Drop mobility on chemically heterogeneous and lubricant-impregnated surfaces

Controlling 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

Droplet breakup driven by shear thinning solutions in a microfluidic T-Junction

Droplet-based microfluidics turned out to be an efficient and adjustable platform for digital analysis, encapsulation of cells, drug formulation, and polymerase chain reaction. Typically, for most biomedical applications, the handling of complex, non-Newtonian fluids is involved, e.g. synovial and salivary fluids, collagen, and gel scaffolds. In this study we investigate the problem of droplet formation occurring in a microfluidic T-shaped junction, when the continuous phase is made of shear thinning liquids. At first, we review in detail the breakup process providing extensive, side-by-side comparisons between Newtonian and non-Newtonian liquids over unexplored ranges of flow conditions and viscous responses. The non-Newtonian liquid carrying the droplets is made of Xanthan solutions, a stiff rod-like polysaccharide displaying a marked shear thinning rheology. By defining an effective Capillary number, a simple yet effective methodology is used to account for the shear-dependent viscous response occurring at the breakup. The droplet size can be predicted over a wide range of flow conditions simply by knowing the rheology of the bulk continuous phase. Experimental results are complemented with numerical simulations of purely shear thinning fluids using Lattice Boltzmann models. The good agreement between the experimental and numerical data confirm the validity of the proposed rescaling with the effective Capillary number.Comment: Manuscript: 11 pages 5 figures, 65 References. Textual Supplemental Material: 6 pages 3 figure. Video Supplemental Materials: 2 movie

Single-file escape of colloidal particles from microfluidic channels

Single-file diffusion is a ubiquitous physical process exploited by living and synthetic systems to exchange molecules with their environment. It is paramount quantifying the escape time needed for single files of particles to exit from constraining synthetic channels and biological pores. This quantity depends on complex cooperative effects, whose predominance can only be established through a strict comparison between theory and experiments. By using colloidal particles, optical manipulation, microfluidics, digital microscopy and theoretical analysis we uncover the self-similar character of the escape process and provide closed-formula evaluations of the escape time. We find that the escape time scales inversely with the diffusion coefficient of the last particle to leave the channel. Importantly, we find that at the investigated {\bf microscale}, bias forces as tiny as $10^{-15}\;{\rm N}$ determine the magnitude of the escape time by drastically reducing interparticle collisions. Our findings provide crucial guidelines to optimize the design of micro- and nano-devices for a variety of applications including drug delivery, particle filtering and transport in geometrical constrictions.Comment: 6 pages, 3 figure

Direct experimental evidence of a growing length scale accompanying the glass transition

Understanding glass formation is a challenge because the existence of a true glass state, distinct from liquid and solid, remains elusive: Glasses are liquids that have become too viscous to flow. An old idea, as yet unproven experimentally, is that the dynamics becomes sluggish as the glass transition approaches because increasingly larger regions of the material have to move simultaneously to allow flow. We introduce new multipoint dynamical susceptibilities to estimate quantitatively the size of these regions and provide direct experimental evidence that the glass formation of molecular liquids and colloidal suspensions is accompanied by growing dynamic correlation length scales.Comment: 5 pages, 2 figure

Single File Flow of Biomimetic Beads for Continuous SERS Recording in a Microfluidic Device

A major challenge in cancer treatment is the quantification of biomarkers associated with a specific cancer type. Important biomarkers are the circulating tumor cells (CTCs) detached from the main cancer and circulating in the blood. CTCs are very rare and their identification is still an issue. Although CTCs quantification can be estimated by using fluorescent markers, all the fluorescence techniques are strongly limited by the number of emissions (therefore markers) that can be discriminated with one exciting line, by their bleaching characteristics, and by the intrinsic autofluorescence of biological samples. An emerging technique that can overcome these limitations is Surface Enhanced Raman Scattering (SERS). Signals of vibrational origin with intensity similar to those of fluorescence, but narrower bandwidths, can be easily discriminated even by exciting with a single laser line. We recently showed the benefit of this method with cells fixed on a surface. However, this approach is too demanding to be applied in clinical routine. To effectively increase the throughput of the SERS analysis, microfluidics represents a promising tool. We report two different hydrodynamic strategies, based on device geometry and liquids viscosity, to successfully combine a microfluidic design with SERS

Thermolubricity of gas monolayers on graphene

Nanofriction of Xe, Kr and N2 monolayers deposited on graphene was explored with a quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene was grown by chemical vapour deposition and transferred to the QCM electrodes with a polymer stamp. Graphene was found to strongly adhere to the gold electrodes at temperatures as low as 5 K and at frequencies up to 5 MHz. At low temperatures, the Xe monolayers are fully pinned to the graphene surface. Above 30 K, the Xe film slides and the depinning onset coverage beyond which the film starts sliding decreases with temperature. Similar measurements repeated on bare gold show an enhanced slippage of the Xe films and a decrease of the depinning temperature below 25 K. Nanofriction measurements of Kr and N2 confirm this scenario. This thermolubric behaviour is explained in terms of a recent theory of the size dependence of static friction between adsorbed islands and crystalline substrates