Leakage Retardation of Static Seals via Surface Roughness and Wettability Modification.

In this paper, the effects of surface roughness and wettability on the leakage performance of static seals were highlighted. The results show that the leakage rate is negatively correlated with the contact pressure and positively correlated with surface roughness. Surface wettability affects leakage performance. Leakage retardation is achieved by modifying the roughness and wettability at the interface. The seal interface consists of two oleophobicity surfaces and exhibits an excellent seal performance, of which the leakage reduction is approximately 64%. A theoretical model based on wettability is established to predict the leakage rates under varying wettability conditions, and its validity is confirmed. This research result is expected to be applied in the field of seals and predict the leakage performance of interfaces with different wettabilities to minimize and reduce the leakage of oil in static sealing systems.

Migration of Liquid Bridges at the Interface of Spheres and Plates with an Imposed Thermal Gradient.

In this paper, we experimentally investigate the migration of liquid bridges at the interface of spheres and plates with an imposed thermal gradient. The key influencing factors of interface gap, sphere material and diameter, liquid viscosity, and thermal gradient on the migration behaviors are highlighted. Furthermore, the physical mechanism of this intriguing interfacial phenomenon is numerically unraveled. The originality of this work lies in the fact that when thermal gradients were encountered, liquid bridges could migrate at the interface of spheres and plates, and a general design philosophy of related parameters for enhancing or weakening this thermal flow is proposed.

On the Thermocapillary Migration on Radially Microgrooved Surfaces.

Thermocapillary migration describes the phenomenon in which a droplet placed on a nonuniformly heated surface can migrate from warm to cold regions. Herein, we report an experimental investigation of the migration of silicone oil droplets on radially microgrooved surfaces subjected to a thermal gradient; the effects of the initial divergence angle and divergent direction on the migration behavior are highlighted. A theoretical model is established to predict the migration velocity considering the thermocapillary, viscous resistance, and radial structure-induced forces; furthermore, the proposed theoretical derivation is validated. This study advances the understanding of this interfacial phenomenon, which has great potential for regulating and controlling liquid motion in lubrication systems, condensation and heat-transfer devices, and open microfluidics.

Magnetically stimulating capillary effect for reversible wet adhesions.

Despite fascinating natural examples of switchable adhesives to wet surfaces, strategies for an artificially switching capillary adhesion system in situ remains a challenge. Here, we develop a smart reversible magnetic fluid (MF) meniscus adhesion system whose capillary effect can be regulated by external magnetic stimuli. It is revealed that the MF filled joint between two solid surfaces undergoes alteration of its adhesive properties in response to the external stimulus of a varying magnetic field. Compared with the original capillary force (without stimuli), the stimulated one increases or decreases depending on the distributions of applied magnetic field intensities, allowing for switchable adhesive behavior. In addition to the Laplace pressure, hydrostatic pressure induced by the intensity difference in the magnetic field between the inner and outer surfaces of the meniscus was found to contribute to wet adhesion, which accounted for the reversibility. Theoretical models of reversible adhesions have been built and solved as well, and agree well with the experiment results. Our findings not only provide a deep understanding of MF capillary adhesion, but also provide a new method to design reversible wet adhesion systems.

Ringlike Migration of a Droplet Propelled by an Omnidirectional Thermal Gradient.

The interfacial phenomenon associated with the ringlike motion of a liquid droplet subjected to an omnidirectional thermal gradient is investigated. An experimentally verified model is proposed for estimating the droplet migration velocity. It is shown that the unbalanced interfacial tension acting on the liquid in the radial direction provides the necessary propulsion for the migration, whereas the internal force acting on the adjoining liquid contributes to the equilibrium condition in the circumferential direction. This study puts forward the understanding of the interfacial spreading phenomenon, the knowledge of which is important in applications where liquid lubricants are encountered with directionally unstable thermal gradients.

Thermocapillary Migration of Liquid Droplets Induced by a Unidirectional Thermal Gradient.

A liquid droplet placed on a nonuniformly heated solid surface will migrate from a high-temperature region to a low-temperature region. This study reports the development of a theoretical model and experimental investigation on the migration behavior of paraffin oil droplets induced by the unidirectional thermal gradient. Thin-film lubrication theory is employed to determine the migration velocity of droplets, and temperature dependence of viscosity is taken into account. Comparisons between experimental and numerical results are presented. An effective approach for estimating the thermocapillary migration velocity of droplets on lubrication is proposed.

A Surface Texture Design to Obstruct the Liquid Migration Induced by Omnidirectional Thermal Gradients.

Thermo-capillary migration is a phenomenon in which surface thermal gradients drive a liquid to flow from warm to cold regions without external forces. It is important to prevent the migration of liquid lubricants on rubbing surfaces. In this paper, a pattern of microdimples was proposed to obstruct the liquid migration induced by an omnidirectional thermal gradient. Microdimple patterns were fabricated on the surfaces of SUS316 stainless steel. Experiments were performed to investigate the influence of microdimple patterns with different geometric parameters on the migration behavior of paraffin oil. In particular, this study focused on the interfacial flowing near the microdimples. The results demonstrated that microdimples have a significant obstructive effect on migration, whereas dimples have a retaining effect, and the adjacent dimples have an interacting effect.

Creating lifting force in liquids via thermal gradients.

In this paper, we explore a concept and present the first experimental evidence to show that it is possible to form a stable liquid film and create lifting force at the interface via thermal gradient to minimize interfacial rubbing of surfaces and the associated wear. The approach is based on manipulating the flow behavior via thermocapillary, which describes how a liquid can be made to flow from warm to cold regions purely by inducing a thermal gradient. We show that liquid bridges between two parallel plates can be manipulated and stabilized under a combined effect of the thermocapillary flow and the Couette flow, which describes the motion of a viscous fluid between two parallel plates in a relative sliding motion. The equilibrium stage is confirmed under different experimental conditions of a thermal gradient, interfacial gap, liquid viscosity, and liquid bridge volume. A strategy is proposed to control liquid motion and create lifting force between two plates. A theoretical model is also presented to illustrate the principle of the equilibrium stage. Creating lifting forces at the interface offers a new thermo-hydrodynamic tool for manipulating liquids behavior. This approach has the potential for controlling liquid motion in mechanical components and nature.

Bioinspired Functional Structures for Lubricant Control at Surfaces and Interfaces: Wedged-Groove with Oriented Capillary Patterns.

In this work, a design concept of bioinspired functional surfaces is proposed for lubricant control at surfaces and interfaces subjected to external thermal gradients. Inspired by the conical structures of cactus and the motion configuration of Centipedes, a bioinspired surface of wedged-groove with an oriented capillary pattern is constructed. The effect of geometrical parameters on the directional lubricant manipulation capacity and sliding anisotropy is discussed. It is found that by regulating the orientation of the capillary pattern, a controllable lubricant self-transport capacity can be achieved for varying conditions from surfaces to interfaces, with or without thermal gradients. The lubricant self-transport process is captured, and the mechanism is revealed. The design philosophy of the proposed bioinspired functional surface is believed to have potential applications for lubricant control in modern machinery and complex liquid control in lab-on-a-chip and microfluidics devices.

Succession of soil bacterial communities and network patterns in response to conventional and biodegradable microplastics: A microcosmic study in Mollisol.

Significant soil contamination of microplastics (MPs) by the application of agricultural mulching films has aroused global concern, however, the effects of conventional and biodegradable MPs on the dynamics of soil microbial communities and network patterns have not been sufficiently reported. In this study, we conducted a soil microcosmic experiment by adding low-density polyethylene and biodegradable MPs (PE and BD) into a black soil at the dosages of 0 % (CK), 0.1 % (low-dose, w/w), 1 % (medium-dose, w/w) and 5 % (high-dose, w/w), and soils were sampled on the 15th, 30th, 60th and 90th day of soil incubation for high-throughput sequencing. The results showed that the incubation time was the most influential factor driving the variations in bacterial community structures, and significant effects of MP dosages and types were also detected. With the increase in MP dosage, bacterial diversity markedly increased and decreased at the beginning (D15) and end of sampling day (D90), respectively. Compared to CK, BD induced a larger community dissimilarity than PE and tended to enrich environmentally friendly taxa, while PE likely promoted the growth of hazardous taxa. Moreover, BD simplified interspecies interactions compared to the networks of PE and CK, and Nitrospira was identified as a keystone species in both PE and BD networks. These findings provide new insights into the influences of conventional and biodegradable MPs on the succession patterns of soil bacterial communities, and further studies are needed to explore the soil metabolic potentials affected by the presence of MPs.

Efficient Bubble Transport on Bioinspired Topological Ultraslippery Surfaces.

Slippery liquid-infused porous surfaces (SLIPS) with micro-/nanostructures inspired by the Nepenthes pitcher plant exhibit excellent characteristics in terms of liquid repellency, self-healing, pressure tolerance, and so forth. In particular, stable bubble transport on SLIPS can be achieved when the surface is submerged in water. However, more precise and sophisticated bubble manipulations on SLIPS still remain challenging. In this research, a three-dimensional topological SLIPS combined with a submillimeter rice leaf-like groove array is fabricated to guide the underwater bubble motion precisely. The dynamic behavior and wetting state of bubbles on SLIPS were investigated experimentally. Furthermore, topological SLIPS with different geometric textures were designed and created for sophisticated bubble manipulations, such as fast bubble directional transport and collection. The results indicated that a lubricant with low surface tension and low viscosity could improve the adhesion force to bubbles and the transport velocity of bubbles, simultaneously. The current findings are helpful to deepen the cognition of interaction between bubbles and SLIPS and to promote their wide applications in the field of smart bubble manipulation and catalytic chemistry.

Non-sticky and Free-forward Performances of Grubs against Soil.

The intriguing non-sticky and free-forward performances of grubs against soil deeply attract our interests. In this study, the life cycle and body morphology of a kind of grubs, larvae of Japanese rhinoceros beetles, are introduced. The uniformly oriented hierarchical micro structures pattern on the back epidermis is firstly reported. The rotating and forwarding motion configuration of grubs in soil is unraveled. The friction and adhesion properties of grubs are evaluated and compared with typical materials. The biological electroosmosis induced adhesion reduction effect and the hierarchical structures pattern induced anisotropic friction feature are highlighted.

Manipulating thermocapillary migration via superoleophobic surfaces with wedge shaped superoleophilic grooves.

HYPOTHESIS: Thermocapillary migration is a phenomenon that liquid droplets can move from warm to cold regions on a nonuniformly heated surface. We expect to construct functional surfaces to manipulate the migration of liquid lubricants on rubbing surfaces. EXPERIMENTS: Superoleophobic surfaces with wedge shaped superoleophilic grooves of varying geometrical parameters are fabricated, and migration experiments of typical liquid lubricants are performed on the designed surfaces. FINDINGS: Manipulation capacity of the designed surfaces on the migration of liquid lubricants is confirmed, and the mechanism is revealed. An effective method using superoleophobic surfaces with wedge shaped superoleophilic grooves to reconcentrate the migrated lubricants is highlighted. Moreover, a general design philosophy for patterns of wedge shaped groove is proposed.

Contact angle hysteresis effect on the thermocapillary migration of liquid droplets.

Thermocapillary migration describes a phenomenon where a liquid droplet spreads from warm to cold regions due to the interfacial tension gradients. Since the contact angle hysteresis effect is involved during the migration process, we consider the hysteresis effect and rectify the theoretical model to predict the migration velocity on solid surfaces. By conducting migration experiments on surfaces with different magnitudes of the hysteresis effect, we verify the validity of the theoretical derivation. This study advances the understanding of the interfacial phenomenon of thermocapillary migration, moreover, offers an insight into the migration capacity of different materials and guides the design of key components associated with the thermal gradients.

Pillar versus dimple patterned surfaces for wettability and adhesion with varying scales.

Inspired by biological topographical surfaces, micropatterned elastomeric surfaces with square pillars and dimples of different geometry scales were fabricated. Their wettability and adhesion properties with various liquids were systematically investigated and compared with flat surfaces. Interesting results were obtained in the case of silicone oil (the toe-pad-like wetting case) in that the scale-dependent wettability and adhesion performed inversely for pillars and dimples. Micropillars significantly enhanced the surface wettability with a geometry scale dependence, whereas the dimples suppressed the wettability independent of the geometry scale. The adhesion force of the micropillars increased with an increase of the geometry scale. However, in the case of the micro-dimples, the adhesion force obviously decreased with an increase of the geometry scale. This behaviour was attributed to the fact that pillars are 'open' to oil but dimples are 'close' to oil, presenting different orientations to the solid-liquid interface.

Effect of wetting case and softness on adhesion of bioinspired micropatterned surfaces.

Inspired by the adhesive ability of amphibian toe-pads, polydimethylsiloxane (PDMS) hexagonal pillar arrayed surfaces with varying softness are fabricated, and their adhesion behaviors in the non-wetting, mostly wetting and totally wetting cases are throughly investigated. Experimental results demonstrate that under a totally wetting case, i.e. the biological toe-pad-like case, besides the long-range capillary force, a short-range interaction caused by close contact plays a significant role for adhesion. Compared with unpatterned surface, hexagonal pillar patterns can lead to a remarkable improvement in both short- and long-range contribution for wet adhesion. Meanwhile, the surface softness performs a beneficial character in the short-range contribution for the adhesion of micropillars. Considering the fact that the soft microstructure and the almost totally wetting case (low surface tension of secretion and high surface energy of epidermis) on the pads of nature species, it is reasonable to suggest that these evolutions are in favor for wet attachments.

On the migration of a droplet on an incline.

A liquid droplet placed on a nonuniformly heated solid surface will migrate from a high temperature region to a low temperature region. The present study reports the results of an experimental investigation on the migration behavior of mineral oil droplets subjected to a thermal gradient on an inclined plane. A particular attention is paid to the relationship between the critical inclination angle and thermal gradients. It is shown that there exists a critical inclination angle at which the droplet migration is halted. This critical inclination angle can be readily predicted using analytical expressions derived in this paper. This study puts forward the understanding of the interface phenomenon of thermocapillary migration on an incline. The knowledge of the critical inclination is important in applications where the migration on an incline must be obstructed to retain adequate lubrication in the desired location.

Covalent attachment of phospholipid analogous polymers to modify a polymeric membrane surface: a novel approach.

A novel method for the surface modification of a microporous polypropylene membrane by tethering phospholipid analogous polymers (PAPs) is given, which includes the photoinduced graft polymerization of N,N-dimethylaminoethyl methacrylate (DMAEMA) and the ring-opening reaction of grafted poly-(DMAEMA) with 2-alkyloxy-2-oxo-1,3,2-dioxaphospholanes. Five 2-alkyloxy-2-oxo-1,3,2-dioxaphospholanes, containing octyloxy, dodecyloxy, tetradecyloxy, hexadecyloxy, and octadecyloxy groups in the molecular structure, were used to fabricate the PAP-modified polypropylene membranes. The attenuated total reflectance FT-IR spectra of the original, poly(DMAEMA)-grafted, and PAP-modified membranes confirmed the chemical changes on the membrane surface. Scanning electron microscope pictures showed that, compared with the original membrane, the surface porosities ofpoly(DMAEMA)-grafted and PAP-modified membranes were somewhat reduced. Water contact angles measured by the sessile drop method on PAP-modified membranes were slightly lower than that on the original polypropylene membrane, but higher than those on poly(DMAEMA)-grafted membranes with the exception of octyloxy-containing PAP-modified membranes. However, BSA adsorption experiments indicated that the five PAP-modified membranes had a much better protein-resistant property than the original polypropylene membrane and the poly(DMAEMA)-grafted membranes. For hexadecyloxy- and octadecyloxy-containing PAP-modified membranes, almost no protein adsorption was observed when the grafting degree was above 6 wt %. It was also found that the platelet adhesion was remarkably suppressed on the PAP-modified membranes. All these results demonstrate that the described approach is an effective way to improve the surface biocompatibility for polymeric membranes.