Session P37: Soft Matter Interfaces: BIo-, Dielectrics, Transport and Other Phenomena

Nonlinear transport of soft droplets in pore networks

A large number of biological and technological processes depend on the transport of soft colloidal particles through porous media; this includes the transport and separation of cells, viruses or drugs through tissues, membranes and microfluidic devices. In these systems, the interactions between soft particles, background fluid and the surrounding pore space yield complex, nonlinear behaviors such as non-Darcy flows, localization and jamming. We devise a computational strategy to investigate the transport of non-wetting and deformable water droplets in a microfluidic device made of a random distribution of cylindrical obstacles. We first derive scaling laws for the entry of the droplet in a single pore and discuss the role of surface tension, contact angle and size in this process. This information is then used to study the transport of multiple droplets in an obstacle network. We find that when the droplet size is close to the pore size, fluid flow and droplet trafficking strongly interact, leading to local redistributions in pressure fields, intermittent clogging and jamming. Importantly, it is found that the overall droplet and fluid transport display three different scaling regimes depending on the forcing pressure, and that these regimes can be related to droplet properties.   

Simulation of non-ionic surfactant micelle formation across a range of temperature and pressure

Non-ionic surfactants can, at certain concentrations and thermodynamic conditions, aggregate into micelles due to their amphiphilic nature. Our work looks at the formation and behavior of micelles at extremes of temperature and pressure. Due to the large system size and simulation time required to study micelle formation, we have developed a coarse-grained (CG) model of our system. This CG model represents each heavy atom with a single CG bead. We use the multibody Stillinger-Weber potential, which adds a three-body angular penalty to a two-body potential, to emulate hydrogen bonds in the system. We simulate the linear surfactant $C_{12}E_{5}$, which has a nonpolar domain of 12 carbons and a polar domain of 5 ethers. Our CG model has been parameterized to match structural properties from all-atom simulations of single and dimer surfactant systems. Simulations were performed using a concentration above the experimental critical micelle concentration at 300K and 1atm. We observe an expected region of stable micelle formation at intermediate temperature, with a breakdown at high and low temperature, as well as at high pressure. The driving forces behind the destabilization of micelles and the mechanism of micelle formation at different thermodynamic conditions will be discussed.   

On the pH of Aqueous Attoliter-Volume Droplets

Droplets of water dispersed in perfluorinated liquids have widespread use including microfluidics, drug delivery and single-molecule measurements. Perfluorinated liquids are distinctly biocompatible due to their stability, low surface tension, lipophobicity, and hydrophobicity. For this reason, the effect of the perfluorinated surface on droplet contents is usually ignored. However, as the droplet diameter is reduced, we expect that any effect of the water/oil interface on droplet contents will become more obvious. We studied the pH of attoliter-volume aqueous droplets in perfluorinated liquids using pH-sensing fluorescent dyes. Droplets were prepared either by sonication or extrusion from buffer and perfluorinated liquids (FC40 or FC77). A non-ionic surfactant was used to stabilize the droplets. Buffer strength, ionic strength, and pH of the aqueous phase were varied and resulting droplet pH compared to the pH of the buffer from which they were formed. Preliminary data are consistent with a pH in droplets that depends on the concentration of non-ionic surfactant. At low surfactant concentrations, the pH in droplets is distinctly lower than the stock buffer. However, as the concentration of non-ionic surfactant is increased the change in pH decreases.   

Electrolyte-mediated adsorption to neutral and dielectric interfaces

Biology relies on electrolytes to regulate molecular interactions and to support functionality in numerous vital processes. Although the role of the electrolyte is generally categorized into two tendencies, namely "salting-out" and "salting-in", the more versatile aspects can be revealed by a more detailed picture of the microscopic ionic structure. We use molecular dynamics simulations and numerical calculations based on liquid state theory, and obtain high-resolution, quantitative information about the spatial structure of primitive model electrolytes in dielectric confinement, up to high concentrations (0.9 M) and strong electrostatic coupling. The theoretical methods also quantify two relevant underlying thermal forces that are highly tunable by the specific selection of electrolytes. The results refine the understanding of the adsorption behavior of ions and macromolecular solutes, and identify tuning parameters for macromolecular assembly, based on ion size, valency, and ionic composition.   

Dipolar fluids near a dielectric surface

The behavior of dipolar fluids near an interface is of fundamental importance in a broad variety of fields, including colloid chemistry, electrochemistry, biochemistry and surface science. The structural properties of such a fluid are affected not only by the presence of surface charge, but also by a dielectric mismatch across the interface. Using large-scale Monte Carlo simulations that explicitly take into account dielectric effects, we investigate a prototypical dipolar fluid. In addition to the organization of the fluid, characterized through the dipolar orientations and spatial correlations, we also calculate the surface tension by employing simulations in the grand-canonical ensemble.   

Dielectric effects on the ion distribution near a Janus colloid

Spherical Janus colloids, particles with two domains of different materials, are typically heterogeneous in permittivity. This dielectric heterogeneity will influence their behavior in electrolytes, ranging from their aggregation to their electrokinetics in external fields. We investigate the structure of the electric double layer around spherical Janus colloids immersed in solution via molecular dynamics simulations. Polarization of the colloidal surfaces by the surrounding ions is calculated dynamically with a boundary-element method based Poisson solver. One observation is that even neutral Janus colloids may carry a net dipole moment in the presence of asymmetric salts. Moreover, we extend this study to incorporate a \emph{spatially varying permittivity of the solvent} near a charged Janus colloid, and demonstrate the effect of this dielectric variation on the electric double layer.   

Preventing Oxide Adhesion of Liquid Metal Alloys to Enable Actuation in Microfluidic Systems

This work explores the wetting behavior of an oxide-coated liquid metal, eutectic alloy of gallium and indium (`EGaIn'), which remains a liquid at room temperature. Liquid metals uniquely combine fluidity with metallic properties. Combined, these properties enable soft, stretchable, and shape reconfigurable electronics with `softer than skin' interfaces. Ga forms spontaneously a thin surface oxide that alters its wetting behavior and makes it difficult to move across surfaces without leaving residue behind. We examine the effects of surface roughness (i.e., Cassie-Baxter state) and lubrication to minimize adhesion of Ga oxide to surfaces. Lubricated surfaces create a `slip-layer' of liquid between the metal and surface that also inhibits wetting. This slip layer allows the metal to move reversibly through microchannels by preventing adhesion of the oxide. The metal may be pumped or moved by using low voltages or pneumatic actuation. Optical microscopy confirms the importance of the slip-layer, which enables non-stick motion of the metal through capillaries. Finally, electrochemical impedance spectroscopy characterizes the electrohydrodynanic motion of EGaIn in capillary systems.   

Confining capillary waves to control aerosol droplet size from surface acoustic wave nebulisation

Aerosols play a significant role in targeted delivery of medication through inhalation of drugs in a droplet form to the lungs. Delivery and targeting efficiencies are mainly linked to the droplet size, leading to a high demand for devices that can produce aerosols with controlled sizes in the range of 1 to 5$\mu $m. Here we focus on enabling the control of the droplet size of a liquid sample nebulised using surface acoustic wave (SAW) generated by interdigitated transducers on a piezoelectric substrate (lithium niobate). The formation of droplets was monitored through a high-speed camera (600,000 fps) and the sizes measured using laser diffraction (Spraytec, Malvern Ltd). Results show a wide droplet size distribution (between 0.8 and 400$\mu $m), while visual observation (at fast frame rates) revealed that the large droplets (\textgreater 100$\mu $m) are ejected due to large capillary waves (80 to 300$\mu $m) formed at the free surface of liquid due to leakage of acoustic radiation of the SAWs, as discussed in previous literature (Qi et al. Phys Fluids, 2008). To negate this effect, we show that a modulated structure, specifically with feature sizes, typically 200$\mu $m, prevents formation of large capillary waves by reducing the degrees of freedom of the system, enabling us to obtain a mean droplet size within the optimum range for drug delivery (\textless 10$\mu $m).   

Scaling Laws for liquid and ion transport in nanochannels grafted with polyelectrolyte brushes

Grafting nanochannels with polyelectrolyte (PE) brushes renders tremendous functionality to the nanochannels, making them capable of applications such as ion manipulation, ion sensing, current rectification, nanofluidic diode fabrication, and~flow control. PE brush is a special case of polymers at interfaces; such brush-like structure is possible only when the grafting density ( $\sigma )$ is beyond a critical value. In this study, we shall propose scaling laws that identify $\sigma $\textbf{-N }(N is the size of the PE molecule) combination that simultaneously ensure that the grafted PE molecules adopt "brush"-like configuration and the height of the PE brushes are smaller than the nanochannel half height. Secondly, we pinpoint the scaling conditions where the electrostatic effects associated with the PE brushes can be decoupled from the corresponding PE excluded volume and elastic effects; such de-coupling has tremendous connotation in context of modeling of electrostatics and transport at PE-brush-covered interfaces. Thirdly, we provide scaling arguments to quantify the dependence of the flow penetration depth into the PE brush as a function of the $\sigma $\textbf{-N} combination. Finally, our scaling estimates pinpoint the conditions where the flow or electric field induced deformation of the grafted nanochannel PE brushes can be neglected while modeling the pressure-driven or electroosmotic transport or ionic current in such nanochannels.   

Entropic changes in liquid gallium clusters: understanding the anomalous melting temperatures

Melting in finite-sized materials differs in two ways from the solid-liquid phase transition in bulk systems. First, there is an inherent scaling of the melting temperature below that of the bulk, known as melting point depression. Secondly, at small sizes, changes in melting temperature become non-monotonic, and show a size-dependence that is sensitive to the structure of the particle. Melting temperatures that exceed those of the bulk material have been shown to occur in vacuum, but have still never been ascribed a convincing physical explanation. Here we find answers in the structure of the aggregate liquid phase in small gallium clusters, based on molecular dynamics simulations that reproduce the greater-than-bulk melting behavior observed in experiments, and demonstrate the critical role of a lowered entropy in destabilising the liquid state.   

Thermocapillary Technique for Shaping and Fabricating Optical Ribbon Waveguides

The demand for ever increasing bandwidth and higher speed communication has ushered the next generation optoelectronic integrated circuits which directly incorporate polymer optical waveguide devices. Polymer melts are very versatile materials which have been successfully cast into planar single- and multimode waveguides using techniques such as embossing, photolithography and direct laser writing. In this talk, we describe a novel thermocapillary patterning method for fabricating waveguides in which the free surface of an ultrathin molten polymer film is exposed to a spatially inhomogeneous temperature field via thermal conduction from a nearby cooled mask pattern held in close proximity. The ensuring surface temperature distribution is purposely designed to pool liquid selectively into ribbon shapes suitable for optical waveguiding, but with rounded and not rectangular cross sectional areas due to capillary forces. The solidified waveguide patterns which result from this non-contact one step procedure exhibit ultrasmooth interfaces suitable for demanding optoelectronic applications. To complement these studies, we have also conducted finite element simulations for quantifying the influence of non-rectangular cross-sectional shapes on mode propagation and losses.   

Relaxations of star-shaped polystyrene melts approaching the colloidal limit

The dynamics of star-shaped polystyrene melts with functionalities ranging from 8\textless f\textless 64 and arm molecular weights ranging from 9 kg/mol \textless M\textless 80 kg/mol were investigated using small amplitude oscillatory shear measurements. The frequency dependent storage, G', and loss, G'', moduli were measured in the linear viscoelastic regime in order to characterize the terminal relaxation behavior of the macromolecules. Our studies reveal gradual, low-frequency deviations away from the Milner-McLeish theory for arm retraction indicating more elastic behavior as functionality is increased. The magnitudes of these deviations diminish with increasing arm molecular weight. These elastic deviations are consistent with the emergence of a relaxation representing cooperative structural rearrangements and colloidal behavior. Our results indicate that changes in the size of the core region for low molecular weight arms leads to a transition in the dynamics from an arm retraction mechanism to a cooperative, structural relaxation mechanism.   

The origin of star-shaped oscillations of Leidenfrost drops

We experimentally investigate the oscillations of Leidenfrost drops of water, liquid nitrogen, ethanol, methanol, acetone and isopropyl alcohol. The drops levitate on a cushion of evaporated vapor over a hot, curved surface which keeps the drops stationary. We observe star-shaped modes along the periphery of the drop, with mode numbers $n \quad =$ 2 to 13. The number of observed modes is sensitive to the properties of the liquid. The pressure oscillation frequency in the vapor layer under the drop is approximately twice that of the drop frequency, which is consistent with a parametric forcing mechanism [1]. However, the Rayleigh and thermal Marangoni numbers are of order 10,000, indicating that convection should play a dominating~role as well. Surprisingly, we find that~the wavelength and~frequency~of the oscillations only depend on the thickness~of the liquid, which is twice the~capillary length, and do not depend on~the mode number, substrate temperature, or the~substrate curvature. This robust behavior suggests that the wavelength for the oscillations is set by thermal convection~inside the drop, and is less dependent on the flow in the vapor layer under the drop.~ [1] P. Brunet and J. H. Snoeijer, Eur. Phys. J. Spec. Top. 192, 207 (2011).   

The intrinsic structure of liquid interfaces

Thermal capillary waves develop spontaneously at fluid/fluid interfaces, and modulate their shapes on scales much larger than the molecular one, thus smearing any density profile measured or calculated using only a global coordinate system\footnote{E. Chacon and P. Tarazona, Phys. Rev. Lett. 91, 166103 (2003)}. In this contribution we present a local picture of several thermodynamic quantities (density, energy, free energy, surface tension) at liquid/vapour interfaces, analyzing them both on a molecular layer-by-layer basis\footnote{M. Sega, B. Fabian and P. Jedlovszky, J. Chem. Phys. 143, 114709 (2015)}, and as a function of the intrinsic distance from the interface\footnote{L. B. Partay, G. Hantal, P. Jedlovszky, A. Vincze, and G. Horvai, J. Comput. Chem. 29, 945 (2008).}, revealing their true, intrinsic structure.   

Out of equilibrium GigaPa Young modulus of water nanobridge probed by Force Feedback Microscopy

Because of capillary condensation, water droplets appear in nano/micropores. We report that dynamical properties of such nanobridge dramatically change when probed at different time scales [1]. Using a Force Feedback Microscope [2], the gap between the nano-tip and the surface is continuously varied, and we observe this change in the simultaneous measurements, at different frequencies, of the stiffness G'(N/m), the dissipative coefficient G''(kg/sec) together with the static force. This is made possible thanks to feedback force which cancels in real time the force acting on the tip. It avoids the mechanical instabilities due to the nucleation of the nanobridge. As the measuring time approaches the microsecond, the liquid droplet exhibits a large positive stiffness (it is small and negative in the long time limit). Although clearly controlled by surface effects, it compares to the stiffness of a solid nanobridge with a 1 GigaPa Young modulus. [1] Carpentier et al. arXiv preprint arXiv:1503.06756, 2015. [2] Rodrigues et al. Applied Physics Letters, 101(20):203105, 2012.