Session W32: Focus Session: Micro/Nanofluidics I

Uncovering stem-cell heterogeneity in the microniche with label-free microfluidics

Better suited for large number of cells from bulk tissue, traditional cell-screening techniques, such as fluorescence-activated cell sorting (FACS) and magnetic-activated cell sorting (MACS), cannot easily screen stem or progenitor cells from minute populations found in their physiological niches. Furthermore, they rely upon irreversible antibody binding, potentially altering cell properties, including gene expression and regenerative capacity. We have developed a label-free, single-cell analysis microfluidic platform capable of quantifying cell-surface marker expression of functional organ stem cells directly isolated from their micro-anatomical niche. With this platform, we have screened single quiescent muscle stem (satellite) cells derived from single myofibers, and we have uncovered an important heterogeneity in the surface-marker expression of these cells. By sorting the screened cells with our microfluidic device, we have determined what this heterogeneity means in terms of muscle stem-cell functionality. For instance, we show that the levels of beta1-integrin can predict the differentiation capacity of quiescent satellite cells, and in contrast to recent literature, that some CXCR4$+$ cells are not myogenic. Our results provide the first direct demonstration of a microniche-specific variation in gene expression in stem cells of the same lineage. Overall, our label-free, single-cell analysis and cell-sorting platform could be extended to other systems involving rare-cell subsets.   

Designing artificial phagocyte that selectively ``ingests'' solutes

We use dissipative particle dynamics to design an active composite vesicle that can controllably and selectively ``ingest'' solutes from the surrounding fluid. The vesicle consists of a lipid membrane that envelops a stimuli-responsive microgel particle. When the microgel swells and increases in size due to an external stimulus, the lipid membrane breaks forming pores that expose a part of the microgel to the external solvent. Solutes initially dispersed in the solvent diffuse and bind to the uncovered surface of microgel particles. After the stimulus is removed and microgel deswells to its original size, the transmembrane pores close isolating the adsorbed solutes inside the vesicle. In our simulations, we formulate the criteria for the controlled pore opening and closing, and probe how this smart vesicle can be harnessed to ``ingest'' specific macromolecules. Our results will be useful for developing a new class of artificial phagocytes for targeted sampling in various biomedical applications.   

Transient Flow Induced by the Adsorption of Particles

When small particles, e.g., glass, flour, pollen, etc., come in contact with a fluid-liquid interface they disperse so quickly to form a monolayer on the interface that it appears explosive, especially on the surface of mobile liquids like water. This is a consequence of the fact that the adsorption of a particle in an interface causes a lateral flow which on the interface away from the particle. In this study we use the particle image velocimetry (PIV) technique to measure the transient three-dimensional flow that arises due to the adsorption of spherical particles. The PIV measurements show that the flow develops a fraction of a second after the adsorption of the particle and persists for several seconds. The fluid below the particle rises upwards and on the surface moves away from the particle. These latter PIV results are consistent with the surface velocity measurements performed in earlier studies. The strength of the induced flow, and the time duration for which the flow persists, both decrease with decreasing particle size. For a spherical particle the flow is axisymmetric about the vertical line passing through the center of the particle.   

Direct measurement of friction of a fluctuating contact line

What happens at a moving contact line, where one fluid displaces another (immiscible) fluid over a solid surface, is a fundamental issue in fluid dynamics. In this presentation, we report a direct measurement of the friction coefficient in the immediate vicinity of a fluctuating contact line using a micron-sized vertical glass fiber with one end glued to an atomic force microscope (AFM) cantilever beam and the other end touching a liquid-air interface. By measuring the broadening of the resonance peak of the cantilever system with varying liquid viscosity $\eta$, we obtain the friction coefficient $\xi_c$ associated with the contact line fluctuations on the glass fiber of diameter $d$ and find it has the universal form, $\xi_c= 0.8\pi d\eta$, independent of the contact angle. The result is further confirmed by using a soap film system whose bulk effect is negligibly small. This is the first time that the friction coefficient of a fluctuating contact line is measured. *Work supported by the Research Grants Council of Hong Kong SAR.   

Giant slip at liquid-liquid interfaces using a hydrophobic ball bearing

We suggest to build an interface where hydrophobic beads maintain a gas layer between two liquids. We show that this interface behaves as a liquid-liquid ball bearing under shear and exhibits giant slip. Such a metastable configuration reminds of pillar-based superhydrophobic surfaces, used to amplify liquid-solid slip. To the advantage of hydrophobic ball bearings, beads are able to roll, thereby reducing friction at the liquid-bead interface. However beads will always penetrate inside the liquid, inducing viscous dissipation and consequently decreasing slippage. The penetration depth being directly controlled by the wetting angle of the liquid at the bead surface, the latter is expected to have a strong influence on the efficiency of the liquid/liquid bearing. We start by quantifying analytically the influence of the wetting angle on liquid/liquid slip in this system. We then confirm the obtained scaling law by means of Molecular Dynamics (MD) simulations. Liquid-liquid bearings open new pathways for micro and nanofluidics. One major direction could be to build fluidic channels without walls, where different liquids in contact could flow independently while maintaining an extremely low interfacial friction, and preventing mixing by diffusion between the different channels.   

A Study of the Concentration Dependent Water Diffusivity in Polymer using Magnetic Resonance Imaging

Hydrogel allows solvent molecules to migrate in and out of the polymer network, often in response to various environmental stimuli such as temperature and pH, resulting in significant volumetric change. Kinetics of penetrants in polymeric network determines time dependent behavior of hydrogel. However, swelling deformation resulting from the solvent uptake in turn significantly changes diffusivity of solvent, and this strong coupling makes it challenging to study dynamic behavior of hydrogels. Here we study concentration dependent diffusivity of water in poly(ethylene glycol) diacrylate (PEGDA) hydrogel using magnetic resonance imaging (MRI). Projection micro-stereolithography is used to fabricate gel samples in which a gradient of water volume fraction occurs. In situ measurement using MRI provides quantitative relationship between diffusivity and volume fraction of water in the gel. This result will help better understand interstitial diffusion behavior of solvent in polymers, which has great implication in board areas such as soft matter mechanics, drug delivery, and tissue engineering.   

Closing the loop in the boundary layer: water slippage, interfacial viscosity and wettability

Understanding and manipulating fluids at the nanoscale is a matter of growing scientific and technological interest. Here, we present experiments showing that the interfacial viscosity of water depends drastically on the wetting properties of the confining surfaces. By using an atomic force microscope (AFM), we have measured the lateral viscous force experienced in water by a nano-size AFM tip while it is sheared in parallel to a smooth solid surface, as a function of the tip-surface distance. The viscous force curves, FL(d), have been measured for five surfaces with various wettabilities. In particular, the experiments indicate that in water lower forces are required to shear a tip very close to a slippery non-wetting surface, yielding to a lower effective viscosity. A modified form of the Newtonian definition of viscosity, which includes slippage, is used to successfully predict the measured shear forces in the boundary layer as a function of surface wettability, and slippage. We prove that this effect is general and can be applied in different contexts such as in explaining the relationship between dissipation and surface wettability for a nano-tip vibrating in proximity of a surface in water.   

Linear Behavior of the Kapitza Jump at Liquid/Solid Interfaces

At macroscale dimensions, it is normally assumed that two distinct materials maintain equal temperature across the surface of contact. Even in the presence of a thermal flux across the interface, the contacting boundary is assumed to maintain thermal equilibrium so long as the interfacial resistance is negligible in comparison to that of the bulk. This has long been assumed an excellent approximation for liquid/solid (L/S) interfaces since liquids conform in shape even to roughened surfaces. Recent molecular dynamics simulations of nanoscale films, however, have revealed the existence of intrinsic temperature jumps at L/S interfaces. While previous studies have shown how stronger interaction potentials between the liquid and solid will diminish temperature jumps, they cannot be altogether eliminated due to commensurability mismatch. Here we show how the magnitude of the thermal jump also is controlled by the applied thermal flux. This finding suggests that temperature jumps across an L/S interface are not simply a local effect due to density mismatch. These jumps are also controlled by the actual rate of heat transfer, underscoring the importance of thermal resistance effects in nanoscale hydrodynamic systems. We thank P. Thompson for assistance in optimizing the simulations.   

Near-wall Brownian motion of anisotropic particles

Anisotropic microscopic objects are ubiquitous such as biological cells, filamentous macromolecules, as well as synthesized nanomaterial. Near interfaces, the thermal motion of these objects is strongly constrained due to the hydrodynamic interactions, impacting the overall behavior of the biophysical and colloidal systems. Thus, understanding this wall-effect is a key to describe many surface-related problems. Unlike the well-studied case of spheres, however, both its experimental and theoretical studies have been elusive due to the intrinsic complexity of the system. Here we present a comprehensive experimental and computational study of the Brownian motion of silicon nanowires tethered on a substrate. A uniquely developed interference method enables the direct visualization of its microscopic rotations in three dimensions with high angular and temporal resolutions. The quantitative measurement at short time scales revealed the anisotropic reduction in their rotational diffusivities as a function of the inclined angles, resulting in the decrease more than 40-80 {\%} at long time scales. We then developed a numerical model from a string-of-beads idealization, which implicitly simulates the complex hydrodynamic interaction and showed excellent agreement with the experimental observations. Our study provides insights into the fundamental diffusive processes, useful for understanding the anisotropic behavior of anisotropic macromolecules near interfaces. The demonstrated methods offers a systematic approach for studying the interfacial rheology of various anisotropic objects.   

Enhancing microscale particle deposition using actuated synthetic cilia

We use three dimensional simulations to examine deposition of diffusive nanoscopic particles suspended in a viscous fluid onto the walls of a microchannel containing an array of actuated synthetic cilia. We model the cilia as elastic filaments attached to the channel walls and actuated by an external periodic force. We use a lattice Boltzmann model coupled with a lattice spring model to simulate the system and investigate the effects of the oscillating cilia on the rate of particle deposition. We consider the effects of variation of cilia properties and spacing, as well as the frequency and amplitude of the applied force on the deposition of particles with different diffusivity. Our findings are useful in understanding how active microscopic structures can be harnessed to design microfluidic devices and surfaces with controllable transport properties.   

Statics and dynamics of polymer droplets on topographically structured substrates

Using Molecular Dynamics simulations of a polymer liquid flowing past flat and patterned surfaces, we investigate the influence of corrugation, wettability and pressure on slippage and friction at the solid-liquid interface. We devise a computational method to compute the interface potential that does not rely on grandcanonical simulation techniques and quantitatively compare droplet profiles obtained in simulations with the predictions of a thin-film equation using the independently determined interface potential. For substrates structured by one-dimensional, rectangular grooves, we observe a gradual crossover between the Wenzel state, where the liquid fills the grooves, and the Cassie state, where the corrugation supports the liquid and the grooves are filled with vapor. Using two independent flow set-ups, we characterize the near-surface flow by the slip length and the position, at which viscous and frictional stresses are balanced according to Navier's partial slip boundary condition. This hydrodynamic boundary position depends on the pressure inside the channel and may be located above the corrugated surface. In the Cassie state, we observe that the edges of the corrugation contribute to the friction.   

The Effect of Polarization on Structure, Dynamics and Electric Double Layer for Interfacial Water near Charged Graphene

A solid surface perturbs water for up to 10-20 {\AA}. Quantifying the structural and dynamics properties of water within this interfacial layer remains crucial for a number of applications, including lab-on-chip and micro- and nano-fluidic devices, and also for designing efficient electric double layers capacitor. As graphene is finding wide applications in the energy sector (batteries and capacitors) we revisited the graphene-water interface. Because at the air-water interface it is known that accounting for the polarization of water and ions is required to properly describe the ions distribution, we conducted a parametric study in which we varied the polarization of carbon atoms on charged graphene. The polarization is described implementing a classic Drude oscillator, which is consistent with the model implemented to describe water and ions. External electric fields are represented by uniform charge distributions on the carbon atoms. The results are quantified in terms of structure and dynamics of interfacial water, as well as of structure of the electric double layer. Comparison with accurate experimental observations is provided.