Orlin D. Velev

North Carolina State University, Raleigh, North Carolina, United States

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Publications (216)1291.41 Total impact

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    ABSTRACT: We report selective and directional actuation of elastomer films utilizing magnetic anisotropy introduced by chains of Fe3O4 magnetic nanoparticles (MNPs). Under uniform magnetic fields or field gradients, dipolar interactions between the MNPs favor magnetization along the chain direction and cause selective lifting. This mechanism is described using a simple model.
    No preview · Article · Dec 2015 · Nanoscale
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    ABSTRACT: Oil foams are composed of gas bubbles dispersed in an oil phase. These systems are scarcely studied despite their great potential in diverse fields such as the food and cosmetic industries. Contrary to aqueous foams, the production of oil foams is difficult to achieve due to the inefficiency of surfactant adsorption at oil-air interfaces. Herein, we report a simple way to produce oil foams from oleogels, whose liquid phase is a mixture of sunflower oil and fatty alcohols. The temperature at which the oleogel formed was found to depend on both fatty alcohol chain length and concentration. The air bubbles in the oleogel foam were stabilized by fatty alcohol crystals. Below the melting temperature of the crystals, oleogel foams were stable for months. Upon heating, these ultrastable foams collapsed within a few minutes due to the melting of the crystal particles. The transition between crystal formation and melting was reversible, leading to thermoresponsive non-aqueous foams. The reversible switching between ultrastable and unstable foam depended solely on the temperature of the system. We demonstrate that these oleogel foams can be made to be photoresponsive by using internal heat sources such as carbon black particles, which can absorb UV light and dissipate the absorbed energy as heat. This simple approach for the formulation of responsive oil foams could be easily extended to other oleogel systems and could find a broad range of applications due to the availability of the components in large quantities and at low cost.
    No preview · Article · Nov 2015 · Langmuir
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    ABSTRACT: We developed a fully biomimetic leaf-like device for hydrogen production which allows incorporated fabric-immobilised microalgae culture to be simultaneously hydrated with media and harvested from the produced hydrogen in a continuous flow regime without the need to replace the algal culture. Our leaf device produces hydrogen by direct photolysis of water resulting from redirecting the photosynthetic pathways in immobilised microalgae due to the lack of oxygen. In contrast to the many other reports in the literature on batch photobioreactors producing hydrogen from suspension culture of microalgae, we present the first report where this is done in a continuous manner from a fabric-immobilised microalgae culture. The reported artificial leaf device maximises the sunlight energy utilisation per gram of algae and can be upscaled cheaply and easily to cover large areas. We compared the production of hydrogen from both immobilised and suspended cultures of C. reinhardtii microalgae under sulphur, phosphorus
    Full-text · Article · Oct 2015 · Journal of Materials Chemistry A
  • Brittany S Mertens · Orlin D Velev
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    ABSTRACT: Understanding of the colloidal interactions of Norovirus particles in aqueous medium could provide insights on the origins of the notorious stability and infectivity of these widespread viral agents. We characterized the effects of solution pH and surfactant type and concentration on the aggregation, dispersion, and disassembly of Norovirus virus-like particles (VLPs) using dynamic light scattering, electrophoretic light scattering, and transmission electron microscopy. Owing to net negative surface charge of the VLPs at neutral pH, low concentrations of cationic surfactant tend to aggregate the VLPs, whereas low concentrations of anionic surfactant tend to disperse the particles. Increasing the concentration of these surfactants beyond their critical micelle concentration leads to virus capsid disassembly and breakdown of aggregates. Non-ionic surfactants, however, had little effect on virus interactions and likely stabilized them additionally in suspension. The data were interpreted on the basis of simple models for surfactant binding and re-charging of the virus capsid. We used zeta potential data to characterize virus surface charge and interpret the mechanisms behind these demonstrated surfactant-virus interactions. The fundamental understanding and control of these interactions will aid in practical formulations for virus inactivation and removal from contaminated surfaces.
    No preview · Article · Sep 2015 · Soft Matter
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    ABSTRACT: The fabrication of multifunctional materials with tunable structure and properties requires programmed binding of their building blocks. For example, particles organized in long-ranged structures by external fields can be bound permanently into stiff chains through electrostatic or van der Waals attraction, or into flexible chains through soft molecular linkers such as surface-grafted DNA or polymers. Here, we show that capillarity-mediated binding between magnetic nanoparticles coated with a liquid lipid shell can be used for the assembly of ultraflexible microfilaments and network structures. These filaments can be magnetically regenerated on mechanical damage, owing to the fluidity of the capillary bridges between nanoparticles and their reversible binding on contact. Nanocapillary forces offer opportunities for assembling dynamically reconfigurable multifunctional materials that could find applications as micromanipulators, microbots with ultrasoft joints, or magnetically self-repairing gels.
    No preview · Article · Aug 2015 · Nature Materials
  • Rachita Sharma · Orlin D. Velev
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    ABSTRACT: The principles and design of “active” self-propelling particles that can convert energy, move directionally on their own, and perform a certain function is an emerging multidisciplinary research field, with high potential for future technologies. A simple and effective technique is presented for on-demand steering of self-propelling microdiodes that move electroosmotically on water surface, while supplied with energy by an external alternating (AC) field. It is demonstrated how one can control remotely the direction of diode locomotion by electronically modifying the applied AC signal. The swimming diodes change their direction of motion when a wave asymmetry (equivalent to a DC offset) is introduced into the signal. The data analysis shows that the ability to control and reverse the direction of motion is a result of the electrostatic torque between the asymmetrically polarized diodes and the ionic charges redistributed in the vessel. This novel principle of electrical signal-coded steering of active functional devices, such as diodes and microcircuits, can find applications in motile sensors, MEMs, and microrobotics.
    No preview · Article · Aug 2015 · Advanced Functional Materials
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    ABSTRACT: Silver nanoparticles have antibacterial properties, but their use has been a cause for concern because they persist in the environment. Here, we show that lignin nanoparticles infused with silver ions and coated with a cationic polyelectrolyte layer form a biodegradable and green alternative to silver nanoparticles. The polyelectrolyte layer promotes the adhesion of the particles to bacterial cell membranes and, together with silver ions, can kill a broad spectrum of bacteria, including Escherichia coli, Pseudomonas aeruginosa and quaternary-amine-resistant Ralstonia sp. Ion depletion studies have shown that the bioactivity of these nanoparticles is time-limited because of the desorption of silver ions. High-throughput bioactivity screening did not reveal increased toxicity of the particles when compared to an equivalent mass of metallic silver nanoparticles or silver nitrate solution. Our results demonstrate that the application of green chemistry principles may allow the synthesis of nanoparticles with biodegradable cores that have higher antimicrobial activity and smaller environmental impact than metallic silver nanoparticles.
    Full-text · Article · Jul 2015 · Nature Nanotechnology
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    Coralie Siebman · Orlin D Velev · Vera I Slaveykova
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    ABSTRACT: An alternative current (AC) dielectrophoretic lab-on-chip setup was evaluated as a rapid tool of capture and assembly of microalga Chlamydomonas reinhardtii in two-dimensional (2D) close-packed arrays. An electric field of 100 V·cm-1, 100 Hz applied for 30 min was found optimal to collect and assemble the algae into single-layer structures of closely packed cells without inducing cellular oxidative stress. Combined with oxidative stress specific staining and fluorescence microscopy detection, the capability of using the 2D whole-cell assembly on-chip to follow the reactive oxygen species (ROS) production and oxidative stress during short-term exposure to several environmental contaminants, including mercury, methylmercury, copper, copper oxide nanoparticles (CuO-NPs), and diuron was explored. The results showed significant increase of the cellular ROS when C. reinhardtii was exposed to high concentrations of methylmercury, CuO-NPs, and 10-5 M Cu. Overall, this study demonstrates the potential of combining AC-dielectrophoretically assembled two-dimensional algal structures with cell metabolic analysis using fluorescence staining, as a rapid analytical tool for probing the effect of contaminants in highly impacted environment.
    Full-text · Article · Jun 2015
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    ABSTRACT: Based on Brownian Dynamics computer simulations in two dimensions we investigate aggregation scenarios of colloidal particles with directional interactions induced by multiple external fields. To this end we propose a model which allows continuous change in the particle interactions from point-dipole-like to patchy-like (with four patches). We show that, as a result of this change, the non-equilibrium aggregation occurring at low densities and temperatures transforms from conventional diffusion-limited cluster aggregation (DLCA) to slippery DLCA involving rotating bonds; this is accompanied by a pronounced change of the underlying lattice structure of the aggregates from square-like to hexagonal ordering. Increasing the temperature we find a transformation to a fluid phase, consistent with results of a simple mean-field density functional theory.
    Preview · Article · May 2015 · Soft Matter
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    ABSTRACT: The operation of digital microfluidic devices with water droplets manipulated by electrowetting is critically dependent on the static and dynamic stability and lubrication properties of the oil films that separate the droplets from the solid surfaces. The factors determining the stability of the films and preventing surface fouling in such systems are not yet thoroughly understood and were experimentally investigated in this study. The experiments were performed using a standard digital microfluidic cartridge in which water droplets enclosed in a thin, oil-filled gap were transported over an array of electrodes. Stable, continuous oil films separated the droplets from the surfaces when the droplets were stationary. During droplet transport, capillary waves formed in the films on the electrode surfaces as the oil menisci receded. The waves evolved into dome-shaped oil lenses. Droplet deformation and oil displacement caused the films at the surface opposite the electrode array to transform into dimples of oil trapped over the centers of the droplets. Lower actuation voltages were associated with slower film thinning and formation of fewer, but larger, oil lenses. Lower ac frequencies induced oscillations in the droplets that caused the films to rupture. Films were also destabilized by addition of surfactants to the oil or droplet phases. Such a comprehensive understanding of the oil film behavior will enable more robust electrowetting-actuated lab-on-a-chip devices through prevention of loss of species from droplets and contamination of surfaces at points where films may break.
    No preview · Article · May 2015 · Biomicrofluidics
  • Bhuvnesh Bharti · Anne-Laure Fameau · Orlin D Velev
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    ABSTRACT: The directed assembly of colloidal particles into linear chains and clusters is of fundamental and practical importance. In this study we characterize and analyse the mechanism of the magnetic field driven assembly of lipid-coated iron oxide nanoparticles into flexible microfilaments. Recently we showed that nanocapillary lipid binding can form a new class of magnetic nanoparticle-lipid microfilaments with unprecedented flexibility and self-healing properties. In the presence of a uniform magnetic field, the magnetophoretic attraction of the particles combined with interparticle dipole-dipole attraction drives the microfilament assembly. The fluid like lipid layer on the particles leads to stickiness on the surface of the filaments and the magnetic field concentration overcomes the potential electrostatic repulsion in the water phase. The lipid capillary bridges formed between the particles facilitate their permanent binding and sustain the flexible microfilament structure. We demonstrate that this surface stickiness combined with the magnetic response of the filaments can be used further to twist, bend and bundle the microfilaments into unusual structures.
    No preview · Article · Apr 2015 · Faraday Discussions
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    ABSTRACT: A simple process for batch or continuous formation of polymer nanofibers and other nanomaterials in the bulk of a sheared fluid medium is introduced. The process could be of high value to commercial nanotechnology as it can be easily scaled up to the fabrication of staple nanofibers at rates that could exceed tens of kilograms per hour. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    No preview · Article · Mar 2015 · Advanced Materials
  • Bhuvnesh Bharti · Orlin D Velev
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    ABSTRACT: Field-directed colloidal assembly has shown remarkable recent progress in increasing the complexity, degree of control, and multiscale organization of the structures. This has largely been achieved by using particles of complex shapes and polarizabilites (Janus, patchy, shaped, and faceted). We review the fundamentals of the interactions leading to the directed assembly of such structures, the ways to simulate the dynamics of the process, and the effect of particle size, shape, and properties on the type of structure obtained. We discuss how directional polarization interactions induced by external electric and magnetic fields can be used to assemble complex particles or particle mixtures into lattices of tailored structure. Examples of such systems include isotropic and anisotropic shaped particles with surface patches, which form networks and crystals of unusual symmetry by dipolar, quadrupolar, and multipolar interactions in external fields. The emerging trends in making reconfigurable and dynamic structures are discussed.
    No preview · Article · Feb 2015 · Langmuir
  • Bhuvnesh Bharti · Orlin D. Velev
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    ABSTRACT: External fields (magnetic and electric) present a simple, robust and efficient route to manipulate and assemble colloidal particles. We report how biparticle dispersions can be assembled into well-defined arrays of tunable morphology using external AC electric field. Binary dispersions of strongly and weakly charged colloidal particles were arranged into linear composite chains via dipole-dipole attraction. The frequency of the applied electric field was the first control parameter for reversibly tuning the biparticle attraction from longitudinal assembly (in the direction of field) to the traverse one (perpendicular to the field). We show that in addition to frequency, spatial limitations play a key role in the assembly process and may assist in the formation of short bidirectional chain-like clusters or characteristic highly structured strings of colloidal triplets. Thus, we control the long-range organization through a combination of particle size ratio, concentration ratio and field frequency. The new strategy to reconfigure the microstructures can find application in better control of the field driven colloidal assembly processes and may be extended to the formation of more complex and precisely arranged particle networks.
    No preview · Article · Jan 2015 · Zeitschrift für Physikalische Chemie
  • Marcel Sperling · Orlin D. Velev · Michael Gradzielski
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    ABSTRACT: The self-assembly of colloidal particles into well-defined structures, such as anisometric ones, has shown to be both a challenging and promising field in today's research. We report a detailed analysis of the processes of drying and the associated colloidal assembly-induced deformation of aqueous droplets containing fumed silica (FS) on a superhydrophobic surface. The use of FS as colloidal building block can lead to the formation of anisometric supraparticles by droplet deformation due to the formation of a dense silica shell at the water-air interface. We demonstrate how this can be done in a controlled way by adjusting the initial suspension's ionic strength using NaCl. An interpretation of the critical deformation point is given and we derive an empirical formula to predict the extent of anisometry depending on the initially applied salt and FS concentration. By addition of polystyrene microspheres we prove the general applicability of this method to droplets containing dispersions of various types of colloidal material, thereby forming hybrid supraparticles. This allows for introducing additional functionality to anisometric particles prepared with FS.
    No preview · Article · Jan 2015 · Zeitschrift für Physikalische Chemie
  • Stephanie Lam · Andrew Tibbits · Orlin D. Velev

    No preview · Conference Paper · Nov 2014
  • Stephanie Lam · Regine von Klitzing · Orlin D. Velev

    No preview · Conference Paper · Nov 2014
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    ABSTRACT: Colloidal assemblies in the form of one-dimensional arrays, chains and gels of particles are of special interest as the core of functional materials with applications in biomedicine, electronics and sensors. One rapid and efficient technique for linear assembly of colloidal particles is the use of external electrical or magnetic field. The attractive dipolar and multipolar interactions induced in the particles under the action of external field determine the interparticle arrangement. However, the resulting ordered structures are temporary and last only as long as external field is present. We report a novel method for permanently assembling superparamagnetic nanoparticles covered by lipid shell into magnetically responsive ultraflexible chains. Initial burst of magnetic field aligns the particles into bundles and after switching off the field the particles retain their linear arrangement by a soft attractive potential induced by the lipid junctions. These soft lipid junctions play the role of nanocapillary bridges and we highlight that the phase of the surface wetting lipid governs the bridge and hence the chain formation. The role of surface wettability and formation of liquid menisci was related to the thermodynamic phase of the surface adsorbed lipid. No nanocapillary bridging was observed until the temperature was less than the gel-fluid phase transition temperature of the corresponding lipid. The presence of nanocapillary bridges provides high flexibility to the resulting chains by allowing for particle rolling and sliding. We evaluate persistence length of the chains by finding the decay in bond correlations along the chain contour. We demonstrate that despite of high linear density, persistence length still remains comparable to persistence length of bio-molecules proving the ultrahigh flexible nature and reconfigurability of the assembled chains. The square-well like "snapping" induced by the lipid shells can be used in making self-closed ring-like objects and self-repairing networks. This assembly mechanism opens new pathways for making multifunctional structures and materials, which can be dynamically reconfigured and programmed, including reassembling micrograbbers and microbots, and self-repairing gels with unusual magnetic and rheological responses.
    No preview · Conference Paper · Nov 2014
  • David Rutkowski · Carol K. Hall · Orlin D. Velev · Sabine Klapp
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    ABSTRACT: Colloids with anisotropic charge distributions hold promise for creating a number of useful new materials including optic materials with novel symmetries, electrical materials for information storage, and dampers for controlling vibrations in structures. Because experimental characterization of the many possible types of multipolar colloidal particles that could form is difficult, the search for novel colloidal materials can be enhanced and guided by simulations of colloidal system assembly. Using a simplified potential, we have simulated a system of dipolar rods with various aspect ratios using discontinuous molecular dynamics (DMD). Each dipolar rod was modeled as several overlapping spheres held in a rod shape to represent excluded volume and two smaller, embedded spheres to represent the charges that make up the extended dipole. We have discovered the existence of fluid, string-fluid, and “gel” phases at low volume fractions and nematic phases at high volume fractions. We have also developed a more realistic discontinuous Yukawa-like potential that allows us to examine colloidal rods that exhibit either head-to-tail or side-by-side configurations depending on the internal charge separation. The percolation probability, maximum cluster size and heat capacity have been monitored to evaluate the aggregation properties of these particles as a function of temperature. The mean squared displacement (MSD) has also been calculated to follow the dynamics. Preliminary results show that at low temperatures the rods assemble to form strands when the head-to-tail configuration is preferred and into rectangular aggregates when the side-by-side configuration is preferred. While the above simulations were all performed in 3d, we have also performed 2d simulations of the same system of dipolar rods corresponding more closely to experiments in which the colloidal particles are confined between plates. Finally, we have also looked into the effect that adding an external electric field has on the system and have observed an increased propensity for chaining in the system.
    No preview · Conference Paper · Nov 2014
  • Stephanie Lam · Anne-Laure Fameau · Saad A. Khan · Orlin D. Velev

    No preview · Conference Paper · Nov 2014

Publication Stats

10k Citations
1,291.41 Total Impact Points

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  • 2002-2015
    • North Carolina State University
      • Department of Chemical and Biomolecular Engineering
      Raleigh, North Carolina, United States
  • 2011
    • Utrecht University
      • Debye Institute for Nanomaterials Science (DINS)
      Utrecht, Utrecht, Netherlands
  • 2007
    • The University of Tennessee Medical Center at Knoxville
      Knoxville, Tennessee, United States
  • 2006
    • Columbia University
      • Department of Biomedical Engineering
      New York, New York, United States
  • 1997-2005
    • University of Delaware
      • Center for Molecular and Engineering Thermodynamics
      Delaware, United States
  • 2003-2004
    • University of Hull
      • Department of Chemistry
      Kingston upon Hull, England, United Kingdom
  • 1992-1999
    • Medical University of Sofia
      • Department of Chemistry
      Ulpia Serdica, Sofia-Capital, Bulgaria
  • 1995
    • University of Patras
      • Department of Chemical Engineering
      Patrís, Kentriki Makedonia, Greece