Solid Surface Research Articles

Hybrid motility mechanism of sperm at viscoelastic fluid-solid interface

To fertilize eggs, sperm must pass through narrow, complex channels filled with viscoelastic fluids in the female reproductive tract. While it is known that the topography of the surfaces plays a role in guiding sperm movement, sperm have been thought of as swimmers, i.e., their motility comes solely from sperm interaction with the surrounding fluid, and therefore, the surfaces have no direct role in the motility mechanism itself. Here, we examined the role of solid surfaces in the movement of sperm in a highly viscoelastic medium. By visualizing the flagellum interaction with surfaces in a microfluidic device, we found that the flagellum stays close to the surface while the kinetic friction between the flagellum and the surface is in the direction of sperm movement, providing thrust. Additionally, the flow field generated by sperm suggests slippage between the viscoelastic fluid and the solid surface, deviating from the no-slip boundary typically used in standard fluid dynamics models. These observations point to hybrid motility mechanisms in sperm involving direct flagellum-surface interaction in addition to flagellum pushing the fluid. This finding signifies an evolutionary strategy of mammalian sperm crucial for their efficient migration through narrow, mucus-filled passages of the female reproductive tract.

N-Alkylation of Dopamine and Its Impact on Surface Coating Properties.

Surface coating with dopamine (DA) has received significant attention over the past decade due to its compatibility with other surface coating techniques and versatility, making it applicable to solid surfaces regardless of substrate and shape. Much effort has been made to elucidate the origin of its surface coating capability, and as a result, many important factors affecting the coating properties have been determined. For example, it has been reported that the length of the carbon chain between catechol and amino groups, the attachment of specific functional groups to the catechol ring and amino group, and the replacement of the amino group with another functional group can affect the surface coating properties of DA. Despite these various attempts, there are still many factors that remain unknown. In this study, we investigate the effect of N-alkylation on DA coating. N-Ethyl-DA, N-propyl-DA, and N-isopropyl-DA are newly synthesized through simple organic reactions, and the coating efficiency of DA derivatives is compared with nucleophilicity and steric bulkiness. As a result, the coating efficiency of N-ethyl-DA and N-propyl-DA is lower than for pristine DA and N-methyl-DA, but it is possible to coat solid surfaces with alkyl-functionalized DA. In contrast, the coating with sterically bulky N-isopropyl-DA is almost unsuccessful.

Functionalization of microbubbles in a microfluidic chip for biosensing application.

Microbubbles are widely used for biomedical applications, ranging from imagery to therapy. In these applications, microbubbles can be functionalized to allow targeted drug delivery or imaging of the human body. However, functionalization of the microbubbles is quite difficult, due to the unstable nature of the gas/liquid interface. In this paper, we describe a simple protocol for rapid functionalization of microbubbles and show how to use them inside a microfluidic chip to develop a novel type of biosensor. The microbubbles are functionalized with biochemical ligand directly at their generation inside the microfluidic chip using a DSPE-PEG-Biotin phospholipid. The microbubbles are then organized inside a chamber before injecting the fluid with the bioanalyte of interest through the static bubbles network. In this proof-of-concept demonstration, we use streptavidin as the bioanalyte of interest. Both functionalization and capture are assessed using fluorescent microscopy thanks to fluorescent labeled chemicals. The main advantages of the proposed technique compared to classical ligand based biosensor using solid surface is its ability to rapidly regenerate the functionalized surface, with the complete functionalization/capture/measurement cycle taking less than 10min.

Direct visualization of viscous dissipation and wetting ridge geometry on lubricant-infused surfaces

Drops are exceptionally mobile on lubricant-infused surfaces, yet they exhibit fundamentally different dynamics than on traditional superhydrophobic surfaces due to the formation of a wetting ridge around the drop. Despite the importance of the wetting ridge in controlling drop motion, it is unclear how it dissipates energy and changes shape during motion. Here, we use lattice Boltzmann simulations and confocal microscopy to image how the wetting ridge evolves with speed, and construct heatmaps to visualize where energy is dissipated on flat and rough lubricated surfaces. As speed increases, the wetting ridge height decreases according to a power law, and an asymmetry develops between the front and rear sides. Most of the dissipation in the lubricant ( >75%) occurs directly in front and behind the drop. The geometry of the underlying solid surface hardly affects the dissipation mechanism, implying that future designs should focus on optimizing the surface geometry to maximize lubricant retention.

Zeta Potentials of Cotton Membranes in Acetonitrile Solutions.

Solid surfaces in contact with nonaqueous solvents play a key role in electrochemistry, analytical chemistry, and industrial chemistry. In this work, the zeta potentials of cotton membranes in acetonitrile solutions were determined by streaming potential and bulk conductivity measurements. By applying the Gouy-Chapman theory and the Langmuir adsorption isotherm of ions to the experimental data, the mechanism of the electrification at the cotton/acetonitrile interface is revealed for the first time to be solely due to ion adsorption on the surface, rather than proton dissociation at the interface. Different salts were found to produce opposite signs of the zeta potentials. This behavior can be attributed to ion solvation effects and the strong ordering of acetonitrile molecules at the interface. Furthermore, a trend of the electroviscous effect was observed, in agreement with the standard electrokinetic theory. These findings demonstrate that electrokinetics in acetonitrile, a polar aprotic solvent, can be treated in the same manner as in water.

Crack densification in drying colloidal suspensions.

As sessile drops of aqueous colloidal suspensions dry, a close-packed particle deposit forms that grows from the edge of the drop toward the center. To compensate for evaporation over the solid's surface, water flows radially through the deposit, generating a negative pore pressure in the deposit associated with tensile drying stresses that induce the formation of cracks. As these stresses increase during drying, existing cracks propagate and additional cracks form, until the crack density eventually saturates. We rationalize the dynamics of crack propagation and crack densification with a local energy balance between the elastic energy released by the crack, the energetic cost of fracture, and the elastic energy released by previously formed cracks. We show that the final spacing between radial cracks is proportional to the local thickness of the deposit, while the aspect ratio of the crack segments depends on the shape of the deposit.

Erosion of surfaces by trapped vortices

Two two-dimensional free boundary problems describing the erosion of solid surfaces by the flow of inviscid fluid in the presence of trapped vortices are considered. The first problem tackles an initially flat, infinite fluid-solid interface with uniform flow at infinity and a vortex in equilibrium above the surface. The second involves flow around a finite body with a trailing Föppl-type vortex pair. The conformal invariance of the complex potential permits both problems to be formulated as a Polubarinova–Galin (PG) type equation in which the time-dependent eroding surface in the physical z-plane is mapped to the fixed boundary of the ζ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\zeta $$\\end{document}-disk. The Hamiltonian governing the equilibrium position of the vortex (or vortex pair in the second problem) is also found from the same map. In each problem, the PG equation giving the conformal map is found numerically and the time-dependent evolution of the interface and vortex location is determined. Different models governing the erosion of the interface are investigated in which the normal velocity of the boundary depends on some given function of the fluid flow velocity at the boundary. Typically, in the infinite surface case, erosion leads to the formation of a symmetric valley beneath the vortex which, in turn, moves downward toward the interface. A finite body undergoes erosion which is asymmetric in the flow direction leading to a flattening of the lee surface of the body so displaying some similarity to the experiments and associated viscous theory of Ristroph et al, Moore et al (Proc Natl Acad Sci 109(48):19606–19609, 2012, Phys Fluids 25(11):116602, 2013).

The role of the water contact layer on hydration and transport at solid/liquid interfaces

Understanding the structure in the nanoscopic region of water that is in direct contact with solid surfaces, so-called contact layer, is key to quantifying macroscopic properties that are of interest to e.g. catalysis, ice nucleation, nanofluidics, gas adsorption, and sensing. We explore the structure of the water contact layer on various technologically relevant solid surfaces, namely graphene, MoS[Formula: see text], Au(111), Au(100), Pt(111), and Pt(100), which have been previously hampered by time and length scale limitations of ab initio approaches or force field inaccuracies, by means of molecular dynamics simulations based on ab initio machine learning potentials built using an active learning scheme. Our results reveal that the in-plane intermolecular correlations of the water contact layer vary greatly among different systems: Whereas the contact layer on graphene and on Au(111) is predominantly homogeneous and isotropic, it is inhomogeneous and anisotropic on MoS[Formula: see text], on Au(100), and on the Pt surfaces, where it additionally forms two distinct sublayers. We apply hydrodynamics and the theory of the hydrophobic effect, to relate the energy corrugation and the characteristic length-scales of the contact layer with wetting, slippage, the hydration of small hydrophobic solutes and diffusio-osmotic transport. Thus, this work provides a microscopic picture of the water contact layer and links it to macroscopic properties of liquid/solid interfaces that are measured experimentally and that are relevant to wetting, hydrophobic solvation, nanofluidics, and osmotic transport.

Impact of lecithin on the lubrication properties of konjac glucomannan gels

The effects of lecithin addition on the properties of konjac glucomannan (KGM) hydrogels prepared by controlled heating were investigated. Weak hydrogels were formed at 1 % KGM, which contained relatively thick strands. The shear viscosity and shear modulus of the hydrogels increased with increasing KGM concentration. The pure KGM hydrogels exhibited relatively poor boundary lubrication at all polysaccharide concentrations studied. The inclusion of lecithin (0.001 % to 0.20 %) in the KGM hydrogels appreciably altered their rheological properties, which could be modulated by varying the lecithin/KGM ratio. Microstructural analysis showed that lecithin caused a substantial restructuring of the strands in the hydrogel network. Lecithin was also found to be a highly effective lubricant in the KGM hydrogels. Incorporation of trace amounts of lecithin led to a significant improvement in the lubricating properties of the KGM hydrogels, especially boundary lubrication. Fourier transform infrared (FTIR) and differential canning calorimetry (DSC) analyses provided information about the molecular interactions between the lecithin and KGM molecules. The ability of lecithin to increase the lubricating performance of the KGM hydrogels was mainly attributed to the adsorption of phospholipid-biopolymer complexes onto solid surfaces, which reduced the friction between them.

Bridging Microscopic Dynamics and Hydraulic Permeability in Mechanically-Deformed Nanoporous Materials.

In the field of nanoconfined fluids, there are striking examples of deformation/transport coupling in which mechanical solicitation of the confining solid and dynamics of the confined fluid impact each other. While this intriguing behavior can be harnessed for applications (e.g., energy storage, phase separation, catalysis), the underlying mechanisms remain to be understood. Here, using molecular simulations, we investigate fluid flow in deformable nanoporous materials subjected to external mechanical stresses. We show that the pore mechanical properties significantly affect fluid flow as they lead to significant pore deformations and different fluid organization at the solid surface. Despite such mechanical effects, we show that the fluid thermodynamic properties (i.e., adsorption) can be linked consistently to Darcy's law for the permeability by invoking a pore size definition based on the concept of Gibbs' dividing surface. In particular, regardless of the solid stiffness and applied external stress, all data can be rationalized by accounting for the fluid viscosity and slippage at the solid surface (independently of a specific pore size definition). Using such a formalism, we establish that the intimate relation─derived using the linear response theory─between collective diffusivity and hydraulic permeability remains valid. This allows linking consistently microscopic dynamics experiments and macroscopic permeability experiments on fluid flow in deformable nanoporous materials.

Hydrogen Adsorption on Solid and Liquid Surfaces of Ga–Sn and Ga–In

AbstractDynamic liquid metal catalysts have remarkable activity and selectivity toward certain reactions, such as carbon dioxide reduction. The surface hydrogen coverage may play a role in this, including controlling competing reactions such as hydrogen evolution. Using a novel method of statistically selecting relevant snapshots from a dynamic liquid metal trajectory, the hydrogen adsorption energy is reported across liquid surfaces of Ga–Sn and Ga–In via density functional theory. A fully dynamic sampling of hydrogen adsorption to the liquid metal is also conducted with ab initio molecular dynamics at temperature. The results indicate that hydrogen only associates weakly with Ga–Sn and Ga–In surfaces, with minimal difference between the two materials. Hydrogen adsorbs only slightly more stably (≈0.1 eV) to the liquid surfaces compared to the solid. A low hydrogen coverage is predicted on the liquid metal of ≈0.03 H Å−2 at potentials of around –1.15 V (vs RHE). However, the mobility of hydrogen on the liquid surface is much greater due to a novel mechanism whereby the dynamic surface rearranges, opening pathways for diffusion. The results suggest that the unique catalytic behavior of these liquid metal materials may be due to changes in adsorbate diffusivity when the metals are melted.

An Atomic-scale Explanation For The High Selectivity Towards Carbon Dioxide Reduction Observed On Liquid Metal Catalysts.

The low-temperature liquid metals Ga-In and Ga-Snhave previously showcased >95% selectivity towards the electrochemical reduction of CO2to formate, occuring only when the alloys are melted, not solid.Here, density functional theory molecular dynamics and metadynamics simulations reveal that CO2does not directly adsorb to the Ga-alloy surface, but instead is reduced indirectly by reaction with an adsorbed hydrogen. The reaction barrier is vastly more favourable when this process occurs at In or Sn sites (average: 0.26 eV), than when it occurs on Ga (average: 0.47 eV). However, there is no difference in barrier between solid and liquid surfaces. Instead, we find that Hadsis mobile only on the liquid surface, travelling due to the motion of the liquid beneath. This process drives Hadsto In/Sn sites, allowing low-barrier CO2reduction to occur only on the liquid. Therefore, the dynamic motion of liquid metal catalysts can underpin their unique reactivity. The result has far reaching implications for any protonation reaction conducted with a liquid metal catalyst.

An Atomic‐scale Explanation For The High Selectivity Towards Carbon Dioxide Reduction Observed On Liquid Metal Catalysts

The low‐temperature liquid metals Ga‐In and Ga‐Sn have previously showcased >95% selectivity towards the electrochemical reduction of CO2 to formate, occuring only when the alloys are melted, not solid. Here, density functional theory molecular dynamics and metadynamics simulations reveal that CO2 does not directly adsorb to the Ga‐alloy surface, but instead is reduced indirectly by reaction with an adsorbed hydrogen. The reaction barrier is vastly more favourable when this process occurs at In or Sn sites (average: 0.26 eV), than when it occurs on Ga (average: 0.47 eV). However, there is no difference in barrier between solid and liquid surfaces. Instead, we find that Hads is mobile only on the liquid surface, travelling due to the motion of the liquid beneath. This process drives Hads to In/Sn sites, allowing low‐barrier CO2 reduction to occur only on the liquid. Therefore, the dynamic motion of liquid metal catalysts can underpin their unique reactivity. The result has far reaching implications for any protonation reaction conducted with a liquid metal catalyst.

Conformational Switching of A Nano-Size Urea-bridged Zn(II)Porphyrin Dimer by External Stimuli.

For the first time, explicit stabilization of all the three conformers, viz. (cis,cis), (cis,trans) and (trans,trans), of a 'nano-sized' highly-flexible urea-bridged Zn(II)porphyrin dimer have been achieved via careful manipulations of external stimuli such as solvent dielectrics, temperature, anionic interactions, axial ligation and surface-induced stabilization. The conformers differ widely in their structures, chemical and photophysical properties and thus have vast potential applicability. X-ray structural characterizations have been reported for the (cis,cis) and (cis,trans)-conformers. While (cis,cis) conformer stabilized exclusively in dichloromethane, more polar solvents resulted in the stabilization of (cis,trans) and (trans,trans)-conformers. Low temperature promotes the stabilization of (cis,trans)-conformer while rise in temperature facilitates flipping to the (cis,cis) one. Significantly, exclusive stabilization of the (trans,trans)-isomer has been illustrated using acetate anion which facilitates H-bonding with the two amide linkages of the urea spacer. Remarkably, HOPG surface facilitates stabilization of the energetically challenging (trans,trans)-conformer via CH···p and p···p interactions with the solid surface to the porphyrinic cores. DFT calculations demonstrate that the relative stability of the conformers can be modulated upon slight external perturbations as also observed in the experiment. Several factors contributing towards the conformational landscape for the highly flexible urea-bridged porphyrin dimers have been mapped.

Surface conversion of the dynamics of bacteria escaping chemorepellents

Flagellar swimming hydrodynamics confers a recognized advantage for attachment on solid surfaces. Whether this motility further enables the following environmental cues was experimentally explored. Motile E. coli (OD ~ 0.1) in a 100 µm-thick channel were exposed to off-equilibrium gradients set by a chemorepellent Ni(NO3)2-source (250 mM). Single bacterial dynamics at the solid surface was analyzed by dark-field videomicroscopy at a fixed position. The number of bacteria indicated their congregation into a wave escaping from the repellent source. Besides the high velocity drift in the propagation direction within the wave, an unexpectedly high perpendicular component drift was also observed. Swimming hydrodynamics CW-bends the bacteria trajectories during their primo approach to the surface (< 2 µm), and a high enough tumbling frequency likely preserves a notable lateral drift. This comprehension substantiates a survival strategy tailored to toxic environments, which involves drifting along surfaces, promoting the inception of colonization at the most advantageous sites.Graphical abstract

Influential mechanism of water occurrence states of waste-activated sludge: Specifically focusing on the pore characteristics dominated by cation-organic interactions

The solid pore characteristics are commonly considered as the important influential factors on waste-activated sludge (WAS) dewaterability, and should be related to the cohesive force of bio-flocs dominated by cation-organic interactions at solid-water interface. This study aimed to establish an approach for regulating the solid pore structure of WAS by cationic regulation. The influential mechanism of WAS dewaterability was accordingly explored from the perspective of the pore characteristics dominated by cation-organic interactions. Primarily, with the gradient removal or addition of bivalent cations, the varying pore structure of WAS flocs was tracked by in-situ synchrotron X-ray computed microtomography imaging technique (CMT). The three-dimensional visual model was established to quantify the pore structure parameters of WAS flocs. Following the visualization analysis, the artificial intelligence means, the gradient-weighted class activation mapping (Grad CAM) of three-dimensional convolutional neural network (3D-CNN), was applied for the first time to explore the linkages among solid surface properties, solid pore structure, water occurrence states and sludge dewaterability. It was found that the number and volume of isolated pores jointly determined the mobility and the fractions of vicinal water and interstitial water (p-value≤0.02); also, the decreasing polar or acid-based interfacial free energy with the cationic addition was accompanied with the decreasing isolated pore mean-volume (Pearson coefficient=-0.77, p-value<0.01). These results indicated that the pore structure characteristics determined the water occurrence states, but the solid porosity strongly depended on the interfacial properties. Accordingly, the molecular docking was applied to explore the interfacial reaction mechanism between Ca2+/Mg2+ and solid compositions in terms of complexation sites, molecular dynamics and free energy calculations. As a result, how the cation-organic interactions affected the pore characteristics through solid surface modification could be clarified, which is expected to serve as theoretical foundation for the development of novel sludge conditioning technologies, i.e., more efforts should be devoted to increasing the dense degree of sludge particles through weakening the hydration repulsion of solid surface.

Bubble adhesion dynamics on solid surfaces: Interfacial behavior, force, and energy perspectives using a self-developed dynamic force testing system

Bubble-solid surface adhesion is crucial for industrial processes such as wastewater treatment, and the separation of minerals, plastics, and biomass. In this paper, a novel dynamic force testing system was developed to provide a detailed understanding of bubble-solid adhesion dynamics by simultaneously collecting “time-displacement-force” signals and capturing real-time images. The interfacial behavior and force mechanisms of bubble adhesion to solid surfaces with varying hydrophobicity were investigated. Higher hydrophobicity results in more pronounced “snap-in” adhesion and greater adhesion strength. Instead of fully detaching, bubble breaks at the capillary neck and leaves a microbubble on the solid surface, whose size increases with hydrophobicity, indicating different interfacial behaviors in subsequent reaction. The evolution of Laplace force and surface tension force was further analyzed and the toroidal approximation shows over 60 μN error in Laplace force calculation for large bubble deformations. Thus, for the first time, the “gorge” method was employed to calculate the bubble adhesion force in the presence of a capillary neck. Error analysis indicates that this method, while maintaining simplicity, achieved higher accuracy (lower error) by calculating the Laplace force based on the neck area and accounting for the surface tension force along the tangent at the neck. Energy analysis show that the detachment energies in the stretching and sliding phases correspond to the energy barrier to be overcome for detachment and the “effective work” of interfacial change, respectively. This provides new insights into detachment mechanisms and industrial process modelling, such as flotation.

Wetting and pressure gradient performance in a lattice Boltzmann color gradient model

An accurate implementation of wetting and pressure drop is crucial to correctly reproduce fluid displacement processes in porous media. Although several strategies have been proposed in the literature, a systematic comparison of them is needed to determine the most suitable for practical applications. Here, we carried out numerical simulations to investigate the performance of two widely used wettability schemes in the lattice Boltzmann color gradient model, namely, the geometrical wetting scheme by Leclaire et al. [Phys. Rev. E 95(3), 033306 (2017)](scheme-I) and the modified direction of the color gradient scheme by Akai et al. [Adv. Water Resour. 116, 56–66 (2018)] (scheme-II). We showed that scheme-II was more accurate in simulating static contact angles of a fluid droplet on a solid surface. However, scheme-I was more accurate in simulating a dynamic case of a binary fluid flow in a horizontal capillary tube described by the Washburn equation. Moreover, we investigated the performance of two popular pressure gradient implementation types. Type-I used the so-called Zou–He pressure boundary conditions at the inlet and the outlet of the domain, while type-II used an external body force as a pressure gradient. We showed that the type-I implementation was slightly more accurate in simulating a neutrally wetting fluid in a horizontal capillary tube described by the Washburn equation. We also investigated the differences between the two types of pressure gradient implementation in simulating two fluid displacement processes in a Bentheimer sandstone rock sample: the primary drainage and the imbibition displacement processes.

Effect of Surface Charges in Polymer Coatings on Antifouling Performance in Marine Environments with Sediment.

The antifouling efficacy of hydrophilic polymer coatings is closely related to their surface charge. Many biological foulants such as mammalian cells and marine microalgae possess a negative surface charge, discouraging the use of positively charged polymer coatings for antifouling purposes. Instead, electrically neutral yet hydrophilic polymers have been widely employed, leveraging hydration layers to create a barrier against fouling. However, challenges arise in marine environments where both living marine organisms and sediments can adhere to solid surfaces, rendering previous findings less directly applicable. This study investigates the impact of a polymer coating surface charge on marine antifouling properties. Polymer brushes with various charges are applied to solid substrates, and the adhesion behavior of marine diatoms is assessed under both marine sediment-free and marine sediment conditions. The results underscore the effectiveness of negatively charged polymer brush coatings in marine antifouling, regardless of sediment presence.

Antiviral Activity of Graphene Oxide-Silver Nanocomposites Against Murine Betacoronavirus.

The high infectivity of coronaviruses has led to increased interest in developing new strategies to prevent virus spread. Silver nanoparticles (AgNPs) and graphene oxide (GO) have attracted much attention in the antiviral field. We investigated the potential antiviral activity of GO and AgNPs combined in the nanocomposite GO-Ag against murine betacoronavirus MHV using an in vitro model. GO, AgNPs, and GO-Ag characterization (size distribution, zeta potential, TEM visualization, FT-IR, and EDX analysis) and XTT assay were performed. The antiviral activity of GO-Ag nanocomposites was evaluated by RT-qPCR and TCID50 assays. The results were compared with free AgNPs and pure GO. Cell growth and morphology of MHV-infected hepatocytes treated with GO-Ag composites were analyzed by JuLI™Br. Immunofluorescence was used to visualize the cell receptor used by MHV. Ultrastructural SEM analysis was performed to examine cell morphology after MHV infection and GO-Ag composite treatment. A significant reduction in virus titer was observed for all nanocomposites tested, ranging from 3.2 to 7.3 log10 TCID50. The highest titer reduction was obtained for GO 5 µg/mL - Ag 25 µg/mL in the post-treatment method. These results were confirmed by RT-qPCR analysis. The results indicate that GO-Ag nanocomposites exhibited better antiviral activity compared to AgNPs and GO. Moreover, the attachment of AgNPs to the GO flake platform reduced their cytotoxicity. In addition, the GO-Ag composite modulates the distribution of the Ceacam1 cell receptor and can modulate cell morphology. Graphene oxide sheets act as a stabilizing agent, inhibiting the accumulation of AgNPs and reducing their cellular toxicity. The GO-Ag composite can physically bind and inhibit murine betacoronavirus from entering cells. Furthermore, the constant presence of GO-Ag can inhibit MHV replication and significantly limit its extracellular release. In conclusion, GO-Ag shows promise as an antiviral coating on solid surfaces to minimize virus transmission and spread.