Stefano Sacanna

• PhD
• Professor (Associate) at New York University

Microscale objects responding to chemical gradients by migrating toward or away from a preferred species is a simple yet constitutive mechanism by which transport occurs in biological organisms. Synthetic chemotaxis provides key physical descriptions of simplified systems that can be used in biological models, or in the creation of advanced respons...

Structures of molecular crystals are identified using scattering techniques because we cannot see inside them. Micrometre-sized colloidal particles enable the real-time observation of crystallization with optical microscopy, but in practice this is still hampered by a lack of ‘X-ray vision’. Here we introduce a system of index-matched fluorescently...

Colloid science has recently grown substantially owing to the innovative use of silane coupling agents (SCAs), especially 3-trimethoxysilylpropyl methacrylate (TPM). SCAs were previously used mainly as modifying agents, but their ability to form droplets and condense onto pre-existing structures has enabled their use as a versatile and powerful too...

Equilibrium gels provide physically attractive counterparts of nonequilibrium gels, allowing statistical understanding and design of the equilibrium gel structure. Here, we assemble two-dimensional equilibrium gels from limited-valency “patchy” colloidal particles and follow their evolution at the particle scale to elucidate cluster-size distributi...

Localized fluxes, production and/or degradation coupled to limited diffusion are well-known to result in stable spatial concentration gradients of biomolecules in the cell. In this study, we demonstrate that this also holds true for small ions, since we found that the close membrane apposition between the membrane of a phagosome and the surface of...

Colloids have a rich history of being used as ‘big atoms’ mimicking real atoms to study crystallization, gelation and the glass transition of condensed matter. Emulating the dynamics of molecules, however, has remained elusive. Recent advances in colloid chemistry allow patchy particles to be synthesized with accurate control over shape, functional...

The immune system has to cope with a wide range of irregularly shaped pathogens that can actively move (e.g., by flagella) and also dynamically remodel their shape (e.g., transition from yeast-shaped to hyphal fungi). The goal of this review is to draw general conclusions of how the size and geometry of a pathogen affect its uptake and processing b...

Using a combination of fluorescence and bright-field optical imaging, the solid-state packing structures of semi-confined two-layer spherical colloidal crystals were observed during modulation of an external AC electric field. Upon increasing field strength, the bottom layer of colloids (layer 1) transitioned from the entropically favored hexagonal...

Self-assembling colloidal particles in the cubic diamond crystal structure could potentially be used to make materials with a photonic bandgap1–3. Such materials are beneficial because they suppress spontaneous emission of light¹ and are valued for their applications as optical waveguides, filters and laser resonators⁴, for improving light-harvesti...

Regioselectivity in colloidal self-assembly typically requires specific chemical interactions to guide particle binding. In this paper, we describe a new method to form selective colloidal bonds that relies solely on polymer adsorption. Mixtures of polymer-coated and bare particles are initially stable due to long-ranged electrostatic repulsion. Wh...

The fundamental and practical importance of particle stabilization has motivated various characterization methods for studying polymer brushes on particle surfaces. In this work, we show how one can perform sensitive measurements of neutral polymer coating on colloidal particles using a commercial zetameter and salt solutions. By systematically var...

From rock salt to nanoparticle superlattices, complex structure can emerge from simple building blocks that attract each other through Coulombic forces1–4. On the micrometre scale, however, colloids in water defy the intuitively simple idea of forming crystals from oppositely charged partners, instead forming non-equilibrium structures such as clus...

Polymer precipitation under turbulent flow is used for the high-throughput synthesis of soft microparticles with fractal coronas that display significant adhesive properties.

In simple fluids, such as water, invariance under parity and time-reversal symmetry imposes that the rotation of constituent ‘atoms’ is determined by the flow and that viscous stresses damp motion. Activation of the rotational degrees of freedom of a fluid by spinning its atomic building blocks breaks these constraints and has thus been the subject...

We describe the synthesis and application of 3-(trimethoxysilyl)propyl methacrylate (TPM) particles as a colloidal model system for three-dimensional (3D) confocal scanning laser microscopy. The effect of the initial TPM concentration on the growth and polydispersity of the particles and a recently developed solvent transfer method to disperse part...

In simple fluids, such as water, invariance under parity and time-reversal symmetry imposes that the rotation of constituent 'atoms' are determined by the flow and that viscous stresses damp motion. Activation of the rotational degrees of freedom of a fluid by spinning its atomic building blocks breaks these constraints and has thus been the subjec...

Self-assembly is the autonomous organization of components into patterns or structures: an essential ingredient of biology and a desired route to complex organization¹. At equilibrium, the structure is encoded through specific interactions2–8, at an unfavourable entropic cost for the system. An alternative approach, widely used by nature, uses ener...

Self-assembly is the autonomous organization of components into patterns or structures: an essential ingredient of biology and a desired route to complex organization. At equilibrium, the structure is encoded through specific interactions, at an unfavorable entropic cost for the system. An alternative approach, widely used by Nature, uses energy in...

A simple yet versatile solution-based process to produce colloidal silica hexapods is developed in which various shapes of silica rods are grown on the faces of cubes in a controlled manner. In the presence of hematite cubic particles, water droplets nucleate on the surface of hematite by phase separation in pentanol. By adjusting the water concent...

The controlled shaping and surface functionalization of colloidal particles has provided opportunities for the development of new materials and responsive particles. The possibility of creating hollow particles with semi-permeable walls allows modulating molecular transport properties on colloidal length scales. While shapes and sizes can typically...

Increasing significance is being placed on the synthesis of smart colloidal particles, since the route to various meta-materials has been outlined through their bottom-up self-assembly. Unfortunately, making particles with well-defined shape and surface chemistry often requires considerable effort and time, and as such, are available only in restri...

We disclose the unique magnetic properties of cubic hematite particles and show that their self-assembly behavior is driven by competing anisotropic interactions caused by their shape and fixed dipole moment.

For magnetite spherical nanoparticles, the orientation of the dipole moment in the crystal does not affect the morphology of either zero field or field induced structures. For non-spherical particles however, an interplay between particle shape and direction of the magnetic moment can give rise to unusual behaviors, in particular when the moment is...

Patches on the surfaces of colloidal particles provide directional information that enables the self-assembly of the particles into higher-order structures. Although computational tools can make quantitative predictions and can generate design rules that link the patch motif of a particle to its internal microstructure and to the emergent propertie...

We report our direct Brownian motion observation of Fe2O3 magnetic cube chains under external magnetic fields, where the possibility of tuning the Brownian motion characteristics, such as diffusivity, by magnetic fields is realized. It has been experimentally demonstrated that the diffusivity is modified with an angular dependency on the magnetic f...

Reversible solid-state phase transitions between open- and close-packed structures in two-dimensional colloidal crystals comprising 1.8 μm dimpled spherical colloids were observed using negative dielectrophoresis. These asymmetrically-shaped colloids adopted lattices with cmm plane group symmetry and a packing fraction, φ, of 0.68 at low electric f...

Colloidal particles of controlled size are promising building blocks for self-assembly of functional materials. Here, we systematically study a method to synthesize monodisperse, micron-sized spheres from 3-(trimethoxysilyl)propylmethacrylate (TPM) in a bench-top experiment. Their ease of preparation, smoothness, and physical properties provide dis...

Figure 1. Artificial Phototactic Microswimmers

Condensation of objects into stable clusters occurs naturally in equilibrium and driven systems. It is commonly held that potential interactions, depletion forces, or sensing are the only mechanisms which can create long-lived compact structures. Here we show that persistent motile structures can form spontaneously from hydrodynamic interactions al...

Condensation of objects into stable clusters occurs naturally in equilibrium and driven systems. It is commonly held that potential interactions, depletion forces, or sensing are the only mechanisms which can create long-lived compact structures. Here we show that persistent motile structures can form spontaneously from hydrodynamic interactions al...

We describe colloidal Janus particles with metallic and dielectric faces that swim vigorously when illuminated by defocused optical tweezers without consuming any chemical fuel. Rather than wandering randomly, these optically-activated colloidal swimmers circulate back and forth through the beam of light, tracing out sinuous rosette patterns. We pr...

The ability to reconfigure elementary building blocks from one structure to another is key to many biological systems. Bringing the intrinsic adaptability of biological systems to traditional synthetic materials is currently one of the biggest scientific challenges in material engineering. Here we introduce a new design concept for the experimental...

Switchable defect. Video microscopy showing a colloidal crystal doped with a shape-shifting particle acting as a point defect. When the crystal is illuminated, the impurity change shape, thus causing the lattice to rearrange and the strain disappears.

Light-reconfigurable colloidal crystal. Video microscopy showing a reconfigurable colloidal crystal (assembled by depletion forces from individual shape-shifting building blocks) that morphs into a striped microstructure after being exposed to light. Video played at 3.7X real time.

Supplementary Figures 1-10, Supplementary Methods, and Supplementary References

Dewetting of oil droplets from Hematite seeds using HCl. Video microscopy showing the HCl-triggered dewetting of oil from colloidal cubes of Hematite. Video played at 10X real time.

Dewetting and detachment of oil droplets from Hematite seeds using light. Video microscopy showing the light-triggered dewetting of oil from peanut-shaped Hematite colloids. Video played at 3X real time.

A new colloidal system which consists of core-shell "probe" particles embedded in an optically transparent "host" particle suspension is developed. This system enables simultaneous fast confocal imaging and optical tweezing in dense 3D colloidal materials.

We describe colloidal Janus particles with metallic and dielectric faces that swim vigorously when illuminated by defocused optical tweezers without consuming any chemical fuel. Rather than wandering randomly, these optically-activated colloidal swimmers circulate back and forth through the beam of light, tracing out sinuous rosette patterns. We pr...

Using confocal microscopy and first passage time analysis, we measure and predict the rates of formation and breakage of polymer-depletion-induced bonds between lock-and-key colloidal particles and find that an indirect route to bond formation is accessed at a rate comparable to that of the direct formation of these bonds. In the indirect route, th...

Motility is a basic feature of living microorganisms, and how it works is often determined by environmental cues. Recent efforts have focused on develop- ing artificial systems that can mimic microorganisms, and in particular their self-propulsion. Here, we report on the design and characterization of syn- thetic self-propelled particles that migra...

Significance Since antiquity it has been known that particle shape plays an essential role in the symmetry and structure of matter. A familiar example comes from dense packings, such as spheres arranged in a face-centered cubic lattice. For colloidal superballs, we observe the transition from hexagonal to rhombic crystals consistent with the denses...

Light-activated self-propelled colloids are synthesized and their active motion is studied using optical microscopy. We propose a versatile route using different photoactive materials, and demonstrate a multiwavelength activation and propulsion. Thanks to the photoelectrochemical properties of two semiconductor materials (α-Fe2O3 and TiO2), a light...

In polymer-colloid mixtures, non-adsorbing polymers dispersed with much larger colloids provide a universal yet specific entropic attraction between the colloids. Such so-called depletion interaction arises from an osmotic-pressure imbalance caused by the polymers and is considered to be independent of temperature. Here we show that, for the most c...

Through the design and manipulation of discrete, nanoscale systems capable of encoding massive amounts of information, the basic components of computation are open to reinvention. These components will enable tagging, memory storage, and sensing in unusual environments - elementary functions crucial for soft robotics and "wet computing". Here we sh...

Colloids with well-defined multicavities are synthesized through the hydrolytic removal of silica cluster templates from organo-silica hybrid patchy particles. The geometry of the cavities stems from the originally assembled cluster templates, displaying well-defined three-dimensional symmetries, ranging from spherical, linear, triangular, tetrahed...

Ultra-smooth, highly spherical monocrystalline gold particles were prepared by a cyclic process of slow growth followed by slow chemical etching, which selectively removes edges and vertices. The etching process effectively makes the surface tension isotropic, so that spheres are favored under quasi-static conditions. It is scalable up to particle...

We study the behaviour of uniformly dimpled colloidal particles in the vicinity of a featureless wall. We use laser scanning confocal microscopy to observe the structures that form in the first layer at the wall for both dimpled colloidal particles and their spherical counterparts. We show that there are notable differences in both the translationa...

We introduce a self-propelled colloidal hematite docker that can be steered to a small parti- cle cargo many times its size, dock, transport the car- go to a remote location, and then release it. The self- propulsion and docking are reversible and activated by visible light. The docker can be steered either by a weak uniform magnetic field or by na...

Fluids of spherical colloids possessing an off-centered embedded magnetic dipole were investigated by using Monte Carlo simulations. Systems of colloids with different strengths and directions of the embedded dipole moment confined in a 2D space without and with an external magnetic field applied were considered. The fluids were characterized by ra...

This article investigates the role of shape in colloidal self-assembly and argues for the importance of a tight synergy between particle design and assembly strategies. To this end, we review synthetic methodologies developed to impart colloidal building blocks with anisotropic shapes and self-assembly mechanisms that exploit geometry to direct and...

'Patchy colloids' is a term that has been recently introduced to indicate specially engineered particles with directional interactions. Based on this concept, a 'bottom-up' process for fabricating functional materials and devices has been envisioned, which employs colloidal building blocks and mimics molecular bonding. This article reviews recent p...

The creation of a new material often starts from the design of its constituent building blocks at a smaller scale. From macromolecules to colloidal architectures, to granular systems, the interactions between basic units of matter can dictate the macroscopic behaviour of the resulting engineered material and even regulate its genesis. Information c...

We probe the rheology of self-assembling chains of ``pacman'' particles using a Zimm viscometer, a modified Couette apparatus. Pacman particles are microscopic spherical particles specially designed to have a spherical indentation on their surface. In the presence of a depletant, overlap between the indentation and another particle's surface maximi...

Colloidal particles with complementary shapes can self-organize into composite structures under the influence of entropic attractions mediated by depletion. What structures can form is governed by the colloidal components' shapes. The structures' stability can be tuned by adjusting the strength of the depletion attraction. Even when a particular co...

Self propelled colloids realize a controlled realization of an artificial bacterium. However living systems present a range of advanced properties such as the migration in gradients, or taxis, based on complex conformational change of proteins. For example, rheotaxis, the directed movement of an organism resulting from a fluid flow, has been report...

Light On Clusters From schools of fish to bacterial colonies, large-scale phenomena—such as swarming or pattern formation—are ubiquitous. In such systems, there is a continuing question as to the relative importance of "intelligence" (biology) of the agents versus purely physical effects. Working with synthetic colloids, Palacci et al. (p. 936 ) sh...

The preparation of anisotropic colloidal particles by a simple yet versatile temperature-controlled swelling process is described. The resulting polymeric particles feature a surface dimple, the size and shape of which were determined by the amount of oil captured in particles and the interfacial tension between the three phases: polystyrene (PS),...

We introduce a new class of spherical colloids that reversibly self-assemble into well-defined nonlinear structures by virtue of "magnetic patches". This assembly is driven by tunable magnetostatic binding forces that originate from microscopic permanent magnets embedded underneath the surface of the particles. The resulting clusters form spontaneo...

A novel type of light activated microswimmers is used to drive the system far from equilibrium. For sufficient concentrations they spontaneously assemble in crystalline clusters of particles. These clusters are mobile, with complex dynamics --explosion, self healing...-- and can reversibly melt down if the activity is shut down. The origin of the a...

We are exploring the self assembly of colloidal matter using building blocks with complex shapes and functionalities. Our toolbox includes particles with tunable cavities and protrusions, particles with flexible ball-and-socket joints, colloidal cubes and particles with magnetic patches. Using these building blocks and a variety of interactions, in...

We present a new type of colloidal artificial swimmers propelled by the decomposition of hydrogen peroxide and activated by light. The effect is reversible and allows external control of the swimming/non swimming behavior of the particles. Moreover the bulk synthesis makes possible the study of very concentrated assemblies of monodisperse and ident...

The lock and key models using Pac-man particles is an alternative identification mechanism for directing the assembly of combined structures. The system was guided by Fischer's lock- and key principle which consisted of colloidal spheres as keys and monodisperse colloidal particles with a spherical cavity as locks that bind. What makes this so spec...

We have studied the crystallization behavior of colloidal cubes by means of tunable depletion interactions. The colloidal system consists of novel micron-sized cubic particles prepared by silica deposition on hematite templates and various non-adsorbing water-soluble polymers as depletion agents. We have found that under certain conditions the cube...

Recent breakthroughs in colloidal synthesis allow the control of particle shapes and properties with high precision. This provides us with a constantly expanding library of new anisotropic building blocks, thus opening new avenues to explore colloidal self-assembly at a higher level of complexity. This article reviews the most recent advances in th...

We developed a new class of colloidal particles that programmably and reversibly self-assemble into well-defined clusters by virtue of "magnetic patches" carrying a permanent magnetic dipole moment. The resulting clusters form spontaneously in a zero external field, and their geometry is entirely determined by the interplay between magnetic, steric...

We have developed a new simple method to fabricate bulk amounts of colloidal spheres with well defined cavities from monodisperse emulsions. Herein, we describe the formation mechanism of “reactive” silicon oil droplets that deform to reproducible shapes via a polymerization-induced buckling instability. Owing to their unique shape, the resulting p...

We report the first observation of chiral colloidal liquid crystals of rod-like particles from a low molecular weight organic compound—phytosterols. Based on the particles shape and crystal structure, we attribute this phenomenon to chiral distribution of surface charge on the surface of neighbouring rods.

Particular types of solid-stabilized emulsions can be thermodynamically stable as evidenced by their spontaneous formation and monodisperse droplet size, which only depends on system parameters. Here, we investigate the generality of these equilibrium solid-stabilized emulsions with respect to the basic constituents: aqueous phase with ions, oil, a...

New functional materials can in principle be created using colloids that self-assemble into a desired structure by means of a programmable recognition and binding scheme. This idea has been explored by attaching 'programmed' DNA strands to nanometre- and micrometre- sized particles and then using DNA hybridization to direct the placement of the par...

We present a refractive-index-matched colloidal system that allows direct observation of critical Casimir induced aggregation with a confocal microscope. We show that in this system, in which van der Waals forces are negligible, a simple competition between repulsive screened Coulomb and attractive critical Casimir forces can account quantitatively...

We study two-dimensional crystallography on a curved oil-water interface. Charged hydrophobic (PMMA) colloids in an oil phase (cyclohexyl bromide) are attracted, without wetting, by image charge effects to an oil-water interface. The micron size spheres form a monolayer on the interface and interact via screened coulomb interactions to form a cryst...

Light scattering techniques are widely used in many fields of condensed and soft matter physics. Usually these methods are based on the study of the scattered light in the far field. Recently, a new family of near field detection schemes has been developed, mainly for the study of small angle light scattering. These techniques are based on the dete...

A novel single-step synthesis of monodisperse latex-based core-shell colloids, polymethyl methacrylate (PMMA) that completely relies on self-assembly of the components involved was reported. 5 ml of TPM (methacryloxy- propyltrimethoxysilane) and 2 ml of MM (methylmethacrylate) were mixed to a water based ferrofluid consisting in 12 nm Fe3O4 particl...

Charge-stabilized dispersions of inorganic colloids are shown to induce spontaneous emulsification of hydrophobic (TPM) molecules to stable oil-in-water emulsions, with monodisperse, mesoscopic oil droplet diameters in the range of 30-150 nm, irrespective of the polydispersity of the starting dispersions. The results for cobalt ferrite particles an...

We report on the preparation of a novel type of particle-stabilized oil-in-water emulsions. The emulsification mechanism comprises partial hydrolysis of the oil phase promoted by the alkaline surface of ellipsoidal hematite colloids stabilized by tetramethylammonium hydroxide. This mechanism yields monodisperse oil droplets with embedded single ell...

We have measured the random packing density of monodisperse colloidal silica ellipsoids with a well-defined shape, gradually deviating from a sphere shape up to prolates with aspect ratios of about 5, to find for a colloidal system the first experimental observation for the density maximum (at an aspect ratio near 1.6) previously found only in comp...

We show that under appropriate conditions, mixtures of oil, water, and nanoparticles form thermodynamically stable oil-in-water emulsions with monodisperse droplet diameters in the range of 30-150 nm. This observation challenges current wisdom that so-called Pickering emulsions are at most metastable and points to a new class of mesoscopic equilibr...

A procedure is presented to determine the permanent magnetic dipole moment of composite microspheres containing magnetic nanoparticles with a blocked magnetic dipole moment. The composite particles are dispersed in a solvent, and the complex magnetic susceptibility is measured from 0.1 to 1000 Hz using a highly sensitive new setup. Composite partic...

The research described in this thesis focuses on novel synthetic strategies for the preparation of well-defined model colloids. The preparation routes, involving self-assembly, emulsions and colloidal templating, are meant to facilitate design and fabrication of complex multi-shell composite colloids with tunable optical, magnetic and morphological...

We report the preparation and properties of monodisperse magnetic poly(methyl methacrylate) latex spheres that exhibit field-induced colloidal crystallization to exotic morphologies controlled by the geometry of the gradient. The magnetic moment of the novel magnetic spheres is due to an inner core of magnetite particles. These particles, obtained...

In this joint experimental-theoretical work we study hydrodynamic interaction effects in dense suspensions of charged colloidal spheres. Using x-ray photon correlation spectroscopy we have determined the hydrodynamic function H(q), for a varying range of electrosteric repulsion. We show that H(q) can be quantitatively described by means of a novel...

We report on the preparation of monodisperse, fluorescent hematite-silica core-shell ellipsoids, with adjustable shapes ranging from spindles to nearly spheres, that are suitable for optical rotational diffusion studies. Hematite cores are grafted with poly(vinylpyrrolidone) which ensures colloidal stability during the silica coating provided by th...

We report experiments on the stability of aqueous mixtures of charged colloidal magnetite and charged silica and silica covered with alumina particles of similar size. First, positively charged magnetite dispersions were mixed with negatively charged silica dispersions at pH 4, at different volume ratios and low colloid volume fractions, producing...

The polymerization in a microemulsion, which provides a useful technique for the synthesis of very small fluorinated latex particles in the size range 17-50 nm, was described. The microemulsion was irradiated at room temperature for 8 h by an 8-W ultraviolet (UV) lightbulb to promote the radical polymerization. Dynamic light scattering (DLS) and tr...

We report an experimental study of rotational and translational diffusion and sedimentation of colloidal tracer spheres in semidilute solutions of the nonadsorbing semiflexible polymer xanthan. The tracers are optically anisotropic, permitting depolarized dynamic light scattering measurements without interference from the polymer background. The xa...

We report the emulsion polymerization and seeded growth of negatively charged, monodisperse fluorinated latex spheres in water with radii in the range 50-700 run. Due to their low refractive index (n(p) = 1.3660), the spheres can be index matched in aqueous media, without any complications due to (optical) polydispersity or specific solvent adsorpt...