Session D15: Thin Films, Surface Flows, and Interfaces I

Physical Limits of Marangoni-driven Patterning

Marangoni-driven patterning is a novel technique that harnesses photochemically-applied surface tension gradients to pattern thin polymer films. When heated, the polymer flows away from regions of lower surface tension and into regions of higher surface tension, thereby generating hill-and-valley patterns. This new technique offers potential advantages over traditional patterning methods in creating functional coatings and flexible electronics at the roll-to-roll scale. To understand the full scope of applications this new patterning method could serve, it is necessary to understand the fundamental limits of pattern aspect ratio and pattern periodicity, both key metrics in evaluating pattern quality. Here, we present a model for Marangoni-driven patterning and perform a nonlinear analysis to determine the physical limits of pattern aspect ratio and feature pitch for an equal line-space system.   

Hydrodynamic and Surface-Wetting Effects on Phase Separation Dynamics in Thin-Film Melts

Understanding the phase separation dynamics in thin films is crucial for the performance of many optoelectronic and photovoltaic device because their functioning relies on a well-defined morphology. The morphology forms in the fluid stages of the production process, and to control and improve the performance of thin-film composites it is vital to predict it quantitatively. Typically, the dynamics of fluid-fluid demixing is modeled using relaxational phase field-type theories that ignore the role of hydrodynamics. We study the impact of hydrodynamics on the demixing of binary fluid mixtures in contact with a wetting substrate by comparing lattice Boltzmann simulations with a diffusion-dominated phase field theory. Special focus is on the impact of a short-range surface interaction that favours one of the two fluids, particularly important in the context of photovoltaics. We find that incorporating hydrodynamics is crucial to quantitatively predict the relevant length scales both in the early and late stages of demixing. Indeed, we find that hydrodynamic processes suppress any dependence of the associated growth exponents on the strength of the substrate interaction predicted by phase field theory. We attribute this to flow-induced transport that significantly speeds up coarsening.   

Molecular Dynamics Simulations of Creep in Silica-Water Systems

Geophysical processes are traditionally studied at the continuum level, but large scale molecular simulations now allow detailed studies of the underlying nanoscale mechanisms responsible for large scale behavior. An outstanding question in geoscience is the dynamics of water in thin water films in geological materials under high pressure. In this work, we demonstrate that molecular dynamics simulations provide new insights on creep in water-wetted silicates. We have performed nonequilibrium molecular dynamics simulations of the creep process in a pair of opposing quartz asperities under constant load. The results are compared with a microphysics-based model for thermally activated creep. The thermal activation energy and the time evolution of the system height agree with theory when there is a vacuum between the asperities. Replacing the vacuum with water drastically alters these results by introducing chemical effects which dominate the creep process. Pressure solution of silica and the formation of an amorphous gel at the asperity interface are found to greatly increase the creep rate.   

Nonlinear Interface Equation For Capillary Driven Flow in 3D Open Curved Trajectories

Capillary driven flow of wetting liquids in open V-grooves and interior corners is known to be an especially robust and rapid means of fluid transport in gravity free environments and in small scale systems where gravity plays a negligible role. Nowadays, such flows are routinely used for propellant management in space based systems, lab-on-a-chip devices and high performance chip with micro heat pipes. The low-order inertia-free model developed by Romero and Yost (1996) and Weislogel (1996) first elucidated the flow behavior of an incompressible Newtonian liquid in a straight V-groove whose length far exceeds the film thickness. Here we present an extension of this classic work to flow in 3D open and curved channels in which the trajectory radius of curvature is larger than the film thickness. Despite the complexity of trajectories allowed by this model, a first-order perturbation analysis of the governing conservation equation yields a surprisingly simple form of the nonlinear equation governing the behavior of the moving interface. This thin film equation can now be used to design determininistic flow trajectories for ultracompact microfluidic systems featuring arbitrarily curved 3D channel flows.   

Theory of the two-dimensional paraelectric structural transformation of ferroelectric group-IV monochalcogenide monolayers

After the initial description of a thermally-driven two-dimensional structural transformation in group-IV monochalcogenide monolayers [1] and its subsequent experimental demonstration [2], an additional publication claimed that the transformation can be determined via a Landau model based on an unidirectional optical phonon mode going soft [3]. In this presentation, we disclose a quadratic dispersion of softening modes and analyze molecular dynamics data to understand the atomistic motions underlying the ferroelectric to paraelectric transformation in these atomically thin membranes. We are then able to characterize a hierarchy of critical temperatures Tc that are dependent on structural constraints. Additional dependencies of Tc on the assumptions used to set interatomic forces are discussed as well [4].

References:
[1] M. Mehboudi et al., Nano Lett. 16, 1704 (2016)
[2] K. Chang et al., Science 353, 274 (2016)
[3] R. X. Fei, W. Kang, L. Yang, PRL 117, 097601 (2016)
[4] J. W. Villanova, P. Kumar, and S. Barraza-Lopez, submitted on September 19, 2019.   

Evolution of elastic moduli through a two-dimensional structural transformation

We use an analytical elastic energy landscape describing SnO monolayers to estimate the softening of elastic moduli through a mechanical instability occurring at finite temperature. Although not strictly applicable due to a quantum paraelastic phase in this material [1], this exercise is relevant as it establishes a conceptual procedure to estimate such moduli straight from a two-dimensional elastic energy landscape. As additional support for the existence of a quantum paraelastic phase, we use a qualitative WKB analysis [2] to estimate escape times from an energy well on the landscape. These results continue to establish a case for the usefulness of soft matter concepts in 2D materials and of the potential lurking of quantum effects in soft matter [3].

[1] T. Bishop et al., Phys. Rev. Lett. 122, 015703 (2019)
[2] D. Griffiths, Introduction to Quantum Mechanics, Pearson international edition (Pearson, Prentice-Hall, Englewood Cliffs, NJ, 2005).
[3] A. Pacheco-Sanjuan et al., Phys. Rev. B. 99, 104108 (2019)   

Laser-induced strong Marangoni effect in deformation and manipulation of ferrofluid

Optical manipulation of fluid or droplet has long been investigated for applications in microfluidics. The light-induced thermocapillary effect is one of the strategies in optical control of liquid. We demonstrate the laser-induced deformation of ferrofluid surfaces with ultra-violet to infra-red lasers. The surface deformation reaches the bottom and breaks the liquid, achieving the highest liquid rupture thickness for over 1000 μm. The deformation process and rupture are related to strong Marangoni effect, high laser absorption and low viscosity of ferrofluid. As applications of the laser-controlled ferrofluid, we show that a ferrofluid droplet in the capillary can be easily moved horizontally and vertically by the illumination of a laser beam. We also demonstrate that letters and patterns can be written on the black surface of ferrofluid thin film with a laser beam even with a common laser pointer. Laser manipulation of ferrofluid also makes it a controlling vehicle for varieties of liquid or droplets.   

Long-time evolution of interfacial structure in dewetting

When a solid plate is withdrawn from a partially wetting liquid, a liquid layer dewets the moving substrate. High-speed imaging reveals alternating thin and thick regions in the entrained layer in the transverse direction at steady state. To quantify the absolute thickness of these steady-state structures precisely, I have developed an interferometric technique taking advantage of a varying angle of incidence. This method allows us to measure the absolute thickness as well as the local slope of the thin film. A new technique of likelihood maximization is applied to detect interference fringes. The result shows that the thicknesses of both regions of the film scale with the capillary number, Ca. In addition, a new region is observed during onset which differs from the behavior predicted by previous models.
  

Flows at Molecular Scales: Probing and Manipulating Ultra-Thin Supported Liquid Films

A hydrodynamical description of a liquid down to molecular levels has to take into account the effects specific to these very small scales. At molecular distances from a solid wall, phenomena such as slip or confinement-induced molecular layering have been observed with liquids in both experiments and numerical simulations. The ability to predict those effects is crucial in many fields involving flows at small scales close to a solid, such as nanofluidics or flows in mesoporous media. However, a unifying picture of the different reported features is still lacking, partly because their manifestations critically depend on the chemical nature of both the liquid and the solid medium.

We have developed a fully non invasive optical technique [Phys. Rev. Lett., 114, 227801 (2015)] in order to characterize the dynamics of a liquid by measuring the spontaneous thermal fluctuations of its free surface. Liquid films lying on solid substrates are formed with a thickness down to 5 nm using a Marangoni (surface tension gradient induced) flow. Using this technique, we evidence disjoining pressure effects, and are able to probe flow properties of ultra-thin liquid films.   

Towards nanoscale confinement in TEM liquid cells

Liquid confinement below micrometer thicknesses is of interest to several scientific disciplines and is inherently challenging to study experimentally. More extreme confinement at the single-digit nanometer scale is possible through the use of self-assembled nanostructures such as carbon nanotubes. Bridging the gap between micro- and nanoscale liquid confinement has been achieved with two-piece liquid cells within a transmission electron microscope (TEM), but irreproducibility and difficulties maintaining uniform liquid layer thickness makes high spatial and spectroscopic resolution challenging. We report on improvements made to a monolithic, in-situ TEM liquid cell¹ with sub 40nm liquid thickness that overcomes the common challenges encountered with two-piece TEM liquid cells.
1. Tanase, M. et al. High-Resolution Imaging and Spectroscopy at High Pressure: A Novel Liquid Cell for the Transmission Electron Microscope. Microsc. Microanal. 21, 1629–1638 (2015).   

Chemical Vapor Deposition Growth of Nickel Sulfide for optoelectronic device applications

Nickel Sulfide (Ni<span style="font-size:10.8333px">S₂)</span>, a Transition Metal Di-chalcogenide (TMDC) has become an interesting material for the scientific community owing to its intrinsically small bandgap within the mid-infrared range [1] and superior charge transport properties. Experimentally, it is challenging to characterize the quality of NiS_2. Although it offers a great opportunity for nanoelectronic devices. However, very few efforts have been given for the growth of NiS₂ thin film for device applications. To date, NiS₂ has been grown by the solution process and hydrothermal method and used for energy storage device applications. Here, we grow the NiS₂ thin film on SiO₂/Si substrate by the chemical vapor deposition (CVD) technique. Several spectroscopic studies reveal the high-quality crystalline nature of CVD grown NiS₂ film. We also fabricated the field-effect device at the nickel sulfide platform with thickness up to few nanometers (~23 nm). The performance of the fabricated device is characterized at room temperature. Nickel Sulfide field-effect device shows the drain current up ~10^-6 A with drain voltage from -2 to +2 V.
References

Mingsheng Long et.al. Adv. Funct. Mater. 2018,   

Colloidal migration in microchannels under combined potential and pressure gradients

Colloidal particle migration in combined Poiseuille and electrokinetic flows away from or towards the microchannel walls has recently been shown to be due to the particle slip velocity with respect to the fluid. The particle migration was attributed to an electrophoretic lift-like force, analogous to the inertial lift forces in sedimentation flows. The migration of colloidal particles towards the walls for dilute colloidal particle suspensions with diameters < 1 micron led to the assembly of distinct colloidal bands within microchannels (100 – 300 microns wide x 34 microns deep x 4cm long). Band formation requires opposing Poiseuille and electrokinetic flows. Band formation also requires a minimum applied potential threshold at a given shear rate. Band formation is a function of particle size and volume fraction, particle and channel wall zeta potential, electrolyte concentration, and the minimum electric field thresholds change non-monotonically for particle mixtures. Here, we discuss the effect of manipulating particle properties and fluid inertia over broad parametric ranges to elucidate robustness of particle migration to and away from walls, and the formation of particle bands.
  

Lattice Boltzmann Method for Designing cellular solids for effective acoustic noise cancellation

Acoustic technology based on Passive Noise Cancellation (PNC) has emerged as an excellent noise reduction strategy for both small and large environments. In general, PNC primarily focuses on designing specialty cellular materials, that can effectively attenuate unwanted sounds by virtue of its intrinsic morphology. Here, we design sound-absorbing materials and establish that by tailoring their morphological features, we can achieve acoustic noise cancellation, effectively. Our approach harnesses Lattice Boltzmann Method (LBM) to simulate sound propagation in two-phase cellular viscoelastic materials and our simulation results are in good agreement with the analytical calculations. In particular, using LBM, we designed various morphologies for cellular viscoelastic solids and demonstrate that the higher frequency waves attenuate faster compared to lower frequency waves. Further, we extend our approach for designing indoor acoustics and use reverberation time to quantify PNC for these systems. Our findings can be used not only to design miniaturized noise-canceling acoustic gadgets but also to design specific sound-absorbing materials for large indoor systems.