Formation Of Liquid Bridges Research Articles

An overview of the role of capillary condensation in wet caking of powders

This work deals with the role of capillary condensation phenomenon on the caking behavior of powders. A theoretical basis is established to provide a quantitative description of the behavior of powders subjected to humidification. This approach is based on the integration of water condensation and liquid bridge formation within binary contacts in regular or random packing of particles. The existence of a minimum and a maximum limiting filling angle is demonstrated based on thermodynamic limitations. The role of the particle mean size, the wetting contact angle, the interparticle gap and the coordination number on limiting filling angles as well as the amount of condensed water at various water activities (aw) is investigated. The results show that for smooth and non soluble particles, capillary condensation plays a significant role only for sub-micronic powders for which the presence of water in liquid state (then liquid bridges) is expected at low water activities, sensibly less than the saturation activity (aw=1).

Experimental characterization and atomistic modeling of interfacial void formation and detachment in short pulse laser processing of metal surfaces covered by solid transparent overlayers

The short pulse laser interaction with metal surfaces covered by solid transparent overlayers is investigated in experiments and atomistic simulations, with a particular aim of revealing the mechanisms responsible for structural modification of the metal–overlayer interfacial regions. Experimental characterization of Al–silica targets modified by single-pulse laser irradiation with the pulse duration of 10 ps reveals the transitions from the generation of extended interfacial voids with internal nanoscale surface roughness to the partial detachment of the overlayer from the metal substrate, and to the cracking/chipping or complete removal of the overlayer as the laser fluence increases. The mechanisms responsible for the appearance, growth, and percolation of the interfacial voids leading to the detachment of the overlayer from the metal substrate are investigated in a large-scale atomistic simulation. The results of the simulation demonstrate that the processes of nucleation and growth of the interfacial voids are driven by the dynamic relaxation of laser-induced stresses proceeding simultaneously with rapid phase transformations and temperature variation in the interfacial region. The growth and coalescence of the interfacial voids results in the formation of liquid bridges connecting the overlayer and the metal substrate, whereas solidification of the transient liquid structures produced by the breakup of the liquid bridges may be responsible for the formation of the nanoscale roughness of the interfacial voids observed in experiments. Computational analysis of the effect of preexisting interfacial voids reveals a complex dynamic picture of the initial expansion and subsequent compaction of the surface region of the metal substrate and suggests a possible scenario for the formation of voids below the metal–overlayer interface.

A model to predict liquid bridge formation between wet particles based on direct numerical simulations

We study dynamic liquid bridge formation, which is relevant for wet granular flows involving highly viscous liquids and short collisions. Specifically, the drainage process of liquid adhering to two identical, non‐porous wet particles with different initial film heights is simulated using Direct Numerical Simulations (DNS). We extract the position of the interface, and define the liquid bridge and its volume by detecting a characteristic neck position. This allows us building a dynamic model for predicting bridge volume, and the liquid remaining on the particle surface. Our model is based on two dimensionless mobility parameters, as well as a dimensionless time scale to describe the filling process. In the present work model parameters were calibrated with DNS data. We find that the proposed model structure is sufficient to collapse all our simulation data, indicating that our model is general enough to describe liquid bridge formation between equally sized particles. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1877–1897, 2016

Anisotropy of a Tensorial Bishop’s Coefficient for Wetted Granular Materials

The objective of this research is to use grain-scale numerical simulations to analyze the evolution of stress anisotropy exhibited in wetted granular matter. Multiphysical particulate simulations of unsaturated granular materials were conducted to analyze how the interactions of contact force chains and liquid bridges affect macroscopic responses under various suction pressure and loading histories. To study how the formation and rupture of liquid bridges affect the mechanical responses of wetted granular materials, a series of suction-controlled triaxial tests were conducted with two grain assemblies, one composed of large particles of similar sizes, the other composed of a mixture of large particles with significant amount of fines. The results indicate that capillary stresses are anisotropic in both sets of specimens, and that stress anisotropy is more significant in granular assemblies filled with fine particles. A generalized tensorial Bishop’s coefficient is introduced to analyze the connections between microstructural attributes and macroscopic responses. Numerical simulations presented in this paper indicate that the principal values and directions of this Bishop’s coefficient tensor are path dependent.

Pattern formation in wet granular matter under vertical vibrations.

Experiments on a thin layer of cohesive wet granular matter under vertical vibrations reveal kink-separated domains that collide with the container at different phases. Due to the strong cohesion arising from the formation of liquid bridges between adjacent particles, the domains move collectively upon vibrations. Depending on the periodicity of this collective motion, the kink fronts may propagate, couple with each other, and form rotating spiral patterns in the case of period tripling or stay as standing wave patterns in the case of period doubling. Moreover, both patterns may coexist with granular "gas bubbles"-phase separation into a liquidlike and a gaslike state. Stability diagrams for the instabilities measured with various granular layer mass m and container height H are presented. The onsets for both types of patterns and their dependency on m and H can be quantitatively captured with a model considering the granular layer as a single particle colliding completely inelastically with the container.

Experimental and Theoretical Investigations on the Nanoscale Kinetic Friction in Ambient Environmental Conditions.

The liquid lubrication, thermolubricity and dynamic lubricity due to mechanical oscillations are investigated with an atomic force microscope in ambient environmental conditions with different relative humidity (RH) levels. Experimental results demonstrate that high humidity at low-temperature regime enhances the liquid lubricity while at high-temperature regime it hinders the effect of the thermolubricity due to the formation of liquid bridges. Friction response to the dynamic lubricity in both high- and low-temperature regimes keeps the same trends, namely the friction force decreases with increasing the amplitude of the applied vibration on the tip regardless of the RH levels. An interesting finding is that for the dynamic lubricity at high temperature, high-humidity condition leads to the friction forces higher than that at low-humidity condition while at low temperature the opposite trend is observed. An extended two-dimensional dynamic model accounting for the RH is proposed to interpret the frictional mechanism in ambient conditions.

Modelling the dynamics of condensation and evaporation of fluids in three-dimensional slit pores

We present an application of dynamic mean field theory (DMFT) for lattice gas models of confined fluids to the case of a fluid in a three-dimensional narrow slit between two plates. We consider a process where the slit is in contact with a subcritical bulk vapour and the bulk chemical potential is changed from a dilute gas value to a value close to that of the bulk saturated vapour so that capillary condensation occurs in the pore. DMFT predicts that the nucleation process during the condensation occurs by the formation of multiple liquid bridges spanning the pore walls, starting in the corners of the slit. These bridges eventually coalesce and the condensation transition terminates through the shrinkage and disappearance of a vapour bubble. We find that the density distribution is sensitive to the dimensions of the slit, with the number of bridges increasing with the slit area.

CAPILLARY FORCES BETWEEN TWO PARALLEL PLATES CONNECTED BY A LIQUID BRIDGE

Liquid flow between porous and nonporous materials plays an important role in many science and engineering applications such as oil recovery from fractured porous media. The capillary continuity between porous matrix blocks via formation of liquid bridges is a key contributor to the gas−oil gravity drainage mechanism in a gas invaded zone of naturally fractured reservoirs, which increases the height of the continuous liquid column in a fractured formation, thereby enhancing the recovery of oil. However, the role of capillary forces information or break-up of liquid bridges between porous matrix blocks remains a controversial topic. In an attempt to improve an understanding of this problem, a force balance is presented for the concave liquid bridges formed between two horizontal parallel plates. The force balance allows development of a simple model that can be used to find a relationship between the net capillary force, contact angle, and liquid bridge volume. Three different regions including: (I) repulsive net capillary force, (II) attractive net capillary force, and (III) nonexistence regions have been identified. Region I is considered as a region of liquid bridge break-up while Region II is considered as a region of liquid bridge formation. The findings improve an understanding of the formation and break-up of the liquid bridges, which is important in oil recovery from naturally fractured reservoirs during a gravity drainage process.

Application of molecular simulations: Insight into liquid bridging and jetting phenomena

Molecular dynamics simulations have been performed on pure liquid water, aqueous solutions of sodium chloride, and polymer solutions exposed to a strong external electric field with the goal to gain molecular insight into the structural response to the field. Several simulation methodologies have been used to elucidate the molecular mechanisms of the processes leading to the formation of liquid bridges and jets (in the production of nanofibers). It is shown that in the established nanoscale structures, the molecules form a chain with their dipole moments oriented parallel to the applied field throughout the entire sample volume. The presence of ions may disturb this structure leading to its ultimate disintegration into droplets; the concentration dependence of the threshold field required to stabilize a liquid column has been determined. Conformational changes of the polymer in the jetting process have also been observed.

Evaporative gold nanorod assembly on chemically stripe-patterned gradient surfaces

Experimentally we explore the potential of using pre-defined motion of a receding contact line to control the deposition of nanoparticles from suspension. Stripe-patterned wettability gradients are employed, which consist of alternating hydrophilic and hydrophobic stripes with increasing macroscopic surface energy. Nanoparticle suspensions containing nanorods and nanospheres are deposited onto these substrates and left to dry. After moving over the pattern and evaporation of the solvent, characteristic nanoparticle deposits are found. The liquid dynamics has a pronounced effect on the spatial distribution. Nanoparticles do not deposit on the hydrophobic regions; there is high preference to deposit on the wetting stripes. Moreover, the fact that distributed nanoparticle islands are formed suggests that the receding of the contact line occurs in a stick-slip like fashion. Furthermore, the formation of liquid bridges covering multiple stripes during motion of the droplet over the patterns is modeled. We discuss their origin and show that the residue after drying, containing both nanoparticles and the stabilizing surfactant, also resembles such dynamics. Finally, zooming into individual islands reveals that highly selective phase separation occurs based on size and shape of the nanoparticles.

Reinfiltration through liquid bridges formed between two matrix blocks in fractured rocks

Summary Liquid reinfiltration is a significant process, which can considerably retard and slowdown the transport of oil, water and contaminants in fractured subsurface formations. However, accurate modeling of the reinfiltration via liquid bridges formed in a horizontal fracture or space between two rock porous blocks remains a controversial topic. In an attempt to improve an understanding of the problem, the reinfiltration from upper to lower matrix blocks through formation of liquid bridges is theoretically modeled by using generalization of the Lucas–Washburn theory for a porous medium, which takes into account pressure differences due to matrix capillary, gravity, inertia, viscous, and fracture capillary forces. The developed model results in a second-order nonlinear ordinary differential equation (ODE), which is solved numerically to obtain depth and rate of the reinfiltrated liquid versus time. The results showed that three reinfiltration regimes: including Early Time Regime (ETR) ( z ∼ t , q = constant ), Middle Time Regime (MTR) ( z ∼ t , q ∼ 1 / t ) and Late Time Regime (LTR) ( z ∼ t , q = constant ) can be observed, where the inertia, viscous, and gravity forces are dominant, respectively. The results also indicated that by increasing the permeability of the porous medium, the durations of the ETR ( z ∼ t , q = constant ) and the LTR ( z ∼ t , q = constant ) are prolonged while the duration of the MTR ( z ∼ t , q ∼ 1 / t ) is reduced. Moreover, the results revealed that by increasing the liquid viscosity, the durations of the ETR ( z ∼ t , q = constant ) and LTR ( z ∼ t , q = constant ) are reduced whereas the duration of the MTR ( z ∼ t , q ∼ 1 / t ) is prolonged. In addition, the results showed that in the case of high permeability of the porous medium when the fracture capillary pressure is strong enough only the LTR ( z ∼ t , q = constant ) can be observed. The MTR ( z ∼ t , q ∼ 1 / t ) and the LTR ( z ∼ t , q = constant ) scalings are of practical significance since the liquid reinfiltration in fractured rocks associated with gas–liquid drainage mechanism is a very slow process. These findings advance the understanding of the two-phase flow in fractured porous media.

Interfacial dynamics of two immiscible fluids in spatially periodic porous media: the role of substrate wettability.

We delineate the contact line dynamics of two immiscible fluids in a medium having spatially periodic porous structures. The flow is driven by an external applied pressure gradient. We bring out the combined consequences of the solid fraction distribution and the substrate wettability on the resulting dynamics of the contact line, by employing phase-field formalism. We capture the sequence of spatiotemporal events leading to formation of liquid bridges by trapping a small amount of displaced phase fluid between two consecutive porous blocks, as dictated by the combinations of substrate wettability and solid fraction. We also demonstrate the existence of a regime of complete interfacial recovery, depending on the parametric space of the governing parameters under concern. Our results essentially demonstrate the intricate mechanisms by virtue of which the wettabilities of the substrates alter the dynamical evolutions of interfaces and the subsequent shapes and sizes of the adsorbed dispersed phases, bearing far-ranging consequences in several practical applications ranging from oil recovery to groundwater flow.

Microscale study of particle agglomeration in oil-based food suspensions: The effect of binding liquid

In highly concentrated oil or fat-based suspensions, the addition of a secondary immiscible liquid binder results in the agglomeration of particles and formation of a spanning filling network. This network gives rise to a transition in the flow behavior of suspensions which can, in turn, be used to create tailored food products.This agglomeration of particles is related to the formation of liquid bridges. The present work investigates this agglomeration process on a microscale. The effect of two binding liquids with different properties (viscosity, interfacial tension and wetting characteristics) on the adhesion of a particle and a flat surface inside oil medium is investigated. Forces due to water and anhydrous glycerol bridges between a spherical glass particle and a flat glass surface inside purified high oleic sunflower oil were measured with an atomic force microscope (AFM), using the colloidal probe technique. Dispersions of droplets of the secondary liquid (water, glycerol) in the continuous phase (oil) were prepared so that bridges between the particles and the glass surface could be created and force curves measured.The measured forces were dominated by capillary attraction when a liquid of high interfacial tension and low viscosity such as water formed the liquid bridges. When a highly viscous liquid formed the bridge (glycerol), a dynamic viscous interaction contributed to the adhesion leading to a higher force, which was less dependent on the volume of the bridge.

Determining the physical stability and water–solid interactions responsible for caking during storage of alpha-anhydrous glucose

Typically, a crystalline powder is considered reasonably stable below it deliquescence point (RH0), however, caking has been reported for some materials below their RH0. The critical relative humidity (RH) values for caking and hydrate formation in alpha-anhydrous glucose (α-AG) and α-AG partitioned into three particle sizes were assessed using saturated salt slurries ranging from 0 to 84 % RH at 25 °C for 20 weeks. The degree of caking was determined by a five-point visual physical stability scale, from free flowing with minimal clumping (1) to fully caked (5), and X-ray powder diffraction was used to determine the composition of the samples. Caking was observed in α-AG during storage at 68 % RH at 25 °C and the severity of caking increased with increasing RH. Fine particle α-AG caked during storage at 64 % RH, whereas medium and large particle α-AG caked at 68 and 75 % RH, respectively, at 25 °C. Caking was observed in the absence of deliquescence, amorphous content, and hydrate formation; therefore, it is proposed that capillary condensation leads to caking in α-AG below its RH0. Capillary condensation caking occurs at a specific RH (termed RHcc) where direct condensation of moisture into confined spaces, such as particle contact points or surface defects, causes the formation of liquid bridges, which may solidify over time without changes in RH or temperature. To avoid caking, α-AG should be stored below its RHcc, which is highly dependent on particle size; and to avoid conversion to glucose monohydrate, α-AG, regardless of particle size, should be stored below 64 % RH at 25 °C.

Study of water adsorption and capillary bridge formation for SiO2nanoparticle layers by means of a combined in situ FT-IR reflection spectroscopy and QCM-D set-up

Water adsorption and capillary bridge formation within a layer of SiO2-nanoparticles were studied in situ by means of a combination of quartz crystal microbalance (QCM-D) with dissipation analysis and Fourier transformation infrared reflection absorption spectroscopy (FT-IRRAS). FT-IR data were employed to distinguish the "ice-like" and "liquid-like" contributions and to support the analysis of the QCM-D data concerning mass change and dissipation. Combined measurements show that for SiO2-nanoparticles with a diameter of about 250 nm, the formation of two adsorbed monolayers of water as well as bulk water leads to a rather linear increase in the dissipation for relative humidity values of up to 60% which is followed by a strong increase in dissipation during the actual liquid bridge formation. Subsequently, the dissipation drops again when the relative humidity is further increased to values >90%.

Observation of Pendular Liquid Bridge Formation in an Environmental Scanning Electron Microscope

AbstractAn environmental scanning electron microscope (ESEM) is used to observe bridge formation between particles under changing conditions. Variable parameters are particle size, shape, roughness, and binding agent. The results will help to determine influences and interactions, demonstrating the practicability of ESEMs to explore the dynamics of particles and particle beds under the influence of evaporation and condensation. The ESEM uses gaseous and liquid water in its chamber combined with a temperature‐controlled stage to condense binding fluids by changing pressure or temperature. For analysis of particle pairs, test setups are created of spherical and nonspherical particles attached to glass wafers. Replacements for water as binding agent are discussed and ranked for further steps.

Preparation and characterization of fuel pellets from woody biomass, agro-residues and their corresponding hydrochars

Two types of biofuel pellets were prepared from raw biomass and the corresponding hydrochar. Fuel qualities of the two types of pellets including pellet density, mechanical strength and combustion characteristics were investigated. The results showed that the hydrochar pellets had higher fixed carbon contents, elevated heating values and enhanced mass densities in comparison to those raw biomass pellets. The presence of extractives led to the low tensile strength of biomass pellets, while the formation of liquid bridge and significantly enhanced attractive forces including H-bonding and van der Waal’s forces mainly contributed to the strong bonding within the matrix of hydrochar pellets. The hydrochar pellets combusted at elevated temperatures and wide temperature ranges compared to biomass pellets. The high mechanical strength, low moisture uptake and improved combustion property indicated that hydrochar pellets were more suitable than raw biomass pellets as solid biofuels. The present study demonstrated that hydrothermal carbonization (HTC) combined with pelletization provides an alternative for solid biofuel production from biomass resources, especially for the abundant agricultural residues.

Dynamics of capillary condensation in lattice gas models of confined fluids: A comparison of dynamic mean field theory with dynamic Monte Carlo simulations

This article addresses the accuracy of a dynamic mean field theory (DMFT) for fluids in porous materials [P. A. Monson, J. Chem. Phys. 128, 084701 (2008)]. The theory is used to study the relaxation processes of fluids in pores driven by step changes made to a bulk reservoir in contact with the pore. We compare the results of the DMFT to those obtained by averaging over large numbers of dynamic Monte Carlo (DMC) simulation trajectories. The problem chosen for comparison is capillary condensation in slit pores, driven by step changes in the chemical potential in the bulk reservoir and involving a nucleation process via the formation of a liquid bridge. The principal difference between the DMFT results and DMC is the replacement of a distribution of nucleation times and location along the pore for the formation of liquid bridges by a single time and location. DMFT is seen to yield an otherwise qualitatively accurate description of the dynamic behavior.

Wet microcontact printing (µCP) for micro-reservoir drug delivery systems

When micro-reservoir-type drug delivery systems are fabricated, loading solid drugs in drug reservoirs at microscale is often a non-trivial task. This paper presents a simple and effective solution to load a small amount of drug solution at microscale using ‘wet’ microcontact printing (µCP). In this wet µCP, a liquid solution containing drug molecules (methylene blue and tetracycline HCl) dissolved in a carrier solvent was transferred to a target surface (drug reservoir) by contact printing process. In particular, we have investigated the dependence of the quantity and morphology of transferred drug molecules on the stamp size, concentration, printing times, solvent types and surfactant concentration. It was also found that the repetition of printing using a non-volatile solvent such as polyethylene glycol (PEG) as a drug carrier material actually increased the transferred amount of drug molecules in proportion to the printing times based on asymmetric liquid bridge formation. Utilizing this wet µCP, drug delivery devices containing different quantity of drugs in micro-reservoirs were fabricated and their performance as controlled drug delivery devices was demonstrated.

Crackling sound generation during the formation of liquid bridges: A lattice gas model

Due to abnormal mechanical instabilities, liquid bridges may form in the small airways blocking airflow. Liquid bridge ruptures during inhalation are the major cause of the crackling adventitious lung sound, which can be heard using a simple stethoscope. Recently, Vyshedskiy and colleagues (2009) [1] described and characterized a crackle sound originated during expiration. However, the mechanism and origin of the expiratory crackle are still controversial. Thus, in this paper, we propose a mechanism for expiratory crackles. We hypothesize that the expiratory crackle sound is a result of the energy released in the form of acoustic waves during the formation of the liquid bridge. The magnitude of the energy released is proportional to the difference in free energy prior and after the bridge formation. We use a lattice gas model to describe the liquid bridge formation between two parallel planes. Specifically, we determine the surface free energy and the conditions of the liquid bridge formation between two parallel planes separated by a distance 2h by a liquid droplet of volume Ω and contact angle Θ, using both Monte Carlo simulation of a lattice gas model and variational calculus based on minimization of the surface area with the volume and the contact angle constrained. We numerically and analytically determine the phase diagram of the system as a function of the dimensionless parameter hΩ−1/3 and Θ. We can distinguish two different phases: one droplet and one liquid bridge. We observe a hysteresis curve for the energy changes between these two states, and a finite size effect in the bridge formation. We compute the release of free energy during the formation of the liquid bridge and discuss the results in terms of system size. We also calculate the force exerted from liquid bridge on the planes by studying the dependence of the free energy on the separation between the planes 2h. The simulation results are in agreement with the analytical solution.