AFM characterization of nanobubble formation and slip condition in oxygenated and electrokinetically altered fluids

A critical review on the properties and applications of bulk micro and nanobubbles for the degradation of organic pollutants in wastewater treatment

The presence of persistent organic pollutants in wastewater streams has presented significant challenges towards their removal. In the recent decade, bulk micro (1-100 μm) and nanobubble (50-150 nm) (MNB) technology has exhibited technological advancements via integration of MNB technology in degrading organic pollutants from wastewater streams. The present review critically analyses the physico-chemical properties such as stability, zeta potential, mass transfer rates, rising velocity and size distribution of MNBs. The paradigm shift from conventional wastewater treatment to more sustainable solution is initiated by the production of OH- ions and free radicals for the degradation of organic pollutants by the MNB technology. Applications of MNBs are also explored in various wastewater treatment processes such as floatation, membrane cleaning, adsorption, aeration, and advanced oxidation processes. Future researches highlighting the challenges in the development of efficient and robust MNB technology and its real-time applications have also been highlighted. It is anticipated that MNBs could be a sustainable and economic solution for wastewater treatment.

Toward a Comprehensive Understanding of the Anomalously Small Contact Angle of Surface Nanobubbles

Experimental studies have demonstrated that the gas phase contact angle (CA) of a surface nanobubble (SNB) is much smaller than that of a macroscopic gas bubble. This reduced CA plays a crucial role in prolonging the lifetime of SNBs by lowering the bubble pressure and preventing gas molecules from dissolving in the surrounding liquids. Despite extensive efforts to explain the anomalously small CA, a consensus about the underlying reasons is yet to be reached. In this study, we conducted experimental investigations to explore the influence of gas molecules adsorbed at the solid-liquid interface on the CA of SNBs created through the solvent exchange (SE) method and temperature difference (TD). Interestingly, no significant change is observed in the CA of SNBs on highly oriented pyrolytic graphite (HOPG) surfaces. Even for nanobubbles on micro/nano pancakes, the CA only exhibited a slight reduction compared to SNBs on bare HOPG surfaces. These findings suggest that gas adsorption at the immersed solid surface may not be the primary factor contributing to the small CA of the SNBs. Furthermore, the CA of SNBs formed on polystyrene (PS) and octadecyltrichlorosilane (OTS) substrates was also investigated, and a considerable increase in CA was observed. In addition, the effects of other factors including impurity, electric double layer (EDL) line tension, and pinning force upon the CA of SNBs were discussed, and a comprehensive model about multiple factors affecting the CA of SNBs was proposed, which is helpful for understanding the abnormally small CA and the stability of SNBs.

Spectral Properties of Foams and Emulsions

The optical and spectral properties of foams and emulsions provide information about their micro-/nanostructures, chemical and time stability and molecular data of their components. Foams and emulsions are collections of different kinds of bubbles or drops with particular properties. A summary of various surfactant and emulsifier types is performed here, as well as an overview of methods for producing foams and emulsions. Absorption, reflectance, and vibrational spectroscopy (Fourier Transform Infrared spectroscopy-FTIR, Raman spectroscopy) studies are detailed in connection with the spectral characterization techniques of colloidal systems. Diffusing Wave Spectroscopy (DWS) data for foams and emulsions are likewise introduced. The utility of spectroscopic approaches has grown as processing power and analysis capabilities have improved. In addition, lasers offer advantages due to the specific properties of the emitted beams which allow focusing on very small volumes and enable accurate, fast, and high spatial resolution sample characterization. Emulsions and foams provide exceptional sensitive bases for measuring low concentrations of molecules down to the level of traces using spectroscopy techniques, thus opening new horizons in microfluidics.

Dynamic interplay between interfacial nanobubbles: oversaturation promotes anisotropic depinning and bubble coalescence

Probing the dynamics of nanobubbles is essential to understand their longevity and behavior. Importantly, such an observation requires tools and techniques having high temporal resolutions to capture the intrinsic characteristics of the nanobubbles. In this work, we have used the in situ liquid-phase electron microscopy (LPEM) technique to gain insights into nanobubbles' behavior and their interfacial dynamics. Interestingly, we could observe a freely growing-shrinking nanobubble and a pinned nanobubble under the same experimental conditions, suggesting the possibility of multiple nanobubble stabilization theories and pathways. Remarkably, the study reveals that a freely growing-shrinking nanobubble induces anisotropic depinning in the three-phase contact line of a strongly pinned neighboring nanobubble. The anisotropic depinning is attributed to the differential local gas saturation levels, depending on the relative positioning of the freely growing-shrinking nanobubble. Furthermore, we also observed a unique pull-push phenomenon exhibited by the nanobubble's interfaces, which is attributed to the van der Waals interactions and the electric double layer collectively. The role of the electric double layer in suppressing and delaying the merging is also highlighted in this study. The present work aims to reveal the role of locally varying gas saturation in the depinning of nanobubbles, their longevity due to the electric double layer, and the consequent coalescence, which is crucial to understand the behavior of the nanobubbles. Our findings will essentially contribute to the understanding of these novel nanoscale gaseous domains and their dynamics.

Generation and characterization of air micro-bubbles in highly hydrophobic capillaries

The generation of air microbubbles in microfluidic systems or in capillaries could be of great interest for transportation (single cell analysis, organite transportation) or for liquid compartmentation. The physicochemical characterization of air bubbles and a better understanding of the process leading to bubble generation during electrophoresis is also interesting in a theoretical point of view. In this work, the generation of microbubbles on hydrophobic Glaco™ coated capillaries has been studied in water-based electrolyte. Air bubbles were generated at the detection window and the required experimental parameters for microbubbles generation have been identified. Generated bubbles migrated against the electroosmotic flow, as would do strongly negatively charged solutes, under constant electric field. They have been characterized in terms of dimensions, electrophoretic mobility, and apparent charge.

Stimuli-responsive nanobubbles for biomedical applications

Stimuli-responsive nanobubbles have received increased attention for their application in spatial and temporal resolution of diagnostic techniques and therapies, particularly in multiple imaging methods, and they thus have significant potential for applications in the field of biomedicine. This review presents an overview of the recent advances in the development of stimuli-responsive nanobubbles and their novel applications. Properties of both internal- and external-stimuli responsive nanobubbles are highlighted and discussed considering the potential features required for biomedical applications. Furthermore, the methods used for synthesis and characterization of nanobubbles are outlined. Finally, novel biomedical applications are proposed alongside the advantages and shortcomings inherent to stimuli-responsive nanobubbles.

Emulsion Electrospinning of Polytetrafluoroethylene (PTFE) Nanofibrous Membranes for High-Performance Triboelectric Nanogenerators

Electrospinning is a simple, versatile technique for fabricating fibrous nanomaterials with the desirable features of extremely high porosities and large surface areas. Using emulsion electrospinning, polytetrafluoroethylene/polyethene oxide (PTFE/PEO) membranes were fabricated, followed by a sintering process to obtain pure PTFE fibrous membranes, which were further utilized against a polyamide 6 (PA6) membrane for vertical contact-mode triboelectric nanogenerators (TENGs). Electrostatic force microscopy (EFM) measurements of the sintered electrospun PTFE membranes revealed the presence of both positive and negative surface charges owing to the transfer of positive charge from PEO which was further corroborated by FTIR measurements. To enhance the ensuing triboelectric surface charge, a facile negative charge-injection process was carried out onto the electrospun (ES) PTFE subsequently. The fabricated TENG gave a stabilized peak-to-peak open-circuit voltage (V_oc) of up to ∼900 V, a short-circuit current density (J_sc) of ∼20 mA m^-2, and a corresponding charge density of ∼149 μC m^-2, which are ∼12, 14, and 11 times higher than the corresponding values prior to the ion-injection treatment. This increase in the surface charge density is caused by the inversion of positive surface charges with the simultaneous increase in the negative surface charge on the PTFE surface, which was confirmed by using EFM measurements. The negative charge injection led to an enhanced power output density of ∼9 W m^-2 with high stability as confirmed from the continuous operation of the ion-injected PTFE/PA6 TENG for 30 000 operation cycles, without any significant reduction in the output. The work thus introduces a relatively simple, cost-effective, and environmentally friendly technique for fabricating fibrous fluoropolymer polymer membranes with high thermal/chemical resistance in TENG field and a direct ion-injection method which is able to dramatically improve the surface negative charge density of the PTFE fibrous membranes.

Solvent Exchange Leading to Nanobubble Nucleation: A Molecular Dynamics Study

The solvent exchange procedure has become the most-used protocol to produce surface nanobubbles, while the molecular mechanisms behind the solvent exchange are far from being fully understood. In this paper, we build a simple model and use molecular dynamics simulations to investigate the dynamic characteristics of solvent exchange for producing nanobubbles. We find that at the first stage of solvent exchange, there exists an interface between interchanging solvents of different gas solubility. This interface moves toward the substrate gradually as the exchange process proceeds. Our simulations reveal directed diffusion of gas molecules against the gas concentration gradient, driven by the solubility gradient of the liquid composition across the moving solvent-solvent interface. It is this directed diffusion that causes gas retention and produces a local gas oversaturation much higher near the substrate than far from it. At the second stage of solvent exchange, the high local gas oversaturation leads to bubble nucleation either on the solid surface or in the bulk solution, which is found to depend on the substrate hydrophobicity and the degree of local gas oversaturation. Our findings suggest that solvent exchange could be developed into a standard procedure to produce oversaturation and used to a variety of nucleation applications other than generating nanobubbles.

Deformation of Surface Nanobubbles Induced by Substrate Hydrophobicity

Recent experimental measurements have shown that there exists a population of nanobubbles with different curvature radii, whereas both computer simulations and theoretical analysis indicated that the curvature radii of different nanobubbles should be the same at a given supersaturation. To resolve such inconsistency, we perform molecular dynamics simulations on surface nanobubbles that are stabilized by heterogeneous substrates either in the geometrical heterogeneity model (GHM) or in the chemical heterogeneity model (CHM) and propose that the inconsistency could be ascribed to the substrate-induced nanobubble deformation. We find that, as expected from theory and computer simulation, for either the GHM or the CHM, there exists a universal upper limit of contact angle for the nanobubbles, which is determined by the degree of supersaturation alone. By analyzing the evolution of the shape of nanobubbles as a function of substrate hydrophobicity that is controlled here by the liquid-solid interaction, two different origins of nanobubble deformation are identified. For substrates in the GHM, where the contact line is pinned by surface roughness, variation in the liquid-solid interaction changes only the location of the contact line and the measured contact angle, without causing a change in the nanobubble curvature. For substrates in the CHM, however, the liquid-solid interaction exerted by the bottom substrate can deform the vapor-liquid interface, resulting in variations in both the curvature of the vapor-liquid interface and the contact angle.

Pinning Down the Reasons for the Size, Shape, and Stability of Nanobubbles

A thermodynamic analysis of the size, shape, and stability of nanobubbles is carried out by modifying classical nucleation theory to include the supersaturation dependence of the surface tension. It is shown that the free energy of a nanobubble is a minimum at the critical radius when the contact line is pinned, that the size of the nanobubble is determined by the decrease in the surface tension caused by the degree of supersaturation, and that together these explain the increased exterior contact angle of a nanobubble compared with the corresponding angle of a macroscopic bubble or droplet on the same surface.

Surface charge-induced EDL interaction on the contact angle of surface nanobubbles

The contact angle (CA) of surface nanobubbles is believed to affect the stability of nanobubbles and fluid drag in micro/nanofluidic systems. The CA of nanobubbles is dependent on size and is believed to be affected by the surface charge-induced electrical double layer (EDL). However, neither of these of attributes are well understood. In this paper, by introducing an EDL-induced electrostatic wetting tension, a theoretical model is first established to study the effect of EDLs formed near the solid-liquid interface and the liquid-nanobubble interface on the gas phase CA of nanobubbles. The size-dependence of this EDL interaction is studied as well. Next, by using atomic force microscopy (AFM), the effect of the EDL on nanobubbles' gas phase CA is studied with variable electrical potential at the solid-liquid interface, which is adjusted by an applied voltage. Both the theoretical and the experimental results show that the EDLs formed near the solid-liquid interface and the liquid-nanobubble interface lead to a reduction of gas phase CA of the surface nanobubbles because of an electrostatic wetting tension on the nanobubble due to the attractive electrostatic interaction between the liquid and nanobubble within the EDL, which is in the nanobubbles' outward direction. An EDL with a larger zeta potential magnitude leads to a larger gas phase CA reduction. Furthermore, the effect of EDL on the nanobubbles' gas phase CA shows a significant size-dependence considering the size dependence of the electrostatic wetting tension. The gas phase CA reduction due to the EDL decreases with increasing nanobubble height and increases with the nanobubble's increasing curvature radius, indicating that a surface charge-induced EDL could possibly explain the size dependence of the gas phase CA of nanobubbles.

Study on Nanobubble-on-Pancake Objects Forming at Polystyrene/Water Interface

Surface nanobubbles, which are the main gaseous state forming at the solid/liquid interface, have received extensive attention due to their peculiar features and potential applications. Nano/micro pancakes and interfacial gas enrichment (IGE) are observed at the water-solid interface, which suggest nanobubbles may coexist with IGE. An intuitive case for the coexistence of nanobubbles and IGE is the nanobubble-on-pancake-like objects. However, it still is not clear whether nanobubbles sit on top of an IGE or the IGE surrounds a nanobubble, which increasingly is seen to be important for understanding the stability and small contact angle of nanobubbles. In this study, the nanobubble-on-pancake-like objects were investigated on a polystyrene (PS) surface. Considering the nanobubble-like objects forming on PS film might be blisters formed because of osmosis, whether such objects are gaseous state or blisters therefore was investigated first. Then, the structure of the nanobubble-on-pancake-like object was analyzed, on the basis of which the stability of nanobubbles under tip perturbation was discussed. The pancake-like domains of the bubble-on-pancake composite disappeared, but the bubble part remained. This indicates that nanobubbles do not sit on top of the pancakes, but are pinned on the solid surface. This is in good agreement with the contact line pinning theory, and is helpful to understanding the abnormal long lifetime (stability) of nanobubbles.

Inertial cavitation initiated by polytetrafluoroethylene nanoparticles under pulsed ultrasound stimulation

Nanoscale gas bubbles residing on a macroscale hydrophobic surface have a surprising long lifetime (on the order of days) and can serve as cavitation nuclei for initiating inertial cavitation (IC). Whether interfacial nanobubbles (NBs) reside on the infinite surface of a hydrophobic nanoparticle (NP) and could serve as cavitation nuclei is unknown, but this would be very meaningful for the development of sonosensitive NPs. To address this problem, we investigated the IC activity of polytetrafluoroethylene (PTFE) NPs, which are regarded as benchmark superhydrophobic NPs due to their low surface energy caused by the presence of fluorocarbon. Both a passive cavitation detection system and terephthalic dosimetry was applied to quantify the intensity of IC. The IC intensities of the suspension with PTFE NPs were 10.30 and 48.41 times stronger than those of deionized water for peak negative pressures of 2 and 5MPa, respectively. However, the IC activities were nearly completely inhibited when the suspension was degassed or ethanol was used to suspend PTFE NPs, and they were recovered when suspended in saturated water, which may indicates the presence of interfacial NBs on PTFE NPs surfaces. Importantly, these PTFE NPs could sustainably initiate IC for excitation by a sequence of at least 6000 pulses, whereas lipid microbubbles were completely depleted after the application of no more than 50 pulses under the same conditions. The terephthalic dosimetry has shown that much higher hydroxyl yields were achieved when PTFE NPs were present as cavitation nuclei when using ultrasound parameters that otherwise did not produce significant amounts of free radicals. These results show that superhydrophobic NPs may be an outstanding candidate for use in IC-related applications.

Characterization of spherical domains at the polystyrene thin film-water interface

Spherical domains that readily form at the polystyrene (PS)-water interface were studied and characterized using atomic force microscopy (AFM). The study showed that these domains have similar characteristics to micro- and nanobubbles, such as a spherical shape, smaller contact angle, low line tension, and they exhibit phase contrast and the coalescence phenomenon. However, their insensitivity to lateral force, absence of long-range hydrophobic attraction, and the presence of possible contaminants and scratches on these domains suggested that these objects are most likely blisters formed by the stretched PS film. Furthermore, the analysis of the PS film before and after contact with water suggested that the film stretches and deforms after being exposed to water. The permeation of water at the PS-silicon interface, caused by osmosis or defects present on the film, can be a reasonable explanation for the nucleation of these spherical domains.

Effect of surface modification on interfacial nanobubble morphology and contact line tension

Past research has confirmed the existence of surface nanobubbles on various hydrophobic substrates (static contact angle >90°) when imaged in air-equilibrated water. Additionally, the use of solvent exchange techniques (based on the difference in saturation levels of air in various solvents) also introduced surface nanobubbles on hydrophilic substrates (static contact angle <90°). In this work, tapping mode atomic force microscopy was used to image interfacial nanobubbles formed on bulk polycarbonate (static contact angle of 81.1°), bromo-terminated silica (BTS; static contact angle of 85.5°), and fluoro-terminated silica (FTS; static contact angle of 105.3°) surfaces when immersed in air-equilibrated water without solvent exchange. Nanobubbles formed on the above three substrates were characterized on the basis of Laplace pressure, bubble density, and contact line tension. Results reported here show that (1) the Laplace pressures of all nanobubbles formed on both BTS and polycarbonate were an order of magnitude higher than those of FTS, (2) the nanobubble number density per unit area decreased with an increase in substrate contact angle, and (3) the contact line tension of the nanobubbles was calculated to be positive for both BTS and polycarbonate (lateral radius, Rs < 50 nm for all nanobubbles), and negative for FTS (Rs > 50 nm for all nanobubbles). The nanobubble morphology and distribution before and after using the solvent exchange method (ethanol-water), on the bulk polycarbonate substrate was also characterized. Analysis for these polycarbonate surface nanobubbles showed that both the Laplace pressure and nanobubble density reduced by ≈98% after ethanol-water exchange, accompanied by a flip in the magnitude of contact line tension from positive (0.19 nN) to negative (-0.11 nN).

Advanced dynamic-equilibrium model for a nanobubble and a micropancake on a hydrophobic or hydrophilic surface

The dynamic-equilibrium model for stabilization of a nanobubble on a hydrophobic surface by Brenner and Lohse [M. P. Brenner and D. Lohse, Phys. Rev. Lett. 101, 214505 (2008)] has been modified taking into account the van der Waals attractive force between gas molecules inside a nanobubble and solid surface. The present model is also applicable to a nanobubble on a hydrophilic surface. According to the model, the pressure inside a nanobubble is not spatially uniform and is relatively higher near the solid surface. As a result, there is gas outflux near a hydrophilic surface, while near a hydrophobic surface there is gas influx which has been already suggested. In the present model, the radius of curvature for a nanobubble depends on the distance from the solid surface because the pressure depends on it. The shape of the micropancake, which is a nearly-two-dimensional bubble, is reproduced by the present model due to the strong dependence of the radius of curvature on the distance from the solid surface. The effect of temperature on the stability of a nanobubble or micropancake is also discussed.

Water-induced blister formation in a thin film polymer

A failure mechanism of thin film polymers immersed in water is presented: the formation of blisters. The growth of blisters is counterintuitive as the substrates were noncorroding and the polymer does not swell in water. We identify osmosis as the driving force behind the blister formation. The dynamics of the blister formation is studied experimentally as well as theoretically, and a quantitative model describing the blister growth is developed, which accurately describes the temporal evolution of the blisters.

Automatic morphological characterization of nanobubbles with a novel image segmentation method and its application in the study of nanobubble coalescence

Nanobubbles (NBs) on hydrophobic surfaces in aqueous solvents have shown great potential in numerous applications. In this study, the morphological characterization of NBs in AFM images was carried out with the assistance of a novel image segmentation method. The method combines the classical threshold method and a modified, active contour method to achieve optimized image segmentation. The image segmentation results obtained with the classical threshold method and the proposed, modified method were compared. With the modified method, the diameter, contact angle, and radius of curvature were automatically measured for all NBs in AFM images. The influence of the selection of the threshold value on the segmentation result was discussed. Moreover, the morphological change in the NBs was studied in terms of density, covered area, and volume occurring during coalescence under external disturbance.

Simple surface treatment of cell-culture scaffolds with ultrafine bubble water

We propose a novel method to treat polymeric scaffold surfaces for cell culture with water containing nanobubbles, called ultrafine bubbles (UFBs), with typical diameters less than 1 μm. A thin film of polystyrene (PS) prepared on a solid substrate was exposed to UFB water for 2 days at room temperature. The PS surface was characterized by X-ray photoelectron spectroscopy (XPS), static contact angle measurements in water, and atomic force microscopy (AFM). The surface chemical composition and wettability of PS films remained unchanged after treatment, so that aggregation states of PS at film surfaces remained unaltered by UFB water. On the other hand, after treatment, many UFBs were adsorbed on hydrophobic PS surfaces. To study the effect of UFBs on scaffold properties, the adsorption behavior of fibronectin, which is a typical extracellular matrix protein involved in cell adhesion and proliferation, was examined. While the effect on the adsorption was unclear, the structural denaturation of fibronectin was enhanced after UFB treatment, so that the proliferation of fibroblast cells on PS surfaces was promoted.

The study of surface wetting, nanobubbles and boundary slip with an applied voltage: A review

The drag of fluid flow at the solid-liquid interface in the micro/nanoscale is an important issue in micro/nanofluidic systems. Drag depends on the surface wetting, nanobubbles, surface charge and boundary slip. Some researchers have focused on the relationship between these interface properties. In this review, the influence of an applied voltage on the surface wettability, nanobubbles, surface charge density and slip length are discussed. The contact angle (CA) and contact angle hysteresis (CAH) of a droplet of deionized (DI) water on a hydrophobic polystyrene (PS) surface were measured with applied direct current (DC) and alternating current (AC) voltages. The nanobubbles in DI water and three kinds of saline solution on a PS surface were imaged when a voltage was applied. The influence of the surface charge density on the nanobubbles was analyzed. Then the slip length and the electrostatic force on the probe were measured on an octadecyltrichlorosilane (OTS) surface with applied voltage. The influence of the surface charge on the boundary slip and drag of fluid flow has been discussed. Finally, the influence of the applied voltage on the surface wetting, nanobubbles, surface charge, boundary slip and the drag of liquid flow are summarized. With a smaller surface charge density which could be achieved by applying a voltage on the surface, larger and fewer nanobubbles, a larger slip length and a smaller drag of liquid flow could be found.

Coalescence and stability analysis of surface nanobubbles on the polystyrene/water interface

In this article, we have studied the surface nanobubbles on polystyrene (PS)/water interfaces using tapping mode atomic force microscopy (TM-AFM). Detailed bubble coalescence phenomenon of differently sized surface nanobubbles (with lateral size up to about ∼10 μm) was obtained. The quantity of gas molecules, before and after coalescence, was calculated. It was found that after coalescence the quantity of gas molecules was increased by approximately 112.5%. The possible reasons for this phenomenon were analyzed and discussed. Our analysis shows that a reasonable explanation should be an influx of gas into the bubble caused by the depinning of the contact line and the decrease in the inner pressure during bubble coalescence. The factors affecting the coalescence speed of surface bubbles were also discussed. It was found that the coalescence speed of larger bubbles is usually slower than that of the smaller ones. We also noticed that it is uncertain whether a larger or smaller bubble will move first to merge into others. This is due to the combined effects of the contact line and the surface properties. Furthermore, the temporal evolution of surface bubbles was studied. The three-phase contact line of bubbles kept the pinning within the incubation time. This was consistent with the contact line pinning theory, based on which the theoretical lifetime of the surface bubbles in our experiments was calculated to be t(b) ≈ 6.9 h. This value is close to the experimental results. Meanwhile, the faster gas diffusion from the oversized bubbles after 12 h of incubation was observed and analyzed. Our results indicate that a viable stability mechanism for surface nanobubbles would be favored simultaneously by the contact line pinning, gas influx near the contact line from an interfacial gas enrichment (IGE), a thin "contaminant film" around the gas/liquid interface, and even the electrostatic effect.

Boundary slip of superoleophilic, oleophobic, and superoleophobic surfaces immersed in deionized water, hexadecane, and ethylene glycol

The boundary slip condition is an important property, and its existence can reduce fluid drag in micro/nanofluidic systems. The boundary slip on various surfaces immersed in water and various electrolytes has been widely studied. For the surfaces immersed in oil, the boundary slip on superoleophilic and oleophilic surfaces has been studied, but there is no data on oleophobic and superoleophobic surfaces. In this paper, experiments are carried out to study electrostatic force and boundary slip on superoleophilic, oleophobic, and superoleophobic surfaces immersed in deionized (DI) water, hexadecane, and ethylene glycol. In addition, the surface charge density of the samples immersed in DI water is quantified. Results show that the electrostatic force and the absolute value of the surface charge density of an octadecyltrichlorosilane surface are larger than that of a polystyrene surface, and the electrostatic force and the absolute value of surface charge density of a superoleophilic surface are larger than that of oleophobic and superoleophobic surfaces. For the same liquid, the larger contact angle leads to a larger slip length at the solid-liquid interface. For the same surface, the larger liquid viscosity leads to a larger slip length. The relevant mechanisms are discussed in this paper.