arXiv subjects

Daniela J. Kraft

Publications and source records attributed to Daniela J. Kraft.

At least 19 recordsLinked to original sources

arXiv2026

Assembly pathways of anisotropic lipid membrane-deforming colloids

Membrane-deformation mediated interactions play an important role in the spatial organization of proteins on the cell membrane. Although interactions between isotropic membrane deformations have been extensively investigated, the role of anisotropic deformations remains largely unexplored despite their prevalence in biological systems. Here, we experimentally investigate the assembly of anisotropic colloidal objects that deform a lipid membrane while being confined underneath it, without direct attachment. Combining experiments and numerical calculations, we analyze how a wide range of shapes, including ellipsoids, dumbbells, cubes, scalene triangles, tetrahedra, and bent rods, interact with each other through the membrane deformations they induce. We find that membrane-deforming objects initially attract through regions of highest curvature and subsequently reorient into close packed arrangements with an approximately spherical circumference. This is achieved through the alignment of flat faces - if possible in register - and locally optimized geometric packing, with regions of high curvature imposing energy barriers that influence the assembly pathway. Our work reveals general principles how anisotropic membrane deformations govern the assembly pathways and final particle arrangements, providing new insights into the behavior of membrane-deforming proteins and other inclusions.

cond-mat.soft↗

arXiv2026

Self-Generated Electric Fields in Polyelectrolyte Gradients Increase Microparticle Transport

There are many situations in nature and industry where small particles are exposed to gradients of charged polymers, such as enzymes in biological gradients of DNA or RNA, virus particles in respiratory droplets, and colloidal particles in stratifying paint layers. Here, we study the phoretic propulsion of charged microparticles in a polyelectrolyte gradient. We theoretically predict the emergence of a macroscopic electric field from charge-separation dynamics in a polyelectrolyte gradient under a continuous diffusive driving force. We confirm the presence of this self-generated electric field experimentally and show that it significantly increases the phoretic velocity of the microparticles. Finally, for high molecular weight polyelectrolytes we observe that propulsion becomes gradient-independent, consistent with diffusiophoretic predictions for asymmetric electrolytes. Our results show that self-generated electric fields in polyelectrolyte gradients can enhance microparticle transport, with potential applicability wherever charged species of different mobility are continuously driven out of equilibrium.

cond-mat.soft↗

arXiv2026

3D microprinting anisotropic and deformable active matter -- A perspective

Active colloidal particles provide versatile model systems for exploring non-equilibrium physics in motile matter. To date, most experimental realizations have focused on spherical particles, largely due to fabrication constraints. However, theoretical and computational studies have long predicted that particle anisotropy and flexibility can dramatically enrich single-particle dynamics, interparticle interactions, and emergent collective behavior. Here, we highlight recent advances in the fabrication of anisotropic active particles and architectures enabled by the unprecedented design freedom of 3D microprinting. We discuss how additive manufacturing is expanding the accessible parameter space of active soft matter, allowing precise control over shape, location of active forces, and functionality at the microscale. These developments establish new model platforms for uncovering fundamental principles of active and soft matter, and simultaneously pave the way toward microrobotic systems with programmable dynamics and emergent functionalities.

cond-mat.soft↗

arXiv2025

Shape-dependent direction reversal in anisotropic catalytic microswimmers

The propulsion direction of active particles is a key feature in self-propelled systems and depends on the propulsion mechanism and environmental conditions. Here, using 3D micro-printed catalytically active particles, we experimentally show that the propulsion direction can change with increasing fuel concentration when the active particle possesses an anisotropic shape. We find that discs, tori, and bent rods reverse their direction of motion with increasing hydrogen peroxide concentration--moving with their inert side forward at low concentrations and with their catalytic side forward at high concentrations. In contrast, spheres and straight rods do not exhibit this reversal. We observe that direction reversal is independent of the base material composition of the swimmer and its size, and only occurs for anisotropic particles where, due to their elongated shape, the location of the solute concentration maximum is unstable and can be shifted by substrate-induced confinements. Our measurements suggest that in addition a change in the platinum-catalyzed reaction of hydrogen peroxide occurs, to which particles with elongated shapes that induce sufficient confinement are more sensitive.

cond-mat.soft↗

arXiv2025

Autonomous life-like behavior emerging in active and flexible microstructures

Many organisms leverage an interplay between shape and activity to generate motion and adapt to their environment. Embedding such feedback into synthetic microrobots could eliminate the need for sensors, software, and actuators, yet current realizations are either active but rigid, or flexible but passive. Here, we introduce micrometer-scale structures that integrate both activity and flexibility by 3D microprinting concatenated units and actuating them with an AC electric field. This minimal yet versatile design gives rise to a rich array of life-like modes of motion - including railway and undulatory locomotion, rotation, and beating - as well as emergent sense-response abilities, which enable autonomous reorientation, navigation, and collision avoidance. Our approach offers a versatile platform for designing biomimetic model systems and autonomously operating microrobots with embodied intelligence.

cond-mat.soft↗

arXiv2025

Colloidal Pivots Enable Brownian Metamaterials and Machines

Biological machines harness targeted deformations that can be actuated by Brownian fluctuations. However, while synthetic micromachines can similarly leverage targeted deformations they are too stiff to be driven by thermal fluctuations and thus require strong forcing. Furthermore, systems that are able to change their conformation by thermal fluctuations do so uncontrollably or require external control. Here we leverage DNA-based sliding contacts to create colloidal pivots, rigid anisotropic objects that freely fluctuate around their pivot point, and use a hierarchical strategy to assemble these into Brownian metamaterials and machines with targeted deformation modes. We realize the archetypical rotating diamond and rotating triangle, or Kagome, geometries, and quantitatively show how thermal fluctuations drive their predicted auxetic deformations. Finally, we implement magnetic particles into the colloidal pivots to achieve an elementary Brownian machine with easily actuatable deformations that can harness Brownian fluctuations. Together, our work introduces a strategy for creating thermal mechanical metamaterials and leverages them for functional Brownian devices, paving the way to materialize flexible, actuatable structures for micro-robots, smart materials, and nano-medicine.

cond-mat.soft↗

arXiv2025

Fabrication and characterization of bimetallic silica-based and 3D-printed active colloidal cubes

Simulations on self-propelling active cubes reveal interesting behaviors at both the individual and the collective level, emphasizing the importance of developing experimental analogs that allow to test these theoretical predictions. The majority of experimental realizations of active colloidal cubes rely on light actuation and or magnetic fields to have a persistent active mechanism, and lack material versatility. Here we propose a system of active bimetallic cubes whose propulsion mechanism is based on a catalytic reaction and study their behavior. We realize such a system from synthetic silica cuboids and 3D printed micro cubes, followed by the deposition of gold and platinum layers on their surface. We characterize the colloids dynamics for different thicknesses of the gold layer at low and high hydrogen peroxide concentrations. We show that the thickness of the base gold layer has only a minor effect on the self propulsion speed and in addition induces a gravitational torque which leads to particles with a velocity director pointing out of the plane thus effectively suppressing propulsion. We find that a higher active force can remedy the effects of torque, resulting in particle orientations that are favorable for in plane propulsion. Finally, we use 3D printing to compare our results to cubes made from a different material, size and roundness, and demonstrate that the speed scaling with increasing particle size originates from the size-dependent drag. Our experiments extend fabrication of active cubes to different materials and propulsion mechanisms and highlight that the design of active particles with anisotropic shapes requires consideration of the interplay between the shape and activity to achieve favorable sedimentation and efficient in plane propulsion.

cond-mat.soft↗

arXiv2024

Repulsion and attraction in the interactions of opposite membrane deformations

Lipid membrane deformations have been predicted to lead to indirect forces between the objects that induce these deformations. Recent experimental measurements have found an attractive interaction between spherical particles that all induce a deformation towards the inside of a giant unilamellar vesicle. Here, we complement these experimental observations by investigating the interactions between deformations pointing in opposite directions with respect to the membrane normal vector. This is experimentally realized by a particle deforming the membrane towards the inside of the GUV and pulling a membrane tube towards the outside of the membrane. Particles completely wrapped by the membrane are repelled from the tube with a strength of 3~k$_B$T at a distance of 0.5~$μ$m. However, particles that strongly curve the membrane by adhering only to a patch of about 50~\% of its surface area are attracted to the center of the tube with a strength of -5.3~k$_B$T at a minimum distance of about 1~$μ$m. We find that such Janus particles also experience attractive interactions when both deforming the membrane in the same way. These quantitative experimental observations provide new insights into interactions between oppositely membrane deforming objects, important for cooperative protein assembly at or interactions of microplastics with cell membranes.

cond-mat.soft↗

arXiv2024

The motion of catalytically active colloids approaching a surface

Catalytic microswimmers typically swim close to walls due to hydrodynamic and/or phoretic effects. The walls in turn are known to affect their propulsion, making it difficult to single out the contributions that stem from particle-based catalytic propulsion only, thereby preventing an understanding of the propulsion mechanism. Here, we use acoustic tweezers to lift catalytically active Janus spheres away from the wall to study their motion in bulk and when approaching a wall. Mean-squared displacement analysis shows that diffusion constants at different heights match with Faxén's prediction for the near-wall hydrodynamic mobility. Both particles close to a substrate and in bulk show a decrease in velocity with increasing salt concentration, suggesting that the dominant factor for the decrease in speed is a reduction of the swimmer-based propulsion. The velocity-height profile follows a hydrodynamic scaling relation as well, implying a coupling between the wall and the swimming speed. The observed speed reduction upon addition of salt matches expectations from a electrokinetic theory, except for experiments in 0.1 wt% hydrogen peroxide in bulk, which could indicate contributions from a different propulsion mechanism. Our results help with the understanding of ionic effects on microswimmers in 3D and point to a coupling between the wall and the particle that affects its self-propulsion speed.

cond-mat.soft↗

arXiv2024

Optimal self-assembly pathways towards colloidal lattices with tunable flexibility

Flexibility governs the many properties of materials and is crucial for the function of proteins and biopolymers. However, how the self-assembly of flexibly bonded particles can lead to larger structures with global reconfigurability is unexplored. We here use a binary colloidal model system equipped with flexible DNA-based bonds to study how regular structures with tunable flexibility can be created through self-assembly. We find that the reconfigurability during lattice growth leads to lattices with square symmetry which are inherently mechanically unstable and hence thermally floppy. By considering the role of size ratio, number ratio, and directionality induced by particle shape, we identify the optimal pathways that maximize the yield and flexibility of these square lattices using a combination of experiments, analytical calculations, and simulations. Our study highlights the crucial role of reconfigurability in systems that are governed by enthalpic and entropic principles, from synthetic to biological, and might be useful for creating materials with novel or reconfigurable properties.

cond-mat.soft↗

arXiv2024

Non-additivity in many-body interactions between membrane-deforming spheres increases disorder

Membrane-induced interactions have been predicted to be important for the organization of membrane proteins. Measurements of the interactions between two and three membrane deforming objects have revealed their non-additive nature. They are thought to lead to complex many-body effects, however, experimental evidence is lacking to date. We here present an experimental method to measure many-body effects in membrane-mediated interactions using colloidal spheres placed between a deflated giant unilamellar vesicles and a planar substrate. The thus confined colloidal particles cause a large deformation of the membrane while not being physochemically attached to it and interact through it. Two particles are found to attract with a maximum force of 0.2~pN. For three particles, we observe a preference for forming compact equilateral triangles over a linear arrangement. We use numerical energy minimization to establish that the attraction stems from a reduction in the membrane-deformation energy caused by the particles. Confining up to 36 particles, we find a preference for hexagonally close packed clusters. However, with increasing number of particles the order of the confined particles decreases, while at the same time, diffusivity of the particles increases. Our experiments for the first time show that the non-additive nature of membrane-mediated interactions affects the interactions and arrangements and ultimately leads to spherical aggregates with liquid-like order of potential importance for cellular processes.

cond-mat.soft↗

arXiv2023

Power-law intermittency in the gradient-induced self-propulsion of colloidal swimmers

Active colloidal microswimmers serve as archetypical active fluid systems, and as models for biological swimmers. Here, by studying in detail their velocity traces, we find robust power-law intermittency with system-dependent exponential cut off. We model the motion by an interplay of the field gradient-dependent active force and the locally fluctuating hydrodynamic drag, set by the wetting properties of the substrate. The model closely describes the velocity distributions of two disparate swimmer systems: AC field activated and catalytic swimmers. The generality is highlighted by the collapse of all data in a single master curve, suggesting the applicability to further systems, both synthetic and biological.

cond-mat.soft↗

arXiv2023

Designing highly efficient lock-and-key interactions in anisotropic active particles

Cluster formation of microscopic swimmers is key to the formation of biofilms and colonies, efficient motion and nutrient uptake, but, in the absence of other interactions, requires high swimmer concentrations to occur. Here we experimentally and numerically show that cluster formation can be dramatically enhanced by an anisotropic swimmer shape. We analyze a class of model microswimmers with a shape that can be continuously tuned from spherical to bent and straight rods. In all cases, clustering can be described by Michaelis-Menten kinetics governed by a single scaling parameter that depends on particle density and shape only. We rationalize these shape-dependent dynamics from the interplay between interlocking probability and cluster stability. The bent rod shape promotes assembly even at vanishingly low particle densities and we identify the most efficient shape to be a semicircle. Our work provides key insights into how shape can be used to rationally design out-of-equilibrium self-organization, key to creating active functional materials and designing targeted two-component drug delivery.

cond-mat.soft↗

arXiv2023

Soft and stiff normal modes in floppy colloidal square lattices

Floppy microscale spring networks are widely studied in theory and simulations, but no well-controlled experimental system currently exists. Here, we show that square lattices consisting of colloid-supported lipid bilayers functionalized with DNA linkers act as microscale floppy spring networks. We extract their normal modes by inverting the particle displacement correlation matrix, showing the emergence of a spectrum of soft modes with low effective stiffness in addition to stiff modes that derive from linker interactions. Evaluation of the softest mode, a uniform shear mode, reveals that shear stiffness decreases with lattice size. Experiments match well with Brownian particle simulations and we develop a theoretical description based on mapping interactions onto linear response to describe the modes. Our results reveal the importance of entropic steric effects, and can be used for developing reconfigurable materials at the colloidal length scale.

cond-mat.soft↗

arXiv2023

Optimality in superselective surface binding by multivalent DNA nanostars

Weak multivalent interactions govern a large variety of biological processes like cell-cell adhesion and virus-host interactions. These systems distinguish sharply between surfaces based on receptor density, known as superselectivity. Earlier experimental and theoretical work provided insights into the control of selectivity: Weak interactions and a high number of ligands facilitate superselectivity. Present experimental studies typically involve tens or hundreds of interactions, resulting in a high entropic contribution leading to high selectivities. However, if, and if so how, systems with few ligands, such as multi-domain proteins and virus binding to a membrane, show superselective behavior is an open question. Here, we address this question with a multivalent experimental model system based on star shaped branched DNA nanostructures (DNA nanostars) with each branch featuring a single stranded overhang that binds to complementary receptors on a target surface. Each DNA nanostar possesses a fluorophore, to directly visualize DNA nanostar surface adsorption by total internal reflection fluorescence microscopy (TIRFM). We observe that DNA nanostars can bind superselectively to surfaces and bind optimally at a valency of three. We quantitatively explain this optimum by extending the current theory with interactions between DNA nanostar binding sites (ligands). Our results add to the understanding of multivalent interactions, by identifying microscopic mechanisms that lead to optimal selectivity, and providing quantitative values for the relevant parameters. These findings inspire additional design rules which improve future work on selective targeting in directed drug delivery.

physics.bio-ph↗

arXiv2023

Three-Body Interactions of Lipid Membrane-Deforming Colloidal Spheres

Many cell functions require a concerted effort from multiple membrane proteins, for example, for signaling, cell division, and endocytosis. One contribution to their successful self-organization stems from the membrane deformations that these proteins induce. While the pairwise interaction potential of two membrane deforming spheres has recently been measured, membrane-deformation induced interactions have been predicted to be non-additive and hence their collective behavior cannot be deduced from this measurement. We here employ a colloidal model system consisting of adhesive spheres and giant unilamellar vesicles to test these predictions by measuring the interaction potential of the simplest case of three membrane-deforming spherical particles. We quantify their interactions and arrangements and for the first time experimentally confirm and quantify the non-additive nature of membrane-deformation induced interactions. We furthermore conclude that there exist two favorable configurations on the membrane: (1) a linear, and (2) a triangular arrangement of the three spheres. Using Monte Carlo simulation we corroborate the experimentally observed energy minima and identify a lowering of the membrane deformation as the cause for the observed configurations. The high symmetry of the preferred arrangements for three particles suggests that arrangements of many membrane-deforming objects might follow simple rules.

cond-mat.soft↗

arXiv2023

Wrapping pathways of anisotropic dumbbell particles by giant unilamellar vesicles

Endocytosis is a key cellular process involved in the uptake of nutrients, pathogens or the diagnosis and therapy of diseases. Most studies have focused on spherical objects, whereas biologically relevant shapes can be highly anisotropic. In this letter, we use an experimental model system based on Giant Unilamellar Vesicles (GUVs) and dumbbell-shaped colloidal particles to mimic and investigate the first stage of the passive endocytic process: engulfment of an anisotropic object by the membrane. Our model has specific ligand-receptor interactions realized by mobile receptors on the vesicles and immobile ligands on the particles. Through a series of experiments, theory and molecular dynamics simulations, we quantify the wrapping process of anisotropic dumbbells by GUVs and identify distinct stages of the wrapping pathway. We find that the strong curvature variation in the neck of the dumbbell as well as membrane tension are crucial in determining both the speed of wrapping and the final states.

cond-mat.soft↗

arXiv2023

Flexible Colloidal Molecules with Directional Bonds and Controlled Flexibility

Colloidal molecules are ideal model systems for mimicking real molecules and can serve as versatile building blocks for the bottom-up self-assembly of flexible and smart materials. While most colloidal molecules are rigid objects, the development of colloidal joints has made it possible to also include conformational flexibility into colloidal molecules. However, their unrestricted range of motion does not capture the restricted motion range and bond directionality that is typical of real molecules. In this work, we create flexible colloidal molecules with an in situ controllable motion range and bond directionality by assembling spherical particles onto cubes functionalized with complementary surface-mobile DNA. We assemble colloidal molecules with different coordination number of spheres by varying the size ratio and find that they feature a constrained range of motion above a critical size ratio. Using theory and simulations, we show that the particle shape together with the multivalent bonds create an effective free-energy landscape for the motion of the sphere on the surface of the cube. We quantify the confinement of the spheres on the surface of the cube and the probability to change facet. We find that temperature can be used as an extra control parameter to switch in situ between full and constrained flexibility of these colloidal molecules. These flexible colloidal molecules with temperature switching motion range can be used to investigate the effect of directional, yet flexible bonds in determining their self-assembly and phase behavior, and may be employed as constructional units in microrobotics and novel smart materials

cond-mat.soft↗