1,721,046 research outputs found
Protein interactions near crystallization: a microscopic approach to the Hofmeister series
No full textThe salting-out effect of simple electrolytes on lysozyme has been studied by measuring the second virial coefficient B-2 Of the osmotic pressure as a function of salt concentration, and for different salts. The aim of this work has been to End a microscopic counterpart of the empirical Hofmeister series for the efficiency of cations and anions in inducing protein crystallization. The experimental results show that, for large enough ionic strengths, B-2 scales linearly with the salt concentration. This trend is common to a number of different monovalent salts, however with efficiency strongly dependent on the specific anion. Conversely, changing the cation does not appreciably affect B-2. The significance of these findings for the investigation-of protein interactions near crystallization is discussed
Drop mobility on chemically heterogeneous and lubricant-impregnated surfaces
No full textControlling the motion of liquid drops in contact with a solid surface has broad technological implications in many different areas ranging from textiles to microfluidics and heat exchangers. The wettability of a surface is determined by specifying the apparent contact angle and contact angle hysteresis (CAH) that depend on the surface chemistry and morphology. The presence of chemical inhomogeneity and morphological disorder usually increases CAH. A liquid substrate, whose surface is atomically flat and homogenous, is then expected to exhibit a very low CAH. Low CAH determines high drop mobility, while high CAH favours drop pinning. Very slippery surfaces with exceptional omniphobicity are obtained by impregnating a textured solid with a lubricant. To guide and control the motion of drops the solid surface can be decorated with suitable chemical patterns. In this review we briefly outline the main results obtained in the past few years to passively control drop motion and produce robust omniphobic surfaces, highlighting some of the most promising applications of these novel functional surfaces
Active Brownian particles escaping a channel in single file
No full textActive particles may happen to be confined in channels so narrow that they cannot overtake each other (single-file conditions). This interesting situation reveals nontrivial physical features as a consequence of the strong interparticle correlations developed in collective rearrangements. We consider a minimal two-dimensional model for active Brownian particles with the aim of studying the modifications introduced by activity with respect to the classical (passive) single-file picture. Depending on whether their motion is dominated by translational or rotational diffusion, we find that active Brownian particles in single file may arrange into clusters that are continuously merging and splitting (active clusters) or merely reproduce passive-motion paradigms, respectively. We show that activity conveys to self-propelled particles a strategic advantage for trespassing narrow channels against external biases (e.g., the gravitational field)
Drop motion induced by vertical vibrations
Full text linkWe have studied the motion of liquid drops on an inclined plate subject to vertical vibrations. The liquids comprised distilled water and different aqueous solutions of glycerol, ethanol and isopropanol spanning the range 1–39 mm2 s^(−1) in kinematic viscosities and 40–72 mN m^(−1) in surface tension. At sufficiently low oscillating amplitudes, the drops are always pinned to the surface. Vibrating the plate above a certain amplitude yields sliding of the drop. Further increasing the oscillating amplitude drives the drop upward against gravity. In the case of the most hydrophilic aqueous solutions, this motion is not observed and the drop only slides downward. Images taken with a fast camera show that the drop profile evolves in a different way during sliding and climbing. In particular, the climbing drop experiences a much bigger variation in its profile during an oscillating period. Complementary numerical simulations of 2D drops based on a diffuse interface approach confirm the experimental findings. The overall qualitative behavior is reproduced suggesting that the contact line pinning due to contact angle hysteresis is not necessary to explain the drop climbing
Depletion-induced fractionation of optically anisotropic particles
No full textWe show that depletion forces induced by surfactant micelles can be profitably exploited to perform an efficient a posteriori size fractionation of suspensions of optically anisotropic colloids. Efficiency and limits of the fractionation protocol are discussed both on 'model' and technical polydisperse lattices. This method can be used to single out fractions composed by long fibrillar particles, showing evidence of mesogenic behaviour. Depolarized dynamic light scattering is used to extract the particle translational and rotational diffusion coefficients D-T and D-R as a function of particle volume fraction. Both the concentration behaviour of D-T and the full shape of the depolarized correlation functions show distinctive and partially unexpected features
Deviation of sliding drops at a chemical step
No full textThe motion of partially wetting liquid drops in contact with a solid surface is strongly affected by contact angle hysteresis and interfacial pinning. However, the majority of models proposed for drops sliding over chemical surface patterns consistently neglect the difference between advancing and receding contact angles. In this article, we present a joint experimental and numerical study of the interaction of gravity-driven drops with a chemical step formed at the junction between a hydrophilic and a hydrophobic region. It demonstrates the strong impact of a contact angle hysteresis contrast on the motion of drops at a linear chemical step. Surprisingly, the smallest driving force required to drag the drop across the step onto the lower hydrophobic surface is not observed at a right angle of incidence. Our model reveals that the non-monotonous response of this passive drop 'filter' is solely due to the higher advancing contact angle on the lower surface, and creates an instance where drop motion is affected by dissipation at the contact line rather than by surface energy
Directional Fluidity of Dense Emulsion Activated by Transverse Wedge-Shaped Microroughness
Full text linkThe handling and fluidization of amorphous soft solids, such as emulsions, foams, or gels, is crucial in many technological processes. This is generally achieved by applying mechanical stress that overcomes a critical threshold, known as yield stress, below which these systems behave as elastic solids. However, the interaction with the walls can facilitate the transition from solid to fluid by activating rearrangements of the fluid constituents close to the wall, resulting in increased fluidity of the system up to distances greater than the spatial scale of the rearrangements. We address the impact of wedge-shaped microroughness on activating the fluidization of emulsion droplets in pressure-driven flow through microfluidic channels. We realize the micro wedges by maskless photolithography to texture one wall of the channel and measure the velocity profiles for flow directed accordingly and against the increasing ramp of the wedge-shaped grooves. We report the enhancement of the emulsion flow in the direction of the climbing ramp of the wedge activated by increasing the magnitude of the pressure gradient. A gain for the volumetric flow rate is registered with respect to the opposite direction as being to 30% , depending on the pressure drop
FbsA-driven fibrinogen polymerization: A bacterial "Deceiving strategy"
No full textWe show that FbsA, a cell wall protein of the bacterium Streptococcus agalactiae, promotes large-scale aggregation of human plasma fibrinogen, leading to the formation of a semiflexible polymerlike network. This extensive aggregation process takes place not only in solution, but also on FbsA-functionalized colloidal particles, and leads to the formation of a thick layer on the bacterial cell wall itself, which becomes an efficient mask against phagocytosis
Generation of water-in-oil and oil-in-water microdroplets in polyester-toner microfluidic devices
No full textThis paper demonstrates that disposable polyester-toner microfluidic devices are suitable to produce either water-in-oil (W/O) or oil-in-water (O/W) droplets without using any surface treatment of the microchannels walls. Highly monodisperse W/O and O/W emulsions were generated in T-junction microdevices by simply adding appropriate surfactants to the continuous phase. The dispersion in size of droplets generated at frequencies up to 500 Hz was always less than about 2% over time intervals of a couple of hours
Realization and characterization of disposable polymer chips for the study of the filling of microchannels at low capillary numbers
No full textA crucial aspect in the use of microfluidic systems is the filling of microchannels with the liquid. In order to better understand this phenomenon, systematic studies of the meniscus dynamics in model system microchannels are being carried out. A critical factor for the reliability of such a study is the capability to control the rounding radius of the micro features used as obstacles. For this purpose a dedicated process chain was developed for the realization of disposable polymer microfluidic chips integrating the desired features and allowing the generation of internal as well as external corners with controlled edge radius. Preliminary test results are presented
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