours and sharp, more brilliant Bragg peaks despite the particle polydispersity and bimodal distribution. Simultaneously, these suspensions appear transparent in transmission. Furthermore, binary AB, A2B and A13B type mixtures of these fluorinated and similarly sized polystyrene particles appeared predominantly white but with clear Bragg peaks indicating a...
arXiv: Soft Condensed Matter, 2020
The interplay between laser light, trapped particles, and fluid flow can produce counterintuitive... more The interplay between laser light, trapped particles, and fluid flow can produce counterintuitive effects in optical tweezing. Here we uncover an attractive, long-ranged, non-equilibrium force field centered on an optically trapped particle near a water-oil interface, produced by local heating and mediated by global fluid flow. This causes surrounding untrapped colloids, tethered to the interface but allowed to diffusely freely along it, to crystallize around the force center. In this configuration, the non-equilibrium force is the gradient of a potential, of strength proportional to the local heating, which, surprisingly, allows for an effective equilibrium description. Our results open unexplored routes to optofluidic manipulation and assembly of colloidal particles.
Optical Trapping and Optical Micromanipulation XV, 2018
One important aspect of the complete physical characterization of novel viscoelastic materials is... more One important aspect of the complete physical characterization of novel viscoelastic materials is the assessment of their response on short timescales. Optical tweezers, equipped with a fast quadrant photodiode, aid in fulfilling this task by providing high-frequency viscoelastic information about the sample. In passive microrheology, this is normally achieved by extracting rheological information from the thermal motion of an optically trapped bead embedded in a test fluid. Here we present the calibration and use of optical tweezers to study the formation of thermally reversible DNA hydrogels. We complement our results with rheological data from dynamic light scattering, video microscopy and conventional bulk rheology. Merging experimental data from different techniques allows us to study the viscoelastic behavior of these DNA networks over a wide frequency-band and the scaling of the complex viscoelastic modulus at the two frequency extremes. By analyzing the high-frequency behavior of our transient network, we prove the semi-flexible polymer nature of DNA and provide an estimate of its persistence length.
"Particles adsorbed at a fluid-fluid interface induce deformations in the shape of the interface.... more "Particles adsorbed at a fluid-fluid interface induce deformations in the shape of the interface. These socalled capillary deformations generate strong and long-range interactions between the particles, driving them to assemble into 2D structures. Therefore they are of crucial relevance in the formation of 2D (or quasi-2D) new materials. Through a recently introduced numerical method [1], we numerically calculate the equilibrium shape of the fluid-fluid interface for a given position and orientation of the adsorbed particles, and from this we obtain the capillary potential between the particles. In this talk we will present results for the capillary interactions and self-assembly of particles with various shapes and contact angles adsorbed at flat or possibly curved fluid-fluid interfaces. In particular, we will present results for adsorbed cubes, showing that, when they have a contact angle close to , they generate a hexapole deformation field in the interface height profile, which drives them to assemble into hexagonal and honeycomb structures [2], as observed experimentally [3]."
Physical Review E, 2021
We measure by experiment and particle-based simulation the rheology of concentrated, non-Brownian... more We measure by experiment and particle-based simulation the rheology of concentrated, non-Brownian droplet emulsions functionalized with surface-bound single-stranded (ss), "sticky," DNA. In the absence of ssDNA, the emulsion viscosity increases with the dispersed phase volume fraction φ, before passing through a liquid-solid transition at a critical φ c related to random close packing. Introducing ssDNA leads to a liquid-solid transition at φ < φ c , the onset being set by the droplet valency N and the ssDNA concentration (or simulated binding strength). Using insight from simulation, we identify three key behaviors: (i) jammed suspensions (φ > φ c ≈ 0.64) show weak effects of functionalization, with elastic rheology instead governed by droplet stiffness; (ii) suspensions with φ < φ c and N = 1, 2 always exhibit viscous rheology, regardless of functionalization; and (iii) for φ < φ c and N > 3, functionalization leads to a controllable viscous-elastic transition. We present state diagrams showing the range of rheological tuning attainable by these means.
Physical Review Letters, 2020
We report experiments that show rapid crystallization of colloids tethered to an oil-water interf... more We report experiments that show rapid crystallization of colloids tethered to an oil-water interface in response to laser illumination. This light-induced transition is due to a combination of longranged thermophoretic pumping and local optical binding. We show that the flow-induced force on the colloids can be described as the gradient of a potential. The non-equilibrium steady state due to local heating thus admits an effective equilibrium description. The optofluidic manipulation explored in this work opens novel ways to manipulate and assemble colloidal particles.
Langmuir, 2020
Today colloids are employed in various products from creams and coatings to electronics. The abil... more Today colloids are employed in various products from creams and coatings to electronics. The ability to control their chemical, optical, or electronic features by controlling their size and shape explains why these materials are so widely employed. Nevertheless, altering some of these properties may also lead to some undesired side effects, one of which is an increase in optical scattering upon concentration. Here, we address this strong scattering issue in films made of colloids with high surface roughness. We focus on raspberry type polymeric particles made of a spherical polystyrene core decorated by small hemispherical domains of acrylate. Owing to their surface charge and model 'roughness', aqueous dispersions of these particles display an unusual stability against aggregation. Under certain angles, their solid films display a brilliant red color due to Bragg scattering but otherwise appear completely white on account of`strong scattering. To suppress the scattering and induce transparency, we prepared films by hybridizing them either with oppositely-charged PSparticles that fit the length-scale of the raspberry roughness or with quantum dots. We report that the smaller PS-particles prevent raspberry particle aggregation in solid films and suppress scattering by decreasing the spatial variation of the refractive index. We believe that the results presented here provide a simple strategy to suppress strong scattering of rough particles and allow for their utilization in optical coatings, cosmetics, or photonics.
Soft Matter, 2020
DNA nanostars with sticky, single-stranded DNA overhangs, are known to form thermally reversible,... more DNA nanostars with sticky, single-stranded DNA overhangs, are known to form thermally reversible, viscoelastic networks. Here we show that introducing the right flexibility will rather lead to a cluster phase with a much reduced viscosity then an elastic hydrogel.
Langmuir, 2018
Many industrial soft materials often include oil-in-water (O/W) emulsions at the core of their fo... more Many industrial soft materials often include oil-in-water (O/W) emulsions at the core of their formulations. By using tuneable interface stabilizing agents, such emulsions can self-assemble into complex structures. DNA has been used for decades as a thermoresponsive highly specific binding agent between hard and, recently, soft colloids. Up until now, emulsion droplets functionalized with DNA had relatively low coating densities and were expensive to scale up. Here a general O/W DNA-coating method using functional non-ionic amphiphilic block copolymers, both diblock and triblock, is presented. The hydrophilic polyethylene glycol ends of the surfactants are functionalized with azides, allowing for efficient, dense and controlled coupling of dibenzocyclooctane functionalized DNA to the polymers through a strain-promoted alkyne-azide click reaction. The protocol is readily scalable due to the triblocks commercial availability. Different production methods (ultrasonication, microfluidics and membrane emulsification) are used with different oils (hexadecane and silicone oil) to produce functional droplets in various size ranges (sub-micron, ∼ 20 µm and > 50 µm), showcasing the generality of the protocol. Thermoreversible sub-micron emulsion gels, hierarchical "raspberry" droplets and controlled droplet release from a flat DNA-coated surface are demonstrated. The emulsion stability and polydispersity is evaluated using dynamic light scattering and optical microscopy. The generality and simplicity of the method opens up new applications in soft matter and biotechnological research and industrial advances.
Journal of Magnetism and Magnetic Materials, 2018
An exact analytical expression for the magnetic field of a cylinder of finite length with a unifo... more An exact analytical expression for the magnetic field of a cylinder of finite length with a uniform, transverse magnetization is derived. Together with known expressions for the magnetic field due to longitudinal magnetization, the calculation of magnetic fields for cylinders with an arbitrary magnetization direction is possible. The expression for transverse magnetization is validated successfully against the well-known limits of an infinitely long cylinder, the field on the axis of the cylinder and in the far field limit. Comparison with a numerical finite-element method displays good agreement, making the advantage of an analytical method over grid-based methods evident.
![Research paper thumbnail of Erratum to “Exact expression for the magnetic field of a finite cylinder with arbitrary uniform magnetization” [J. Magnet. Magnet. Mater. 456 (2018) 423–432]](https://attachments.academia-assets.com/102099814/thumbnails/1.jpg)
](Journal of Magnetism and Magnetic Materials, 2018
Erratum to ''Exact expression for the magnetic field of a finite cylinder with arbitrary uniform ... more Erratum to ''Exact expression for the magnetic field of a finite cylinder with arbitrary uniform magnetization" [
Nature Communications, 2018
Aqueous colloidal suspensions, both man-made and natural, are part of our everyday life. The appl... more Aqueous colloidal suspensions, both man-made and natural, are part of our everyday life. The applicability of colloidal suspensions, however, is limited by the range of conditions over which they are stable. Here we report a novel type of highly monodisperse raspberry-like colloids, which are prepared in a single-step synthesis that relies on simultaneous dispersion and emulsion polymerisation. The resulting raspberry colloids behave almost like hard spheres. In aqueous solutions, such prepared raspberries show unexpected stability against aggregation over large variations of added salt concentrations without addition of stabilisers. We present simple Derjaguin–Landau–Verwey–Overbeek (DLVO) calculations performed on raspberry-like and smooth colloids showing that this stability results from our raspberries’ unique morphology, which extends our understanding of colloidal stability against salting. Further, the raspberries’ stability facilitates the formation of superspheres and thin ...
Proceedings of the National Academy of Sciences, 2018
Significance While widely known as the molecule of life, DNA is also an amazing building block at... more Significance While widely known as the molecule of life, DNA is also an amazing building block at the nanoscale, since it allows us to design and program the structure and dynamics of functional nanomaterials. We exploit the programmability of DNA to achieve control over the rheology of self-assembled hydrogels, which have elastic or viscous behavior (similar to that of slime) that is finely regulated by temperature. Using microrheology to investigate the mechanical properties of DNA hydrogels at the microlength scale, we map the viscoelastic response over a broad range of frequencies and temperatures. The deep understanding in the fundamental physics provides a way to design DNA-based materials with precise control over the structure stability and rigidity at molecular level.
Polymers
Nano-sized particles functionalised with short single-stranded (ss)DNAs can act as detectors of c... more Nano-sized particles functionalised with short single-stranded (ss)DNAs can act as detectors of complementary DNA strands. Here we consider tri-block-copolymer-based, self-assembling DNA-coated nanoparticles. The copolymers are chemically linked to the DNA strands via azide (N3) groups. The micelles aggregate when they are linked with complementary ssDNA. The advantage of such block-copolymer-based systems is that they are easy to make. Here we show that DNA functionalisation results in inter-micellar attraction, but that N3-groups that have not reacted with the DNA detector strands also change the phase behaviour of the tri-block polymer solution. We studied the triblock copolymer, Pluronic® F108, which forms spherical micelles in aqueous solutions upon heating. We find that the triblock chains ending with either an N3 or N3-DNA complex show a dramatic change in phase behaviour. In particular, the N3-functionalisation causes the chain ends to cluster below the critical micelle temp...
arXiv (Cornell University), Nov 30, 2022
Soft' patchy surfactant micelles have become an additional building tool in selfassembling system... more Soft' patchy surfactant micelles have become an additional building tool in selfassembling systems. The triblock copolymer, Pluronic ® F108, forms spherical micelles in aqueous solutions upon heating leading to a simple phase diagram with a micellar crystalline solid at higher temperatures and concentrations. Here we report the strong influence of end-functionalising the chain ends either with an azide or azide-DNA complex on the systems phase behaviour. We find that the azide(N 3)functionalisation renders the chain ends weakly hydrophobic at lower temperatures, causing them to self-assemble into flower-micelles. This hydrophobicity increases with increasing temperature and poses a competing self-assembling mechanism to the solvent induces hydrophobic interactions between the middle-blocks of F108 at higher temperatures and leads to a macroscopic phase separation that is absent in the pure F108 system. However, when we attached short, hydrophilic single-stranded (ss)DNA to the azide groups via click chemistry the chain ends became 'sticky due to DNA hybridisation below the melting temperature of the complementary ssDNA ends while reverting to hydrophilic behaviour above. We characterise their structural and rheological properties via Dynamic Light Scattering (DLS) and DLS-based passive microrheology with an improved time-frequency domain inversion step. We present the structural behaviour of dilute and semi-dilute solutions of the original F108 system and compare the results with solutions containing either the F108azide (F108-N 3) or partially DNA-functionalised F108-azide chains. Our DLS and microrheology studies inform us on how the attachment of azide groups on F108 changes the mechanical and structural properties of micellar fluids pioneering further characterisation and design of these hybrid systems.
arXiv (Cornell University), Oct 25, 2022
We characterize the self-assembly of colloidal particles with surface mobile DNA linkers under ki... more We characterize the self-assembly of colloidal particles with surface mobile DNA linkers under kinetically limited valence conditions. For this, we put forward a computer simulation model that captures quantitatively the interplay between the main dynamic processes governing these systems and allows the simulation of the long time scales reached in experiments. The model is validated by direct comparison with available experimental results, showing an overall good agreement that includes measurements of the average effective valence and its probability distribution as a function of the density of DNA linkers on the particles surface. Finally, simulation results are used to evidence the opposite impact of particle density and characteristic DNA hybridization time on the effective valence.
arXiv (Cornell University), Jun 29, 2018
Today colloids are employed in various products from creams and coatings to electronics. The abil... more Today colloids are employed in various products from creams and coatings to electronics. The ability to control their chemical, optical, or electronic features by controlling their size and shape explains why these materials are so widely employed. Nevertheless, altering some of these properties may also lead to some undesired side effects, one of which is an increase in optical scattering upon concentration. Here, we address this strong scattering issue in films made of colloids with high surface roughness. We focus on raspberry type polymeric particles made of a spherical polystyrene core decorated by small hemispherical domains of acrylate. Owing to their surface charge and model 'roughness', aqueous dispersions of these particles display an unusual stability against aggregation. Under certain angles, their solid films display a brilliant red color due to Bragg scattering but otherwise appear completely white on account of`strong scattering. To suppress the scattering and induce transparency, we prepared films by hybridizing them either with oppositely-charged PSparticles that fit the length-scale of the raspberry roughness or with quantum dots. We report that the smaller PS-particles prevent raspberry particle aggregation in solid films and suppress scattering by decreasing the spatial variation of the refractive index. We believe that the results presented here provide a simple strategy to suppress strong scattering of rough particles and allow for their utilization in optical coatings, cosmetics, or photonics.
arXiv (Cornell University), Aug 19, 2018
The intensity and brilliance of such photonic crystals require high size-monodispersity of the co... more The intensity and brilliance of such photonic crystals require high size-monodispersity of the colloids, a low number of lattice defects and disorder, as well as a relatively large refractive index contrast between the scattering colloids and the continuous background. Here we present the unexpected photonic properties of aqueous suspensions of charge-stabilized, 186 nm large, fluorinated colloids with a refractive index of 1.37. Employing reflectivity, optical observation, small angle x-ray scattering measurements and reflectivity modeling, we demonstrate that these suspensions become partially transparent while showing strong, almost angle-independent color in reflection despite the very small refractive index difference. Under certain conditions additional sharp Bragg reflections are observed. We were able to tune the observed structural colors continuously across the entire visible range by simply changing the volume fraction of these colloidal suspensions, which show a white appearance when dilute, structural color and Bragg peaks when concentrated enough to form Wiegner crystal, and angle-independent color when very concentrated and in a glassy phase.
arXiv (Cornell University), Mar 23, 2017
We demonstrate the realization of (laterally) optically bounded colloidal structures on a liquidl... more We demonstrate the realization of (laterally) optically bounded colloidal structures on a liquidliquid interface of an emulsion droplet. We use DNA tethers to graft the particles to the droplet surface, effectively confining them to a quasi-2D plane with minimum constraints on the lateral movement even when optically trapped in a common single-beam configuration. We show that relatively weak interactions such as depletion can be measured in the optically bounded crystals by video-microscopy imaging and analysis. This illustrates the possible use of our system as template to study optically controlled quasi two-dimensional colloidal assembly on liquid-liquid interfaces. Optical tweezing is an established tool to examine interactions at the micro and nanometer level and between biomolecules, whose interactions are typically in the range from pico-to femtoNewtons [1-4]. Such forces are typically assessed by using an optically trapped particle as a probe [5]. Since opticaltrap sizes are usually on the same order of magnitude as the bead diameter, multiple particles can enter the same trap. This results in multiple scattering between the (dielectric) particles themselves, which display the tendency to reversibly form ordered structures: the phenomenon is dubbed optical binding [4]. The ordered structures are either chains of particles longitudinal to the direction of propagation of the light or two dimensional crystalline patterns perpendicular to the laser beam [6-8]. The latter, referred to as lateral optical binding, is of particular interest as the creation of ordered, quasi-2D patterns of many distinct colloids or nano-particles is one of the key challenges in complex self-assembly. However, its previous realization involved either complex multiple beam trapping setups [8, 9] or the use
Proceedings of the National Academy of Sciences of the United States of America, Oct 3, 2019
Frontiers in Physics, 2022
Spatially ordered arrangements of spherical colloids are known to exhibit structural colours. The... more Spatially ordered arrangements of spherical colloids are known to exhibit structural colours. The intensity and brilliance of these structural colours typically improve with colloidal monodispersity, low concentrations of point and line defects and with increasing refractive index contrast between the colloids and the embedding medium. Here we show that suspensions of charge stabilised, fluorinated latex particles with low refractive-index contrast to their aqueous background form Wigner crystals with FCC symmetry for volume fractions between 13 and 40%. In reflection they exhibit both strong, almost angle-independent structural colours and sharp, more brilliant Bragg peaks despite the particle polydispersity and bimodal distribution. Simultaneously, these suspensions appear transparent in transmission. Furthermore, binary AB, A2B and A13B type mixtures of these fluorinated and similarly sized polystyrene particles appeared predominantly white but with clear Bragg peaks indicating a...
arXiv: Soft Condensed Matter, 2020
The interplay between laser light, trapped particles, and fluid flow can produce counterintuitive... more The interplay between laser light, trapped particles, and fluid flow can produce counterintuitive effects in optical tweezing. Here we uncover an attractive, long-ranged, non-equilibrium force field centered on an optically trapped particle near a water-oil interface, produced by local heating and mediated by global fluid flow. This causes surrounding untrapped colloids, tethered to the interface but allowed to diffusely freely along it, to crystallize around the force center. In this configuration, the non-equilibrium force is the gradient of a potential, of strength proportional to the local heating, which, surprisingly, allows for an effective equilibrium description. Our results open unexplored routes to optofluidic manipulation and assembly of colloidal particles.
Optical Trapping and Optical Micromanipulation XV, 2018
One important aspect of the complete physical characterization of novel viscoelastic materials is... more One important aspect of the complete physical characterization of novel viscoelastic materials is the assessment of their response on short timescales. Optical tweezers, equipped with a fast quadrant photodiode, aid in fulfilling this task by providing high-frequency viscoelastic information about the sample. In passive microrheology, this is normally achieved by extracting rheological information from the thermal motion of an optically trapped bead embedded in a test fluid. Here we present the calibration and use of optical tweezers to study the formation of thermally reversible DNA hydrogels. We complement our results with rheological data from dynamic light scattering, video microscopy and conventional bulk rheology. Merging experimental data from different techniques allows us to study the viscoelastic behavior of these DNA networks over a wide frequency-band and the scaling of the complex viscoelastic modulus at the two frequency extremes. By analyzing the high-frequency behavior of our transient network, we prove the semi-flexible polymer nature of DNA and provide an estimate of its persistence length.
"Particles adsorbed at a fluid-fluid interface induce deformations in the shape of the interface.... more "Particles adsorbed at a fluid-fluid interface induce deformations in the shape of the interface. These socalled capillary deformations generate strong and long-range interactions between the particles, driving them to assemble into 2D structures. Therefore they are of crucial relevance in the formation of 2D (or quasi-2D) new materials. Through a recently introduced numerical method [1], we numerically calculate the equilibrium shape of the fluid-fluid interface for a given position and orientation of the adsorbed particles, and from this we obtain the capillary potential between the particles. In this talk we will present results for the capillary interactions and self-assembly of particles with various shapes and contact angles adsorbed at flat or possibly curved fluid-fluid interfaces. In particular, we will present results for adsorbed cubes, showing that, when they have a contact angle close to , they generate a hexapole deformation field in the interface height profile, which drives them to assemble into hexagonal and honeycomb structures [2], as observed experimentally [3]."
Physical Review E, 2021
We measure by experiment and particle-based simulation the rheology of concentrated, non-Brownian... more We measure by experiment and particle-based simulation the rheology of concentrated, non-Brownian droplet emulsions functionalized with surface-bound single-stranded (ss), "sticky," DNA. In the absence of ssDNA, the emulsion viscosity increases with the dispersed phase volume fraction φ, before passing through a liquid-solid transition at a critical φ c related to random close packing. Introducing ssDNA leads to a liquid-solid transition at φ < φ c , the onset being set by the droplet valency N and the ssDNA concentration (or simulated binding strength). Using insight from simulation, we identify three key behaviors: (i) jammed suspensions (φ > φ c ≈ 0.64) show weak effects of functionalization, with elastic rheology instead governed by droplet stiffness; (ii) suspensions with φ < φ c and N = 1, 2 always exhibit viscous rheology, regardless of functionalization; and (iii) for φ < φ c and N > 3, functionalization leads to a controllable viscous-elastic transition. We present state diagrams showing the range of rheological tuning attainable by these means.
Physical Review Letters, 2020
We report experiments that show rapid crystallization of colloids tethered to an oil-water interf... more We report experiments that show rapid crystallization of colloids tethered to an oil-water interface in response to laser illumination. This light-induced transition is due to a combination of longranged thermophoretic pumping and local optical binding. We show that the flow-induced force on the colloids can be described as the gradient of a potential. The non-equilibrium steady state due to local heating thus admits an effective equilibrium description. The optofluidic manipulation explored in this work opens novel ways to manipulate and assemble colloidal particles.
Langmuir, 2020
Today colloids are employed in various products from creams and coatings to electronics. The abil... more Today colloids are employed in various products from creams and coatings to electronics. The ability to control their chemical, optical, or electronic features by controlling their size and shape explains why these materials are so widely employed. Nevertheless, altering some of these properties may also lead to some undesired side effects, one of which is an increase in optical scattering upon concentration. Here, we address this strong scattering issue in films made of colloids with high surface roughness. We focus on raspberry type polymeric particles made of a spherical polystyrene core decorated by small hemispherical domains of acrylate. Owing to their surface charge and model 'roughness', aqueous dispersions of these particles display an unusual stability against aggregation. Under certain angles, their solid films display a brilliant red color due to Bragg scattering but otherwise appear completely white on account of`strong scattering. To suppress the scattering and induce transparency, we prepared films by hybridizing them either with oppositely-charged PSparticles that fit the length-scale of the raspberry roughness or with quantum dots. We report that the smaller PS-particles prevent raspberry particle aggregation in solid films and suppress scattering by decreasing the spatial variation of the refractive index. We believe that the results presented here provide a simple strategy to suppress strong scattering of rough particles and allow for their utilization in optical coatings, cosmetics, or photonics.
Soft Matter, 2020
DNA nanostars with sticky, single-stranded DNA overhangs, are known to form thermally reversible,... more DNA nanostars with sticky, single-stranded DNA overhangs, are known to form thermally reversible, viscoelastic networks. Here we show that introducing the right flexibility will rather lead to a cluster phase with a much reduced viscosity then an elastic hydrogel.
Langmuir, 2018
Many industrial soft materials often include oil-in-water (O/W) emulsions at the core of their fo... more Many industrial soft materials often include oil-in-water (O/W) emulsions at the core of their formulations. By using tuneable interface stabilizing agents, such emulsions can self-assemble into complex structures. DNA has been used for decades as a thermoresponsive highly specific binding agent between hard and, recently, soft colloids. Up until now, emulsion droplets functionalized with DNA had relatively low coating densities and were expensive to scale up. Here a general O/W DNA-coating method using functional non-ionic amphiphilic block copolymers, both diblock and triblock, is presented. The hydrophilic polyethylene glycol ends of the surfactants are functionalized with azides, allowing for efficient, dense and controlled coupling of dibenzocyclooctane functionalized DNA to the polymers through a strain-promoted alkyne-azide click reaction. The protocol is readily scalable due to the triblocks commercial availability. Different production methods (ultrasonication, microfluidics and membrane emulsification) are used with different oils (hexadecane and silicone oil) to produce functional droplets in various size ranges (sub-micron, ∼ 20 µm and > 50 µm), showcasing the generality of the protocol. Thermoreversible sub-micron emulsion gels, hierarchical "raspberry" droplets and controlled droplet release from a flat DNA-coated surface are demonstrated. The emulsion stability and polydispersity is evaluated using dynamic light scattering and optical microscopy. The generality and simplicity of the method opens up new applications in soft matter and biotechnological research and industrial advances.
Journal of Magnetism and Magnetic Materials, 2018
An exact analytical expression for the magnetic field of a cylinder of finite length with a unifo... more An exact analytical expression for the magnetic field of a cylinder of finite length with a uniform, transverse magnetization is derived. Together with known expressions for the magnetic field due to longitudinal magnetization, the calculation of magnetic fields for cylinders with an arbitrary magnetization direction is possible. The expression for transverse magnetization is validated successfully against the well-known limits of an infinitely long cylinder, the field on the axis of the cylinder and in the far field limit. Comparison with a numerical finite-element method displays good agreement, making the advantage of an analytical method over grid-based methods evident.
Journal of Magnetism and Magnetic Materials, 2018
Erratum to ''Exact expression for the magnetic field of a finite cylinder with arbitrary uniform ... more Erratum to ''Exact expression for the magnetic field of a finite cylinder with arbitrary uniform magnetization" [
Nature Communications, 2018
Aqueous colloidal suspensions, both man-made and natural, are part of our everyday life. The appl... more Aqueous colloidal suspensions, both man-made and natural, are part of our everyday life. The applicability of colloidal suspensions, however, is limited by the range of conditions over which they are stable. Here we report a novel type of highly monodisperse raspberry-like colloids, which are prepared in a single-step synthesis that relies on simultaneous dispersion and emulsion polymerisation. The resulting raspberry colloids behave almost like hard spheres. In aqueous solutions, such prepared raspberries show unexpected stability against aggregation over large variations of added salt concentrations without addition of stabilisers. We present simple Derjaguin–Landau–Verwey–Overbeek (DLVO) calculations performed on raspberry-like and smooth colloids showing that this stability results from our raspberries’ unique morphology, which extends our understanding of colloidal stability against salting. Further, the raspberries’ stability facilitates the formation of superspheres and thin ...
Proceedings of the National Academy of Sciences, 2018
Significance While widely known as the molecule of life, DNA is also an amazing building block at... more Significance While widely known as the molecule of life, DNA is also an amazing building block at the nanoscale, since it allows us to design and program the structure and dynamics of functional nanomaterials. We exploit the programmability of DNA to achieve control over the rheology of self-assembled hydrogels, which have elastic or viscous behavior (similar to that of slime) that is finely regulated by temperature. Using microrheology to investigate the mechanical properties of DNA hydrogels at the microlength scale, we map the viscoelastic response over a broad range of frequencies and temperatures. The deep understanding in the fundamental physics provides a way to design DNA-based materials with precise control over the structure stability and rigidity at molecular level.