Liquid-substrate Interface Research Articles

Scalable substrate development for aqueous sample preparation for atom probe tomography.

Reliable and consistent preparation of atom probe tomography (APT) specimens from aqueous and hydrated biological specimens remains a significant challenge. One particularly difficult process step is the use of a focused ion beam (FIB) instrument for preparing the required needle-shaped specimen, typically involving a "lift-out" procedure of a small sample of material. Here, two alternative substrate designs are introduced that enable using FIB only for sharpening, along with example APT datasets. The first design is a laser-cut FIB-style half-grid close to those used for transmission-electron microscopy, that can be used in a grid holder compatible with APT pucks. The second design is a larger, standalone self-supporting substrate called a "crown," with several specimen positions that self-aligns in APT pucks, prepared by electrical discharge machining (EDM). Both designs are made nanoporous, to provide strength to the liquid-substrate interface, using chemical and vacuum dealloying. We select alpha brass a simple, widely available, lower-cost alternative to previously proposed substrates. We present the resulting designs, APT data, and provide suggestions to help drive wider community adoption. Lay Description Atom probe tomography (APT) maps the composition of solid materials in three dimensions with sub-nanometre resolution. Preparation of specimens from aqueous specimens remains a significant challenge. To aid with preparation, we introduce here two substrate designs that can hold suitable volumes of liquid for freezing, prior to shaping the final specimen into a sharp needle for APT by using cryogenic focused ion beam milling. We also describe the complete setup that we designed for handling these substrates, including dedicated sample holders. Both substrates are made nanoporous, to provide strength to the liquid-substrate interface, using chemical and vacuum dealloying of alpha brass a simple, widely available, low-cost material. We finally showcase APT data and provide suggestions to help drive wider community adoption. This article is protected by copyright. All rights reserved.

Collagen-like Motifs of SasG: A Novel Fold for Protein Mechanical Strength

The Staphylococcus aureus surface protein G (SasG) is associated with host colonisation and biofilm formation. As colonisation occurs at the liquid-substrate interface bacteria are subject to a myriad of external forces and, presumably as a consequence, SasG displays extreme mechanical strength. This mechanical phenotype arises from the B-domain; a repetitive region composed of alternating E and G5 subdomains. These subdomains have an unusual structure comprising collagen-like regions capped by triple-stranded β-sheets. To identify the determinants of SasG mechanical strength, we characterised the mechanical phenotype and thermodynamic stability of 18 single substitution variants of a pseudo-wildtype protein. Visualising the mechanically-induced transition state at a residue-level by ϕ-value analysis reveals that the main force-bearing regions are the N- and C-terminal ‘Mechanical Clamps’ and their side-chain interactions. This is tailored by contacts at the pseudo-hydrophobic core interface. We also describe a novel mechanical motif – the collagen-like region and show that glycine to alanine substitutions, analogous to those found in Osteogenesis Imperfecta (brittle bone disease), result in a significantly reduced mechanical strength.

Droplet trampolining on heated surfaces in the transitional boiling regime

Droplet dynamics on a heated surface plays an important role in applications ranging from spray cooling to solidification and pharmaceuticals. Here, we show that in the transitional boiling regime, an unconstrained droplet exhibits trampolining where the droplet bounces to higher heights with each consecutive contact with the substrate. A water droplet of volume 1 μL bounces to a height as high as 6-7 times of its diameter and the height of bounce increases to 18–20 times the diameter as the volume reduces to ∼ 0.1μL. The dynamics of the droplet in the transitional boiling regime results in a non-monotonic variation in the total evaporation time of the droplet with an increase in substrate temperature – an observation that is absent for a constrained droplet. We develop a force-based model to explain the trampolining behavior of the droplet and propose that the gain in momentum of the droplet that results in trampolining is caused by bubble nucleation and escape at the liquid-substrate interface prior to droplet takeoff.

Alteration of freezing paradigms of an impact water droplet on different cold surfaces

The influence of surface properties on the dynamics and freezing of an impact water droplet is investigated under various impact velocities and surface temperatures. Three distinct icing paradigms are found, namely the widely observed Central-Pointy Icing paradigm (CPI), the annular paradigm with Exterior Freezing front First (EFF), and the annular paradigm with Interior Freezing front First (IFF). High impact velocities and low surface temperatures are beneficial to the IFF icing paradigm. Upon decreasing the impact velocity and/or increasing the surface temperature, the IFF paradigm transits to EFF and ultimately to CPI. The interior freezing front occurring prior to the exterior one is mainly attributed to the small difference of the thickness between the central and peripheral liquid, and the large temperature difference between the droplet and liquid-substrate interface. Raising the impact velocity and surface wettability enhances droplet spreading, resulting in a small thickness difference. The liquid-substrate interface temperature is collectively determined by surface temperature, density, thermal conductivity and specific heat capacity of the solid surface. A freezing paradigm map described by the dimensionless temperature and the maximum spreading diameter is proposed to evaluate the conditions under which one of the three distinct freezing paradigms appears. A large spreading area and a low liquid-substrate interface temperature are prerequisites for the emergence of IFF paradigm.

Laser-induced assembly of biological cells and colloids onto a candle soot coated substrate

Recently, laser-induced heating assisted assembly of colloidal particles is emerging as a popular technique against direct light-assisted particle assembling methods. Herein, we demonstrate the trapping and assembly of yeast cells and upconversion particles at laser excitation intensities as low as 11 μW/μm2 via localized heating of non-plasmonic candle soot coated biocompatible polymer, polydimethylsiloxane. The thermal convective flow around the irradiation zone plays a vital role in trapping and assembling of the objects. The flow velocity profile is symmetrical around the irradiation zone, and the velocity increases drastically near the irradiation zone and with laser intensity. An increase in laser intensity leads to formation of an air bubble at the liquid-substrate interface and subsequent assembly of the objects at the interface. The tunneling of objects towards the irradiation zone across the barrier is demonstrated. Finally, with the localized laser-induced heating, the surface charge independent swarm of upconversion particle is shown, and the size and range of which found to be laser intensity dependent. With the wavelength-independent low power operation in the near-infrared range through simple optics, the results presented here may open avenues for a wide range of applications in diverse areas, including life sciences, colloidal science, nanoscience, photonics, and material science.

Principle analysis for the micromanipulation probe-type ultrasonic nanomotor

The micromanipulation probe-type (MMP-type) ultrasonic nanomotor (UNM) has proven to be a type of robust and versatile tool for controllable rotary driving, dynamic trapping and high-precision orientation of a single one-dimensional (1D) nano object. The manipulation functions of the MMP-type UNM are implemented in the probe-liquid-substrate (PLS) system, in which a MMP is inserted into a liquid film of nano suspension on a stationary substrate. When the MMP elliptically vibrates parallel to the substrate surface, the acoustic streaming can be engendered to rotate a single silver nanowire (AgNW) at the liquid-substrate interface. Although the experimental results have demonstrated the effectiveness and high performance of the developed UNM, there have been no simulation results and quantitative analysis method to show the details of acoustic streaming field and to perform principle analysis for the MMP-type UNM, which has hindered a deeper understanding of the underlying physical mechanisms as well as optimization for the MMP-type UNM. In this work, to deeply analyze the principle for our developed UNM, we carry out numerical investigations on the thermo-viscous acoustic field and acoustic streaming field in the PLS system of the MMP-type UNM based on the finite element method (FEM). The simulation result shows that the elliptical vibration of the MMP parallel to the substrate surface can induce the reversed acoustic streaming vortex (compared to the MMP’s vibration trajectory direction) at the liquid-substrate interface, which enables controllable rotary driving of a single AgNW. With the simulation results of acoustic streaming fields, we theoretically predict the driving angular velocities of the AgNW at the liquid-substrate interface, and verify that the quantitative simulation results agree well with the reported experimental results. Moreover, the effects of device’s parameters and working conditions such as the distance between the MMP’s tip and substrate surface, the MMP’s length and radius and the liquid film’s thickness on the acoustic streaming field and the angular velocity of the AgNW are analyzed and clarified through both simulation results and experimental verifications, and the effect of the MMP’s material is also predicted via simulations.

Unraveling the film‐formation kinetics of interfacial polymerization via low coherence interferometry

AbstractInterfacial polymerization (IP) is one of the most important methods for fabricating thin film composite (TFC) membranes. Understanding the film‐formation mechanisms is of great value for developing membranes with enhanced performance. This work proposed a novel method to in situ characterize the film‐formation kinetics via low coherence interferometry (LCI). The polyamide film formed at the liquid–substrate interface was scanned in real time; the polymerization induced significant variations in the optical properties around the reaction zone. After mitigating the effects of the perturbed interface, the surface‐averaged intensity profiles provide a solid basis for analyzing the film‐formation kinetics at various depths. In particular, the effects of the monomer concentrations were investigated to reveal the asymmetric growth and development of irregular substructures. All the characterization results confirm that the LCI‐based characterization is a powerful tool for studying the structural evolution of the IP layer and thereby providing deeper insights for optimizing TFC membranes.

Interplay of substrate wettability and surfactant distributions in controlling interfacial instability

The current work investigates the combined effects of substrate wettability patterns and preset surfactant distributions on the dynamics and rupture of thin liquid films. The configuration considered here is a liquid film with a free surface above, and bounded below by a solid substrate with alternate zones of less and more wettable areas. This wettability-based patterning triggers an instability of the liquid film, caused by the outward flow from less to more wettable regions at the liquid–substrate interface. Introducing surfactants in the liquid film also affects the interfacial dynamics through Marangoni flow at the air–liquid interface. A variety of distribution functions are used as initial conditions for the surfactant concentration, to suitably modify the final configuration of a flat film. A set of two evolution equations for the film height and surfactant concentration are developed as partial differential equations to be solved numerically and simultaneously. Results indicate that the interplay between substrate wettability and surfactant distributions may significantly alter the ruptured film configurations. Typical scenarios include multiple near-rupture events in place of a single rupture, asymmetry in the ruptured film profile, and reversal of flow leading to film rupture on a more wettable region of the substrate. The results also provide insights into the dependence of rupture time scales on the inherent length scales in the problem, such as the pattern feature widths and the size of the surfactant-depleted zones.

Interaction Between Liquid Steel and AlN Substrate Containing Al-Y-Oxides

Reactions between molten iron, molten silicon steel, and ceramic materials are of great importance from the perspective of the corrosion of refractory material in the steelmaking and continuous casting processes. Reactions between liquid steel and aluminum nitride (AlN) substrate containing Al-Y-oxides were investigated using the sessile drop method at 1833 K (1560 °C). The liquid–substrate interface was observed with the increasing holding time, and the reaction mechanism was studied. In the case of the liquid iron and the silicon steel on the AlN substrate containing Al-Y-oxides, the reaction products on the steel side of the interface between the steel and the substrate were Al-rich Al-Y-oxides and Al2O3 particles. For silicon steel and pure AlN substrate, Al2O3 particles were the main products. Contact angles were lower for liquid iron than for silicon steel due to the higher oxygen content in iron. In addition, the solid–liquid interfacial tension and the work of adhesion between AlN and steel were measured.

Noble Metal Nanostructure Synthesis at the Liquid-Substrate Interface: New Structures, New Insights, and New Possibilities.

Modern technologically driven societies could not exist in their current form if it were not for a great many synthetic achievements reliant on solution-based chemistry and substrate-based processing techniques. It is, hence, not surprising that these same materials preparation techniques have given rise to an impressive list of functional nanomaterials including those derived from noble metals, a class of materials renowned for their extraordinary optical and catalytic properties. Acting as the foundation for substrate-based processing is a collection of techniques such as physical and chemical vapor deposition, epitaxy, self- and directed assembly, and a host of lithographic methods. These techniques allow for precise control over nanostructure placement, but where the fabrication of sophisticated architectures and sub-50 nm feature sizes are often unattainable or reliant on the use of technically demanding cost-prohibitive routes. In contrast, solution-based chemistry allows for the formation of complex nanostructures while maintaining synthetic ease, cost-effectiveness, and exacting control over monodispersity, size, shape, composition, and crystallinity. While many methods exist for the dispersal of colloids onto substrates, few are capable of achieving nanostructure ensembles where nanostructure placement allows for true long-range order as well as control over the crystallographic alignment of the nanostructures relative to each other and the underlying substrate. A more exhaustive comparison of these two approaches reveals that, more often than not, a weakness of substrate-based processing is a strength of colloidal synthesis and vice versa. In this Account, we describe a synthetic strategy devised and validated by the Neretina laboratory that integrates the competencies of substrate-based techniques with colloidal chemistry and, in doing so, brings this rich and exciting chemistry and its associated functionalities to the substrate surface. The strategy takes advantage of an impressive collection of seed-mediated solution-based protocols in which dispersed seeds direct noble metal nanostructure formation along orderly reaction pathways. It, however, replaces the seed colloid with substrate-immobilized templates formed in periodic arrays where the crystallographic orientation of the templates is defined by an epitaxial relationship with the substrate. Demonstrated are syntheses at the liquid-substrate interface in which organized surfaces of crystalline templates formed through templated dewetting are subjected to galvanic replacement, preferential etching, and/or heterogeneous deposition facilitated by redox reactions in both the presence and absence of capping agents. While the protocols utilized are adapted from some of the most well-studied colloidal syntheses, in no case do they yield reaction products that are identical since the substrate inflicts asymmetries onto the growth mode. We believe that the strategy described herein not only demonstrates a family of nanostructures unobtainable through other means but also establishes a synthetic foundation that offers unprecedented flexibility, expands the palette of accessible template materials, provides a new vantage point from which complex reactions occurring in liquid media can be examined, and has the potential to underpin photovoltaic, catalytic, and sensing applications reliant on substrate-based noble metal nanostructures.

Regimes of water droplet evaporation on copper substrates

Distilled water droplet evaporation has been studied on copper substrate surfaces with different degrees of roughness. Data on variations in the contact diameter have been employed to distinguish between the regimes of distilled water droplet spreading over the copper surfaces that proceed after the viscous regime. For each isolated regime, the duration has been determined as a fraction of the total evaporation time and the main physical processes have been described. Variations in contact angles have been analyzed as depending on copper surface temperature. It has been established that, as the substrate temperature is elevated, wetting becomes better, while the adhesion work remains almost unchanged, thereby indicating the absence of chemical and structural transformations at the liquid–substrate interface.

Direct Observation of the Dynamics of Self-Assembly of Individual Solvation Layers in Molecularly Confined Liquids.

Confined liquids organize in solidlike layers at the liquid-substrate interface. Here we use force-clamp spectroscopy AFM to capture the equilibrium dynamics between the broken and reformed states of an individual solvation layer in real time. Kinetic measurements demonstrate that the rupture of each individual solvation layer in structured liquids is driven by the rupture of a single interaction for 1-undecanol and by two interactions in the case of the ionic liquid ethylammonium nitrate. Our results provide a first description of the energy landscape governing the molecular motions that drive the packing and self-assembly of each individual liquid layer.

Four degree of freedom liquid dispenser for direct write capillary self-assembly with sub-nanoliter precision

Capillary forces provide a ubiquitous means of organizing micro- and nanoscale structures on substrates. In order to investigate the mechanism of capillary self-assembly and to fabricate complex ordered structures, precise control of the meniscus shape is needed. We present a precision instrument that enables deposition of liquid droplets spanning from 2 nl to 300 μl, in concert with mechanical manipulation of the liquid-substrate interface with four degrees of freedom. The substrate has sub-100 nm positioning resolution in three axes of translation, and its temperature is controlled using thermoelectric modules. The capillary tip can rotate about the vertical axis while simultaneously dispensing liquid onto the substrate. Liquid is displaced using a custom bidirectional diaphragm pump, in which an elastic membrane is hydraulically actuated by a stainless steel syringe. The syringe is driven by a piezoelectric actuator, enabling nanoliter volume and rate control. A quantitative model of the liquid dispenser is verified experimentally, and suggests that compressibility in the hydraulic line deamplifies the syringe stroke, enabling sub-nanoliter resolution control of liquid displacement at the capillary tip. We use this system to contact-print water and oil droplets by mechanical manipulation of a liquid bridge between the capillary and the substrate. Finally, we study the effect of droplet volume and substrate temperature on the evaporative self-assembly of monodisperse polymer microspheres from sessile droplets, and demonstrate the formation of 3D chiral assemblies of micro-rods by rotation of the capillary tip during evaporative assembly.

The dynamics of the impact and coalescence of droplets on a solid surface

A simple experimental setup to study the impact and coalescence of deposited droplets is described. Droplet impact and coalescence have been investigated by high-speed particle image velocimetry. Velocity fields near the liquid-substrate interface have been observed for the impact and coalescence of 2.4 mm diameter droplets of glycerol∕water striking a flat transparent substrate in air. The experimental arrangement images the internal flow in the droplets from below the substrate with a high-speed camera and continuous laser illumination. Experimental results are in the form of digital images that are processed by particle image velocimetry and image processing algorithms to obtain velocity fields, droplet geometries, and contact line positions. Experimental results are compared with numerical simulations by the lattice Boltzmann method.

Water Freezes Differently on Positively and Negatively Charged Surfaces of Pyroelectric Materials

Although ice melts and water freezes under equilibrium conditions at 0 degrees C, water can be supercooled under homogeneous conditions in a clean environment down to -40 degrees C without freezing. The influence of the electric field on the freezing temperature of supercooled water (electrofreezing) is of topical importance in the living and inanimate worlds. We report that positively charged surfaces of pyroelectric LiTaO3 crystals and SrTiO3 thin films promote ice nucleation, whereas the same surfaces when negatively charged reduce the freezing temperature. Accordingly, droplets of water cooled down on a negatively charged LiTaO3 surface and remaining liquid at -11 degrees C freeze immediately when this surface is heated to -8 degrees C, as a result of the replacement of the negative surface charge by a positive one. Furthermore, powder x-ray diffraction studies demonstrated that the freezing on the positively charged surface starts at the solid/water interface, whereas on a negatively charged surface, ice nucleation starts at the air/water interface.

Intermediate-asymptotic structure of a dewetting rim with strong slip

When a thin viscous liquid film dewets, it typically forms a rim which spreads outwards, leaving behind a growing dry region. We consider the dewetting behavior of a film, when there is strong slip at a liquid-substrate interface. The film can be modeled by two coupled partial differential equations (PDEs) describing the film thickness and velocity. Using asymptotic methods, we describe the structure of the rim as it evolves in time and the rate of dewetting, in the limit of large slip lengths. An inner region emerges, closest to the dewetted region, where surface tension is important; in an outer region, three subregions develop. This asymptotic description is compared with numerical solutions of the full system of PDEs.

Steam Laser Cleaning of Plasma-Etch-Induced Polymers from Via Holes

As the wafer industry enters into submicron processes and below, the demand for new cleaning technology after plasma etching increases. The cleanliness of via holes becomes very crucial for the success of low-resistance interconnecting high-density ultra-large-scale integrated devices. In this study, a relatively new approach in removing the sidewall and bottom polymers resulting from reactive ion etching of via holes, using dry and steam laser cleaning techniques is investigated. The presence of a layer of isopropanol (IPA) film on the wafer surface during excimer laser irradiation is found to improve the removal efficiency greatly even at fluences as low as 80–100 mJ cm-2–much lower than the damage threshold of the underlying Al–Cu metal film with titanium nitride anti-reflective coating of 250–280 mJ cm-2. The explosive evaporation of IPA and the creation of bubbles at the liquid-substrate interface were proposed to be the reason for the improvement. Experimental results show however that the presence of a layer of acetone film does not improve but even impedes the laser cleaning process. An explanation is offered for this phenomenon in terms of the difference in the absorbance of the two liquids at the laser wavelength.

RAYLEIGH-BÉNARD-MARANGONI INSTABILITIES DURING EVAPORATION OF AQUEOUS ALCOHOL SOLUTIONS, DETECTED BY PYROELECTRIC SENSORS

A simple pyroelectric sensor is presented for on-line monitoring of Rayleigh-Bénard -Marangoni convective instabilities in methanol, ethanol and isopropanol aqueous solutions, caused by alcohol evaporation from the free surface of the liquid. The signals measured at the liquid-substrate interface represent thermal fluctuations of the order of 10−2 K, having a frequency spectrum between 0.08 Hz and 0.4 Hz. The numerical estimation of the Rayleigh and Marangoni numbers leads to the conclusion that the latter is dominant in the conditions of the experiment. The amplitude and duration of the thermal noise are investigated as a function of alcohol concentration. An attempt is made to determine characteristic invariant quantities.

Synthesis of Silver Nanoparticles by a Laser-Liquid-Solid Interaction Technique

Silver nanoparticles of high chemical homogeneity have been synthesized by a novel laser–liquid–solid interaction technique from a solution composed of silver nitrate, distilled water, ethylene glycol, and diethylene glycol. Rotating nickel, niobium, stainless steel, and ceramic Al2O3 substrates were irradiated using a continuous-wave CO2 laser and Q-switched Nd–YAG laser (λ = 1064 and 532 nm). The silver nanoparticles were characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe x-ray microanalysis (EPMA). The shape of silver particles was dependent on the chemical composition and laser parameters. The synthesis mechanism of silver nanoparticles has been proposed to occur primarily at the laser–liquid–substrate interface by a nucleation and growth mechanism.

Behavior of substrate-confined liquids for in situ crystallization of semiconductors

Millimeter- and micrometer-sized concavities, mostly in the form of close-packed arrays, were produced in SiO 2 and graphite substrates and filled with germanium and related liquids. A variety of processing conditions were studied for controlled filling and for clearing of the intermediate (field) regions. Germanium crystal appears to nucleate, at least from some liquids, from within the fluid phases rather than at the liquid-substrate interface. Crystal growth to sizes comparable with that of the concavity was observed.