Thin Water Film Research Articles

Mechanistic Model Describes Wettability Change in Sandstones and Carbonates

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 190236, “Modeling Wettability Change in Sandstones and Carbonates Using a Surface-Complexation-Based Method,” by Aboulghasem K.N. Korrani, SPE, and Gary R. Jerauld, SPE, BP, prepared for the 2018 SPE Improved Oil Recovery Conference, Tulsa, 14–18 April. The paper has not been peer reviewed. Previously proposed models of wettability change have not been tied to the chemistry that controls wettability but instead were driven by simplistic criteria such as salinity level or concentration of an adsorbed species. In this paper, after testing proposed models in the literature on sandstones and carbonates, the authors propose a mechanistic surface-complexation-based model that describes observations quantitatively for ionically treated waterfloods. To the best of the authors’ knowledge, this is the first surface-complexation-based model that describes fully ionic compositional dependence observed in ionically treated waterfloods in both sandstones and carbonates. Introduction While some debate remains about the underlying mechanisms of ionically tuned waterflooding, the geochemical reactions that control the wetting of crude oil on the rock surface are likely to be central to a detailed description of the process. Models of wettability change often have been simplistic and not tied to the chemistry that controls wettability, instead driven by a simplistic criterion such as salinity level or concentration of an adsorbed species. Such models are inadequate for modeling the effect of compositional changes in brine, which is key to optimizing ionic design. One problem has been the lack of reservoir models that included geochemistry. The oilfield reactive-transport simulators are simplistic; in these, either aqueous reactions are assumed to be ideal (i.e., with aqueous activity coefficients of unity) or they lack important geochemical features such as kinetics or surface-complexation reactions. In a previous work, the authors assumed that decrease in the total ionic strength is the driving mechanism for wettability change during low-salinity waterflooding. The reason was that the total ionic strength is the controlling parameter for the double-layer expansion. Double-layer thickness increases as the brine total ionic strength decreases. The thickness of the double layer is an inverse function of the square root of the total ionic strength. However, the proposed model was too simple and was unable to predict many low-salinity or modified-salinity waterflooding observations, such as the detrimental effect of divalents on oil recovery in sandstones, the positive effect of sulfate in chalks, the positive effect of high pH in sand-stones, and the detrimental effect of sodium chloride in chalks. In this study, the authors identified and tested an alternative approach based on a stability analysis of thin water film, which yields a dimensionless group, a ratio of electrostatic-to-Van der Waals (VdW) forces called the stability number. The stability number can be used as a measure of wettability. The authors use simulation software to model surface reactions that can estimate surface charge and potential from surface-complexation models, demonstrating that this process can be used in a model to match a wide variety of data in sandstones and carbonates found in models in the literature. The complete paper provides details of these comparisons.

Ice Melting with Allowance for Selective Absorption in the Medium

Melting of an ice layer on a vertical substrate heated by long-wave radiation is numerically simulated in a one-phase formulation of the Stefan problem. A simple model is proposed to take into account the presence of a thin water film on the irradiated surface. Ice is considered as a selectively absorbing material with two absorption bands. The calculated results are in reasonable agreement with the experiment.

Crystallization kinetics of thin water films on Pt(111): effects of oxygen and carbon-monoxide adspecies.

This paper describes nucleation, epitaxial growth, and wettability of water on Pt(111) and how they are influenced by oxygen and carbon-monoxide adspecies, based on reflection high energy electron diffraction (RHEED), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and temperature-programmed desorption (TPD). Amorphous solid water deposited onto the pristine Pt(111) substrate crystallizes into ice Ih together with a 2D layer at 150 K, whereas ice Ic (stacking disordered ice or a mixture of ice Ic and Ih) is formed preferentially onto oxygenated Pt(111) (CO-adsorbed Pt(111)) at 155-160 K (150 K). The ice nucleation and epitaxial growth tend to be hampered on the oxygenated Pt(111) surface via hydrogen bond formation with chemisorbed oxygen. The CO-adsorbed Pt(111) surface is hydrophobic, as evidenced by the fact that water forms a complex with CO during evaporation of crystallites at 160-165 K. A disordered 2D layer remains on pristine Pt(111) up to 175 K, whereas an ordered 2D layer exhibiting the (√3 ×√3)R30° structure formed on oxygenated Pt(111) up to 200 K.

Modelling Low-Salinity Oil Recovery Mechanisms Using an Ion Dissociation Model

Experimental evidence shows that injecting low-salinity water during the oil recovery process can lead to an increase in the amount of oil recovered. Numerous mechanisms have been proposed to explain this effect, and, in recent years, two which have gained notable support are multicomponent ionic exchange (MIE) and pH increase. Both mechanisms involve ion exchange reactions within the thin film of water separating the oil in a reservoir from the clay minerals on the surface of the reservoir rock. Since the reactions occur on the molecular scale, an upscaled model is required in order to accurately determine the dominant mechanism using centimetre-scale experiments. In this paper, we develop the first stages of this upscaling process by modelling the pore-scale motion of an oil slug through a clay pore throat. We use a law-of-mass-action approach to model the exchange reactions occurring on the oil–water and clay–water interfaces in order to derive expressions for the surface charges as functions of the salinity. By balancing the disjoining pressure in the water film with the capillary pressure across the oil–water interface, we derive an expression for the salinity-dependent film thickness. We compare the two mechanisms by modifying an existing model for the velocity of an oil slug through a pore throat. Numerical results show that the velocity increases as the salinity decreases. The percentage increase is larger for the MIE mechanism, suggesting that MIE may be the dominant causal mechanism; however, this will vary depending on the particular clay and oil being studied.

Modeling wettability change in sandstones and carbonates using a surface-complexation-based method

After testing proposed models in the literature on sandstones and carbonates, we propose a mechanistic surface-complexation-based model that quantitatively describes observations for ionically treated waterfloods. We model wettability change by directly linking wettability to brine chemistry using detailed colloidal science. Brine has charged ions that interact with polar acidic/basic components at the oil-water interface and rock surface and therefore oil/brine and rock/brine interfaces are charged and exert both Van der Waals and electrostatic forces on each other. If the net result of the forces is repulsive, the thin water film between the two interfaces is stable (i.e., the rock is water-wet) otherwise, the thin water film is unstable and the rock becomes oil-wet. We describe a ratio of electrostatic force to Van der Waals force with a dimensionless group, called “wetting film stability number,” where rock wettability is water-wet for values greater than one and oil-wet for values less than one. For sandstones, the zeta potentials of oil/brine and rock/brine interfaces become more negative/less positive by diluting or softening the brine and/or increasing pH. Similarly, for carbonates, dilution and/or sulfate enrichment of brine makes surface potentials more negative. Such brine modification can therefore be used to improve oil recovery. We implemented the improved wettability change model in a comprehensive coupled reservoir simulator, UTCOMP-IPhreeqc, in which oil/brine and rock/brine zeta potentials are modelled using the IPhreeqc surface complexation module. Surface potentials obtained from the geochemical model are used to calculate the dimensionless group controlling wettability change, which is dynamically modelled in the transport simulator. The model is validated in sandstones and carbonates by simulating an inter-well test, and several corefloods and imbibition tests reported in the literature. For sandstones, we model Kozaki (2012) and BP's Endicott trial. For simple dilution in carbonates we model experiments by Shehata et al. (2014) and Yousef et al. (2010). For enrichment with sulfate we model Zhang and Austad (2006) and for increasing total ionic strength via sodium chloride enrichment, Fathi et al. (2010a).

Wettability of Bare and Graphene-Adsorbed Pt(111) during Glass–Liquid Transition, Crystallization, and Premelting of Water

Properties of thin water films on Pt(111) with and without monolayer graphene (MLG) in between were investigated using reflection high-energy electron diffraction, time-of-flight secondary-ion mass...

Nanosecond laser ablation of Ag–Au films in water for fabrication of nanostructures with tunable optical properties

In this study, we present results on nanosecond laser ablation of Ag–Au bimetallic thin films in double-distilled water. A standard on-axis pulsed laser deposition technology is used for the thin-film deposition. The targets used in the deposition stage were composed of two sections, each containing Au or Ag. Varying the area of the sections of the target, thin bimetallic films with different ratio of Ag:Au (72:28, 56:44, 50:50, and 34:66) were obtained. As-deposited thin films were immersed in double-distilled water and undergo irradiation with nanosecond laser pulses. The optical absorption spectra of the obtained bimetallic colloids show a single-surface plasmon resonance (SPR) band, positioned between SPR bands of the monometallic Ag and Au nanoparticles (NPs), proving the formation of Ag–Au alloyed NPs. A linear dependence of the SPR maximum of the resulting colloidal NPs on the metal concentration of the deposited thin films was established. The influence of the laser wavelength (the four harmonics of Nd:YAG laser), laser fluence and film thickness on the morphology, particle-size distribution, and optical properties of the obtained colloidal Ag–Au NPs was also studied. TEM analyses of the dried drops of the colloids reveal mainly two types of shapes of the produced nanostructures depending on the processing conditions: spherical NPs and network of nanowires. A characteristics size of the formed nanostructures of about a few nanometers was estimated in the cases of ablation at all the wavelengths used. However, a broader size distribution and presence of larger amount of bigger particles (diameters up to 50 nm) were observed in the case of ablation at 1064 nm. The laser fluence and film thickness are also found to influence the nanostructure size and morphology. The proposed technique can be a basis for the fabrication of complex nanostructure colloids with application in biophotonics, sensor design, and catalysis.

Solid-solid hydration and dehydration of Mars-relevant chlorine salts: Implications for Gale Crater and RSL locations

Water vapor is likely being exchanged between the regolith and the atmosphere on Mars, according to evidence from multiple landing sites (including Gale Crater), satellite measurements and numerical modeling. The mechanism of this exchange is largely unknown but could involve the formation of water frost, the adsorption of thin films of water onto mineral surfaces, or the deliquescence or hydration of salts. Hydration, a solid-solid phase transition during which water molecules are incorporated into a crystal structure in stoichiometric amounts, is possible for many salts found at Gale Crater and elsewhere on Mars. These salts may therefore be acting as a source and sink for water vapor in the Martian regolith. Furthermore, salt hydration state may be used as a marker for the presence of liquid water on present-day Mars. For example, the hydrated, crystalline perchlorate and chloride salts detected at recurring slope lineae (RSL) locations have been proposed to form from liquid brines rather than by experiencing hydration by atmospheric water vapor. Here we use an environmental cell coupled to a Raman microscope to experimentally study the hydration and dehydration of magnesium chloride (MgCl2), calcium perchlorate (Ca(ClO4)2), and calcium chloride (CaCl2) with the goal of determining which of these salts are capable of experiencing hydration on diurnal time scales. Other potential hydration phase transitions of chlorine-containing salts are thought to be less likely. Specifically, we study the transition between magnesium chloride tetrahydrate and hexahydrate, anhydrous calcium perchlorate and hydrated calcium perchlorate, and calcium chloride dihydrate and hexahydrate. We find that under conditions measured by the REMS instrument at Gale Crater, some chlorine-containing salts can readily hydrate and other salts can readily dehydrate, but no salt system studied here is likely to undergo both processes at the surface on diurnal time scales. With respect to RSL formation, these experiments suggest that atmospheric hydration of these salt systems may be too slow or otherwise not feasible. Hence, hydrated salts formed recently at active RSL locations on Mars may indeed be an indicator of the presence of liquid water.

Simulation of ice melting in a single-phase statement of the Stefan problem

The problem of radiation-conductive heating and subsequent melting of ice in a climatic chamber in a single-phase approximation of the Stefan problem with allowance for the emerging thin water film on the irradiated surface was solved by mathematical modeling methods. Fields of temperature and flux density of the resulting radiation are obtained, as well as the rate of melting and heating of the non-irradiated ice surface. Comparison with experiment showed satisfactory agreement.

On the Relation between the Continuum and Molecular Dynamic Descriptions of the Explosive Boiling of a Liquid Film on Heated Substrate

A continuum description of the explosive boiling of a thin water film on a heated gold substrate is proposed based on previous molecular dynamics calculations. It is shown that the continuum model in use provides under certain conditions agreement (with an error of about 10%) with the molecular dynamics results on the heating rate of a film contacting a substrate and the dynamics of film removal after explosive boiling.

Experimental study on wet skid resistance at different water-film thicknesses

PurposeDriving safety on a wet road is closely related to the wet skid resistance of tires. The purpose of this paper is to reveal the evolution of wet skid resistance at different water film thicknesses and provide some guidance on the design of a tread pattern with improved traction on rainy roads.Design/methodology/approachBrake tests are performed in a laboratory with a viscoelastic tribotester at various water film thicknesses. The initial water film thickness is 3 mm, which decreases with an increase in the test number. Brake friction force is dynamically measured at water film thicknesses ranging from 0 mm to 3 mm.FindingsThe results show that water film thickness exerts a great influence on the forms of tire motion and slip ratio. The tire is much easier to slide on the road with thick water film and also with a considerably thin water film (about 0-1 mm) during a sharp braking process. The brake traction can be very low under this road condition despite the apparently safe quality of the road.Originality/valueThe authors design and establish a new viscoelastic tribotester which is used to simulate the real braking sliding process and study the tribological properties between tire rubber and road surface. The variation in the wet friction coefficient and slip ratio at different water film thicknesses have a great influence on the design of a tread pattern with improved traction on rainy roads.

Ultrafast water harvesting and transport in hierarchical microchannels.

Various natural materials have hierarchical microscale and nanoscale structures that allow for directional water transport. Here we report an ultrafast water transport process in the surface of a Sarracenia trichome, whose transport velocity is about three orders of magnitude faster than those measured in cactus spine and spider silk. The high velocity of water transport is attributed to the unique hierarchical microchannel organization of the trichome. Two types of ribs with different height regularly distribute around the trichome cone, where two neighbouring high ribs form a large channel that contains 1-5 low ribs that define smaller base channels. This results in two successive but distinct modes of water transport. Initially, a rapid thin film of water is formed inside the base channels (Mode I), which is followed by ultrafast water sliding on top of that thin film (Mode II). This two-step ultrafast water transport mechanism is modelled and experimentally tested in bio-inspired microchannels, which demonstrates the potential of this hierarchal design for microfluidic applications.

Laser-Ablated Vortex Fluidic-Mediated Synthesis of Superparamagnetic Magnetite Nanoparticles in Water Under Flow.

Selective formation of only one iron oxide phase is a major challenge in conventional laser ablation process, as is scaling up the process. Herein, superparamagnetic single-phase magnetite nanoparticles of hexagonal and spheroidal-shape, with an average size of ca. 15 nm, are generated by laser ablation of bulk iron metal at 1064 nm in a vortex fluidic device (VFD). This is a one-step continuous flow process, in air at ambient pressure, with in situ uptake of the nanoparticles in the dynamic thin film of water in the VFD. The process minimizes the generation of waste by avoiding the need for any chemicals or surfactants and avoids time-consuming purification steps in reducing any negative impact of the processing on the environment.

The exact regulation of temperature evolutions for droplet impact on ultrathin cold films at superhydrophilic surface

The present study reports a study of water droplets impacting on cold thin water films on a superhydrophilic surface. A thermal infrared imager was used to record the surface temperature distribution after the droplet impact on the liquid films. A ring-shaped high temperature zone was found after impact with the temperature first increasing and then gradually decreasing in the radial direction. Numerical simulations were then used to study the velocity distribution and the droplet motion inside the liquid film. The droplet spreading motion was restricted by liquid in the film for lower We which then reduces the heat transfer in that area and causes the formation of a ring-shaped high temperature distribution. The ring structure shape then changes with the increasing of We. This hot ring becomes wider and the temperature difference between the ring and the impact center decrease with the increasing of We. As We continues to increase, the ring-shaped temperature distribution disappears with the highest temperature at the center and the temperature in radial direction monotonically decreasing from the center to the edge. The initial film thickness also affects the temperature distribution after droplet impact. A thicker film causes the ring-shaped hot region to gradually move inward until it reaches the center and forms a hottest region in the impact center. Thus, the ring-shaped temperature distribution only occurs for lower We and thinner films. The shape and position of the temperature distribution after droplet impact on the cold, thin liquid films can then be precisely controlled by regulating the impact We and the initial film thickness. These results will greatly facilitate the design of precision spraying processes.

Numerical simulation of air–steam mixture condensation flows in a vertical tube

In this study, a fully compressible three-phase numerical model for steam condensation flow in the presence of air is presented. The model solves the Reynolds-averaged Navier–Stokes equation coupled with two interface advection equations. The Lee condensation model was applied to simulate the mass and energy transfer processes via interfaces. Several test cases were simulated for the air–steam mixture condensation flow in a vertical tube with different air mass fractions to verify the numerical model. The validity was demonstrated through comparisons of the centerline temperatures along the tube and experimental data; the results were in good agreement with experimental data. A detailed radial profile of the temperature, species mass fraction, and velocity was analyzed. A developing thin condensate water film along the tube and the thickest film layer at the outlet tube wall of approximately 4.0 mm were observed. Air accumulation at the steam–water interface was also represented. In addition, the effects of air mass fraction on the characteristics of the condensation rate were investigated.

MARMIT: A multilayer radiative transfer model of soil reflectance to estimate surface soil moisture content in the solar domain (400–2500 nm)

Surface soil moisture content (SMC) is known to impact soil reflectance at all wavelengths of the solar spectrum. As a consequence, many semi-empirical methods aim at inferring SMC from soil reflectance, but very few rely on physically-based models. This article presents a multilayer radiative transfer model of soil reflectance called MARMIT (multilayer radiative transfer model of soil reflectance) as a function of SMC given on a mass basis and a method called MARMITforSMC to estimate it from soil reflectance spectra. This model depicts a wet soil as a dry soil covered with a thin film of water. It is used to assess SMC over seven independent laboratory datasets gathered from the literature. A learning phase is required to link the thickness of the water film with the SMC. For that purpose, a sigmoid function, the parameters of which are related to soil physical and chemical properties such as porosity, grain size and mineralogy composition, is fitted. SMC can be inferred with good accuracy (RMSE ≈ 3%) if the learning step is applied soil by soil. The link between SMC and water thickness actually depends on soil texture and chemical composition. If the soils are divided into classes and if the learning phase is applied to a class, the RMSE slightly increases up to 5%. Finally, MARMITforSMC provides lower RMSE than any other existing semi-empirical or physically-based method.

A three-dimensional mathematical model for simulating ice accretion on helicopter rotors

A new three-dimensional mathematical model is proposed to predict the shape of accreted ice on helicopter rotors. The model assumes that ice accretion on a rotor’s surface occurs by flow and solidification of a thin water film on its surface. The model is developed by analyzing conservation of mass, momentum, and energy in the thin water film on a rotating curved surface. The model is expressed in rotating body-fitted nonorthogonal curvilinear coordinates, and the Coriolis-force term in the momentum equations is neglected because its effect is relatively small compared with that of the centrifugal force. The computational method is presented to solve the model, and the model is validated by comparing its results with published experimental data. The results show that the proposed model accurately predicts both the water-film thickness and the ice-layer thickness on a rotating surface. Additionally, the model has been used to study how the rotor angular speed and MVD (Median Volumetric Diameter) of the droplets affect ice accretion. The results show that an increase in both angular speed and MVD can lead to a thicker layer of ice, while the effect of MVD on ice accretion near the stagnation point becomes unobvious when MVD is larger enough.

Effect of Environment Variables and Material Properties on the Limiting Current Density for Oxygen Reduction

For aircraft alloys that have received surface treatments, primers, and topcoats to control self-corrosion due to precipitate phases, high-strength fasteners made of stainless steel or titanium alloys are the most obvious point of attack due to galvanic mismatches between the materials. However, the electrolytes that form from atmospheric processes can differ widely in composition and thickness. Dissolved oxygen concentrations vary due to temperature and salinity changes in the electrolyte. Recent attempts to model atmospheric corrosion have suggested that, at high salt loadings on the surface, electrolytes can form that have water layer thicknesses that appear as bulk electrolytes in terms of oxygen transport1. Experiments using rotating disk electrode (RDE) measurements and polarization curve data,2 have suggested an upper bound of approximately 800 mm on the diffusion layer thickness for oxygen that can be expected to form in a thin film of water. At distances beyond this thickness, convective flow is expected to dominate oxygen transport. In this report, we compare the results of RDE experiments that simulate thin film behavior on different substrate materials in different electrolytes with polarization data obtained under actual electrolyte thin films equilibrated at different relative humidity values, and bulk polarization measurements, as shown in Figure 1, in order to determine the effects of ohmic drop on the measurements and isolate relevant kinetic parameters. Isolating these effects is important in order to accurately model the corrosion currents arising from galvanic couples in which the oxides at which the reduction reactions are occurring can be changing in response to the changes in the electrolyte as the environment changes. Figure 1. Bulk cathodic polarization curves in 0.6M NaCl at 25◦C for Pt, UNS S31600, and UNS R56400. This work was sponsored by the Office of Naval Research, ONR, under grant/contract number N0001418WX00826; the views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy or the U.S. government. References 1 Van den Steen, N., Simillion, H., Dolgikh, O., Terryn, H. & Deconinck, J. An integrated modeling approach for atmospheric corrosion in presence of a varying electrolyte film. Electrochimica Acta 187, 714-723, doi:10.1016/j.electacta.2015.11.010 (2016). 2 Liu, C., Srinivasan, J. & Kelly, R. G. Electrolyte Film Thickness Effects on the Cathodic Current Availability in a Galvanic Couple. J Electrochem Soc 164, C845-C855, doi:10.1149/2.1641713jes (2017). Figure 1

(Invited) Potential of Double Network Gel as a Tribological Material Realizing Low Friction in Water

Introduction Soft matter enables stress relaxation at contact point and hydrogel, one of soft matter which consists of gel network and water, is expected as sliding material in water due to its high water content since hydrophilic surface is known to form water film between sliding parts and thus realizes low friction in water. Especially, double network (DN) gel is highlighted since mechanical strength of DN gel is tough, realized by combination of hard network and soft network, compared to other hydrogel gel such as PVA gel. In this study, effect of properties of counter material, such as surface roughness or wettability, is examined to its potential of practical usage. Also, friction property of DN gel against silicon carbide (SiC) in water is examined under varied sliding condition such as sliding speed and load. Specimen and experimental setup DN gel sheet is fixed and slid against SiC disk rotated by servo motor. Friction experiments were carried out in water and friction force is measured by load cell and thus friction coefficient was calculated with applied load. Friction tests using PVA gel sheet, its tensile strength is lower by 1 digit than that of DN gel, were also carried out to investigate effect of tensile strength of gel material on friction property. All experimental procedures were done at room temperature. Effect of wettability of counter surface on friction property Varied wettability of sliding counterpart indicates that hydrophilic disk slid against DN gel realizes lower friction coefficient around 0.01 while hydrophobic disk increases friction coefficient up to 0.34. Roughened DN gel surface was fabricated by means of laser cutter and textured DN gel realized lower friction coefficient around 0.01 even disk is hydrophobic. These results indicate that maintenance of water between DN gel and sliding counterpart is crucial at practical usage to realize low friction in water. Generation of low friction by formation of damaged layer Varied sliding speed and load exhibits that friction coefficient of DN gel/SiC in water shows running-in behavior which consists of twice decrease, then finally induces low friction coefficient less than 0.001 under medium sliding severity between boundary lubrication and hydrodynamic lubrication. On the other hand, PVA gel does not show such running-in behavior within examined sliding condition and severe wear occurs after sliding test. In-situ friction apparatus revealed that DN gel surface is roughened during 1st decrease and 2nd decrease starts as soon as roughening process is finished. Also, when green DN gel and red water, DN gel surface become darker during 2nd decrease, indicating that water between DN gel and sliding counterpart intrude in DN gel surface during 2nd decrease. Furthermore, cross section of DN gel indicated that moist layer with thickness of several-tens mm is formed on DN gel surface by friction. It is known that applied tensile stress causes destruction and elongation of each network to the depth of 100 mm approximately, called as damaged region, on DN gel surface. Also, mechanical property of damaged region is softer and sparser compared to original DN gel. Therefore it is estimated that damaged region is formed beneath DN gel surface during 1st decrease. Also, water between DN gel and sliding counter material penetrated into damaged region on DN gel surface and formed thin water film enables generation of lower friction coefficient than 0.001. It is understood that higher tensile strength of DN gel maintained damaged region formed beneath DN gel surface and kept damaged region played a role as water reservoir to form water film when DN gel experiences higher contact pressure, as a result, adhesion between DN gel and sliding counterpart is prohibited and low friction coefficient is realized. Conclusions Friction tests using DN gel sheet and SiC disk were carried out in water and following conclusions were achieved. Roughened DN gel decreases friction coefficient around 0.01 with hydrophobic disk where non-textured DN gel shows higher friction coefficient around 0.04.DN gel/SiC shows running in behavior which consists of decrease of friction coefficient twice in water, then friction coefficient exhibits lower than 0.001.Damaged region formed beneath DN gel surface during 1st decrease and water between DN gel and sliding counterpart penetrates into damaged region.

Active particle aggregate on complex bubble surfaces

Recently, colloids have been shown to form complex structures on bubble surfaces on demand. With the help of a high power pulse laser shining on a thin water film, water bubbles can be formed and heat unbalance creates a convective flow, which carries colloids on the surface of these water bubbles to form aggregates. Here, active particles are studied in a similar setup and conditions are laid out to form aggregates on water bubble surfaces. The effect of motility and chirality of active particles on formation of aggregate are discussed. The simulation results obtained here will hopefully help the experimental endeavors in future.