Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves

Separation and sorting of micron-sized particles has great importance in diagnostics, chemical and biological analyses, food and chemical processing and environmental assessment. By employing the unique characteristics of microscale flow phenomena, various techniques have been established for fast and accurate separation and sorting of microparticles in a continuous manner. The advancements in microfluidics enable sorting technologies that combine the benefits of continuous operation with small-sized scale suitable for manipulation and probing of individual particles or cells. Microfluidic sorting platforms require smaller sample volume, which has several benefits in terms of reduced cost of reagents, analysis time and less invasiveness to patients for sample extraction. Additionally, smaller size of device together with lower fabrication cost allows massive parallelization, which makes high-throughput sorting possible. Both passive and active separation and sorting techniques have been reported in literature. Passive techniques utilize the interaction between particles, flow field and the channel structure and do not require external fields. On the other hand, active techniques make use of external fields in various forms but offer better performance. This paper provides an extensive review of various passive and active separation techniques including basic theories and experimental details. The working principles are explained in detail, and performances of the devices are discussed.

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Particle separation and sorting in microfluidic devices: a review

After more than 20 years of steady progress, lithium-ion batteries still exhibit modest energy capacities that seem to have reached their asymptotic values with the present combination of graphite at the anode and insertion oxide or phosphate materials at the cathode. New applications, particularly for all-electric vehicles are pushing the development of electrode materials with higher Li storage capabilities, for both electrodes. Silicon, which exhibits the highest known Li-alloying capacity is one of the most promising anode materials. However, Li alloying with Si is accompanied by a large volume change which induces cracking and rapid pulverization of Si-based anodes. Significant improvements in the anode's lifetime as well as charge–discharge rates have been obtained over the past few years by employing Si nanostructures, particularly nanowires. In this paper, we present the main synthesis methods for Si nanowires as well as the alloying properties of Li with Si and review how the use of Si-based nanowires has evolved, thanks to sophisticated material/structure combinations, including core–shell nanowires, composites as well as hollowed nanotube-like approaches.

Silicon nanowires for Li-based battery anodes: a review

Droplet microfluidics has rapidly emerged as one of the key technologies opening up new experimental possibilities in microbiology. The ability to generate, manipulate and monitor droplets carrying single cells or small populations of bacteria in a highly parallel and high throughput manner creates new approaches for solving problems in diagnostics and for research on bacterial evolution. This review presents applications of droplet microfluidics in various fields of microbiology: i) detection and identification of pathogens, ii) antibiotic susceptibility testing, iii) studies of microbial physiology and iv) biotechnological selection and improvement of strains. We also list the challenges in the dynamically developing field and new potential uses of droplets in microbiology.

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Droplet microfluidics for microbiology: techniques, applications and challenges

The complex wrinkling patterns produced when a sheet or membrane is compressed are often difficult to predict. Observations of unexpected spatial period-doubling bifurcation instability in the wrinkling of a rigid membrane attached to a soft substrate can be described within a framework similar to that used for the parametric resonance of nonlinear oscillators.

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Multiple-length-scale elastic instability mimics parametric resonance of nonlinear oscillators

In this letter we propose an original, in situ, approach to tune the optical properties of an optical cavity, based on the wrinkling of compressed metal/polymer multilayer thin films This phenomenon is conceptually described, simulated, and experimentally confirmed The main idea is to use wrinkling to modulate the effective refractive index of the upper interface This modulation induces a spectral shift of the cavity modes The work presented here constitutes a first step to the development of stretchable and curved photonics

In situ tuning the optical properties of a cavity by wrinkling

Performing detailed studies of viscoelastic dewetting of thin polystyrene films on solid substrates, we demonstrate the existence of residual stress due to strongly out of equilibrium chain conformations and a reduced entanglement density resulting from film preparation by spin coating. The ratio of stress over elastic modulus was found to increase strongly with decreasing film thickness and increasing chain length. Full equilibration of chain conformations required long times comparable to bulk reptation times. However, for chains longer than about 3000 monomers, the residual stress relaxed faster, at a rate independent of chain length.

Relaxation of residual stress and reentanglement of polymers in spin-coated films.

Physico-chemical and biochemical factors in the local cellular microenvironment are known to impact on multiple aspects of cell behaviour through specific signal pathways. These mechanotransduction cues can couple each other to regulate cell fate, and it remains unclear whether mechanotransduction in different contexts shares common mechanisms. Undoubtedly, a challenge will involve the further characterization of such cooperative mechanisms, as well as clearly defining the individual role of each mechanical and biochemical parameter. To control these mechanotransduction cues in an independent manner, we developed a simple and efficient strategy to immobilize any desired nature of proteins on polyacrylamide hydrogels and independently control various parameters of the cell microenvironment, such as matrix stiffness, cell-binding ligand density and confined adhesiveness. This novel platform is validated by conducting single-cell experiments and opens a broad avenue for studying complex interplays involved in mechanotransduction with a facile and versatile approach.

A simple route to functionalize polyacrylamide hydrogels for the independent tuning of mechanotransduction cues

Wrinkling patterns at the metallized surface of thin polymer films are shown to be sensitive to the sticky or slippery character of the polymer-substrate interface. Existing theoretical models were expanded to specific boundary conditions (adhesive versus slippery) in order to rationalize these observations. Based on this concept, we were able to propose a new and simple method to orient the wrinkles by chemically patterning the substrate with regions of high and low adhesion.

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Slippery or Sticky Boundary Conditions: Control of Wrinkling in Metal-Capped Thin Polymer Films by Selective Adhesion to Substrates

Thin films of fluids are playing a leading role in countless natural and industrial processes. Here we study the stability and dewetting dynamics of viscoelastic polymer thin films. The dewetting of polystyrene close to the glass transition reveals unexpected features: asymmetric rims collecting the dewetted liquid and logarithmic growth laws that we explain by considering the nonlinear velocity dependence of fric- tion at the fluid/solid interface and by evoking residual stresses within the film. Sys- tematically varying the time so that films were stored below the glass-transition tem- perature, we studied simultaneously the probability for film rupture and the dewetting dynamics at early stages. Both approaches proved independently the significance of re- sidual stresses arising from the fast solvent evaporation associated with the spin-coat- ing process. V C 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3022-3030, 2006

/pdf/viscoelastic-dewetting-of-constrained-polymer-thin-films-p087h49gz4.pdf

Sylvain Desprez

Papers

In situ tuning the optical properties of a cavity by wrinkling

Relaxation of residual stress and reentanglement of polymers in spin-coated films.

A simple route to functionalize polyacrylamide hydrogels for the independent tuning of mechanotransduction cues

Slippery or Sticky Boundary Conditions: Control of Wrinkling in Metal-Capped Thin Polymer Films by Selective Adhesion to Substrates

Viscoelastic dewetting of constrained polymer thin films