Alexander Alexeev

Deformations in Si-Li anodes upon electrochemical alloying in nano-confined space.

The energy density of Li-ion batteries can be increased if graphitic anodes are replaced with nanostructured Si-based materials. Design of efficient Si anodes requires a better fundamental understanding of the possible changes in Si-Li alloy morphology during cycling. Here we propose a simple elastoplastic model to predict morphological changes in Si upon electrochemical reaction with Li in a confined geometry, such as a pore of a carbon nanotube (CNT). Our experiments with CNTs having inner Si coatings of different thicknesses confirmed the theoretical predictions and demonstrated irreversible shape changes in the first cycle and fully reversible shape changes in subsequent cycles. During the first lithiation, Si was found to adapt to the restricted shape of the rigid CNT pore and plastically deform during electrochemical alloying with Li. The sequential Li insertion and extraction periodically alters the tube size between the expanded and contracted states. The produced samples of porous Si with rigid CNT outer shell showed capacity up to 2100 mAh/g, stable performance for over 250 cycles, and outstanding average Coulombic efficiency in excess of 99.9%. CNT walls were demonstrated to withstand stresses caused by the initial Si expansion and Li intercalation.

Harnessing Janus Nanoparticles to Create Controllable Pores in Membranes

We use a coarse-grained numerical simulation to design a synthetic membrane with stable pores that can be controllably opened and closed. Specifically, we use dissipative particle dynamics to probe the interactions between lipid bilayer membranes and nanoparticles. The particles are nanoscopic Janus beads that comprise both hydrophobic and hydrophilic portions. We demonstrate that when the membrane rips and forms a hole due to an external stress, these nanoparticles diffuse to the edge of the hole and form a stable pore, which persists after the stress is released. Once the particle-lined pore is formed, a small increase in membrane tension readily reopens the pore, allowing transport through the membrane. Besides the application of an external force, the membrane tension can be altered by varying, for example, temperature or pH. Thus, the findings provide guidelines for designing nanoparticle-bilayer assemblies for targeted delivery, where the pores open and the cargo is released only when the local environmental conditions reach a critical value.

Ultrasoft microgels displaying emergent platelet-like behaviours

Efforts to create platelet-like structures for the augmentation of haemostasis have focused solely on recapitulating aspects of platelet adhesion; more complex platelet behaviours such as clot contraction are assumed to be inaccessible to synthetic systems. Here, we report the creation of fully synthetic platelet-like particles (PLPs) that augment clotting in vitro under physiological flow conditions and achieve wound-triggered haemostasis and decreased bleeding times in vivo in a traumatic injury model. PLPs were synthesized by combining highly deformable microgel particles with molecular-recognition motifs identified through directed evolution. In vitro and in silico analyses demonstrate that PLPs actively collapse fibrin networks, an emergent behaviour that mimics in vivo clot contraction. Mechanistically, clot collapse is intimately linked to the unique deformability and affinity of PLPs for fibrin fibres, as evidenced by dissipative particle dynamics simulations. Our findings should inform the future design of a broader class of dynamic, biosynthetic composite materials.

Anisotropic Micro‐ and Nano‐Capsules

In this work, we introduce anisotropically shaped, ultrathin micro- and nano-capsules fabricated by layer-by-layer approach. The original cubic and tetrahedral shapes of the template particles were replicated to produce hollow capsules with well-defined edges. Introducing tannic acid as a component of LbL shells resulted in enhanced chemical stability of these hollow polymer structures under a wide pH range due to high pK(a) value. Computational studies demonstrated increased mechanical stability of the anisotropic capsules under osmotic pressure variation due to sharp edges and vertices acting as a reinforcing frame in contrast to spherical microcapsules that undergo random buckling.

Controlled release of nanoparticles and macromolecules from responsive microgel capsules.

Using a mesoscale computational model, we probe the release of nanoparticles and linear macromolecules from hollow microgel capsules that swell and deswell in response to external stimuli. Our simulations reveal that responsive microcapsules can be effectively utilized for steady and pulsatile release of encapsulated solutes. Swollen gel capsules allow steady, diffusive release of nanoparticles and polymer chains, whereas gel deswelling causes burst-like discharge of solutes driven by an outward flow of the solvent enclosed within a shrinking capsule. We demonstrate that this hydrodynamic release can be regulated by introducing rigid microscopic rods in the capsule interior. Thus, our findings disclose an efficient approach for controlled release from stimuli-responsive microcarriers that could be useful for designing advanced drug delivery systems.

Healing substrates with mobile, particle-filled microcapsules: designing a 'repair and go' system

We model the rolling motion of a fluid-driven, particle-filled microcapsule along a heterogeneous, adhesive substrate to determine how the release of the encapsulated nanoparticles can be harnessed to repair damage on the underlying surface. We integrate the lattice Boltzmann model for hydrodynamics and the lattice spring model for the micromechanics of elastic solids to capture the interactions between the elastic shell of the microcapsule and the surrounding fluids. A Brownian dynamics model is used to simulate the release of nanoparticles from the capsule and their diffusion into the surrounding solution. We focus on a substrate that contains a damaged region (e.g. a crack or eroded surface coating), which prevents the otherwise mobile capsule from rolling along the surface. We isolate conditions where nanoparticles released from the arrested capsule can repair the damage and thereby enable the capsules to again move along the substrate. Through these studies, we establish guidelines for designing particle-filled microcapsules that perform a ‘repair and go’ function and thus, can be utilized to repair damage in microchannels and microfluidic devices.

Mesoscale modeling: solving complex flows in biology and biotechnology

Fluids are involved in practically all physiological activities of living organisms. However, biological and biorelated flows are hard to analyze due to the inherent combination of interdependent effects and processes that occur on a multitude of spatial and temporal scales. Recent advances in mesoscale simulations enable researchers to tackle problems that are central for the understanding of such flows. Furthermore, computational modeling effectively facilitates the development of novel therapeutic approaches. Among other methods, dissipative particle dynamics and the lattice Boltzmann method have become increasingly popular during recent years due to their ability to solve a large variety of problems. In this review, we discuss recent applications of these mesoscale methods to several fluid-related problems in medicine, bioengineering, and biotechnology.

Designing synthetic, pumping cilia that switch the flow direction in microchannels.

Using computational modeling, we simulate the 3D movement of actuated cilia in a fluid-filled microchannel. The cilia are modeled as deformable, elastic filaments, and the simulations capture the complex fluid-structure interactions among these filaments, the channel walls, and the surrounding solution. The cilia are tilted with respect to the surface and are actuated by a sinusoidal force that is applied at the free ends. We find that these cilia give rise to a unidirectional flow in the system and by simply altering the frequency of the applied force we can controllably switch the direction of the net flow. The findings indicate that beating, synthetic cilia could be harnessed to regulate the fluid streams in microfluidic devices.

Marangoni-induced deformation and rupture of a liquid film on a heated microstructured wall

Thermocapillary instability is one of the primary causes of a spontaneous rupture of thin films on heated walls. The film rupture may lead to an appearance of uncontrolled dry patches that significantly deteriorate the heat and mass transfer. In the present paper the thermocapillarity-induced film flow on a microstructured wall is studied in the framework of the long-wave theory. When the wall is heated or cooled, the solution predicts a film deformation caused by thermocapillarity. The linear stability analysis shows that the films on heated microstructured walls are less stable to the long-wave disturbances compared to the films on flat walls. The time-dependent film evolution is simulated and the effect of the wall structure on the film thinning and rupture is analyzed. It is shown that the wall topography exerts a profound effect on the dynamics of the film deformation and rupture, as well as on the size and the location of the dry patches. The full-scale direct volume-of-fluid simulations are used to verity the predictions of the long-wave theory. Good agreement is found for the small ratios between the groove depth and period. The agreement is further improved by including the effect of the convection heat transfer into the long-wave model.

Marangoni convection and heat transfer in thin liquid films on heated walls with topography: Experiments and numerical study

Thermocapillary-induced motion in thin liquid films on a heated horizontal wall with parallel grooves on its upper surface is studied experimentally and numerically. The results of velocity and temperature measurements are reported. A numerical model for a liquid film on a structured wall is developed. The full incompressible Navier–Stokes equations and the energy equation are integrated by a finite difference algorithm, whereas the mobile gas-liquid interface is tracked by the volume-of-fluid method. The numerical model is verified by comparison with the experimental data showing a good agreement. The model is used to study flow patterns and film rupture caused by the thermocapillary forces. Heat transfer in the liquid is also investigated. In particular, it is found that the thermocapillary convection enhances heat transfer in liquid, though the effect depends on the shape of the wall surface.