Boundary Slip of Superoleophilic, Oleophobic, and Superoleophobic

Article pubs.acs.org/Langmuir

Boundary Slip of Superoleophilic, Oleophobic, and Superoleophobic Surfaces Immersed in Deionized Water, Hexadecane, and Ethylene Glycol Dalei Jing†,‡ and Bharat Bhushan*,†,‡ †

School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, People's Republic of China Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics (NLB2), The Ohio State University, 201 West 19th Avenue, Columbus, Ohio 43210-1142, United States

‡

ABSTRACT: 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.

1. INTRODUCTION The reduction of fluid drag in micro/nanofluidic systems widely used in chemical, biology, and medical fields is of interest.1,2 The boundary slip condition can affect the fluid drag in micro/ nanofluidic systems. The existence of the boundary slip condition implies that the velocity of the fluid near the solid wall is not zero and is characterized by slip length.3,4 The measured slip lengths on various surfaces range from several nanometers to hundreds of micrometers.5−8 For the boundary slip on various solid−liquid interfaces, studies have been widely carried out. Table 1 provides a representative summary of slip length measurement results on different solid−liquid interfaces. For the boundary slip in deionized (DI) water, Wang et al.9 studied the boundary slip on samples with different degrees of hydrophobicity using an atomic force microscope (AFM). They reported a no-slip boundary condition on the hydrophilic sample, whereas about 44 and 257 nm of slip lengths were reported on the hydrophobic and superhydrophobic samples, respectively. Bhushan et al.10 reported a no-slip boundary condition on a hydrophobic polystyrene (PS) sample immersed in DI water. The slip length on a smooth selfassembled monolayer (SAM) of an octadecyltrichlorosilane (OTS) sample was measured to study the effect of surface roughness, and a slip length of 28 nm was obtained, which suggests that a smoother surface results in a larger slip length.11 They also © 2013 American Chemical Society

found that the slip length decreases with the increasing magnitude of surface charge deposited on the OTS sample.11 Oils are widely used in many fluid flow applications, so the study of boundary slip on samples immersed in oils is important to reduce the fluid drag. For the boundary slip in oil, studies have focused on superoleophilic and oleophilic surfaces. Cho et al.12 studied the boundary slip of a number of liquids on a hexadecyltrichlorosilane surface. They found that for nonpolar liquids with small contact angles from 13.5° to 39.4°, the slip length increased with increasing contact angle. However, for polar liquids with contact angles from 61.8° to 97.5°, the slip length did not depend on contact angle. Instead, it was controlled by the dipole−dipole interaction (electrostatic interactions of permanent dipoles in molecules); the increasing dipole−dipole interaction makes it more difficult to shear the liquid molecules adjacent to the solid−liquid interface, which decreases the slip length.12 Schmatko et al.13 measured the slip lengths with hexadecane and squalane, which have the same surface tension, on bare sapphire with a contact angle of about 0°, on SiH with a contact angle of about 20°, and on OTS with a contact angle of about 40°. They found that the slip length was lower for squalane Received: August 13, 2013 Revised: October 25, 2013 Published: October 30, 2013 14691

dx.doi.org/10.1021/la4030876 | Langmuir 2013, 29, 14691−14700

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Table 1. Summary of Contact Angle and Slip Length Measurements on Different Solid−Liquid Interfaces substrate mica (RMS ∼ 0.2 nm) n-hexatriacontane and lotus wax (RMS ∼ 11 nm) n-hexatriacontane and lotus wax (RMS ∼ 178 nm) polystyrene (RMS ∼ 0.3 nm) octadecyltrichlorosilane (RMS ∼ 0.1 nm) hexadecyltrichlorosilane (Ra ∼ 0.3 nm)

sapphire α-Al2O3{0001} (Ra ∼ 0.4 nm) SiH (Ra ∼ 0.4 nm) octadecyltrichlorosilane (Ra ∼ 0.3 nm) n-hexadecyltrichlorosilane (Ra ∼ 0.3 nm) chlorotrimethylsilane (Ra ∼ 0.3 nm)

liquid and contact angle (deg)

surface tension of liquid (mN/m)

slip length measurement technique

slip length (nm)

DI water, 0 DI water, 91 DI water, 167

72.8

AFM9

0 44 ± 10 257 ± 22

DI water, 94 DI water, 106 nonpolar, n-alkanes octane, 13.5 dodecane, 32 tridecane, 35.3 tetradecane, 37 pentadecane, 39.1 n-hexadecane, 39.4 nonpolar, ring shape cyclohexane, 25.5 benzene, 32.5 polar aniline, 64.1 water, 97.5 benzaldehyde, 61.8 nitrobenzene, 63.2 2-nitroanisole, 69.8 n-hexadecane, 0 squalane, 0 n-hexadecane, 20 n-hexadecane, 40 squalane, 40 n-alkanes, 0−40 n-alcohols, 30−50 n-pentane, 11 n-hexane, 11 n-hexadecane, 11

72.8 72.8

AFM10 AFM11 AFM12

∼0 28 ± 5

21.1 24.9 25.6 26.1 26.6 27.5

0 0 10 15 10 20

24.7 28.2

10 50

42.1 72.8 38 43.5 45.7 27.5 27.4 27.5 27.5 27.4 20.1−27.5 22.1−29.9 15.8 18.4 27.5

50 30 20 10 0 110 ± 50 40 ± 50 230 ± 50 350 ± 50 180 ± 50 5−20 0−40 15 ± 8 13 ± 3 −5 ± 3

near field laser velocimetry13

AFM14 AFM15

references. Two organics of scientific interest with different surface tensions and viscosities, hexadecane and ethylene glycol, were selected. In addition, DI water was used as a reference. The electrostatic forces of the samples immersed in DI water, hexadecane, and ethylene glycol were measured. The surface charge density of the samples immersed in DI water was quantified. The slip lengths on superoleophilic, oleophobic, and superoleophobic surfaces immersed in DI water, hexadecane, and ethylene glycol were measured and analyzed. The relevant mechanisms are discussed in this paper.

compared to that of hexadecane because of the different molecular conformations of the liquids, and that the slip length increased with the increasing contact angle for both of the liquids. McBride and Law14 measured the slip length of 18 Newtonian liquids from two homologous series, the n-alkanes and n-alcohols, on smooth n-hexadecyltrichlorosilane coated samples with a contact angle range from 0° to 50°. They found 5−20 nm slip lengths for the n-alkanes and 0−40 nm slip lengths for the n-alcohols. Moreover, they found that slip length increased with the increasing contact angle and a linear dependence of log (slip length) on log (viscosity).14 Bowles et al.15 measured the slip length on a chlorotrimethysilane coated surface with n-pentane, n-hexane, and n-hexadecane. They found a slip length of 15 ± 8 nm for n-pentane, a slip length of 13 ± 3 nm for n-hexane, and a no-slip condition for n-hexadecane, and an activation model for sheardriven molecular motion was proposed to explain the results.15 Although oil slip has been studied, to date studies have focused on superoleophilic and oleophilic surfaces, but there is no data on oleophobic and superoleophobic surfaces. In this paper, the boundary slip on superoleophilic, oleophobic, and superoleophobic surfaces was studied. PS, which is widely used in biological and medical applications,1,16 is a hydrophobic surface that has been used to study the boundary slip.10,17 OTS is a commonly used organosilane to prepare a smooth hydrophobic SAM with a lower surface roughness compared to the PS and has been used to study the boundary slip.11,18 Because of the wide application of PS and OTS, these two surfaces were used as

2. EXPERIMENTAL SECTION The samples prepared included one superoleophilic sample, one oleophobic sample, and one superoleophobic sample. All three samples were found to be superhydrophobic. In addition, two hydrophobic samples, PS and OTS, with different surface roughness were prepared. Two organics of scientific interest, hexadecane and ethylene glycol, were selected. DI water was used as a reference. The surface tension and viscosity of the liquids used in this paper at room temperature are shown in Table 2.19 These properties of the liquids can affect the boundary slip condition. From the data presented in Table 2, DI water has the largest

Table 2. Parameters of Liquids Used in the Experiments19

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liquid

surface tension (mN/m)

dynamic viscosity (mPa s)

DI water hexadecane ethylene glycol

72.8 27.5 47.7

0.98 3.03 16.1


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superhydrophobicity and good adhesion of nanoparticles on the substrate. After sonication, an additional 10 mL of fresh 40%/60% (v/v) THF/IPA solvent was added to the above mixture. Then, the coating was prepared at a speed of 10 cm/min using the dip coating technique. After coating, the sample was put in an oven to heat at about 40 °C for 10 min to cure the binder.22 To prepare the superoleophobic samples, 300 mg of SiO 2 nanoparticles were dispersed in a 98%/2% (w/w) acetone/formic acid solvent with a concentration of 32 mg/mL, and then sonicated for 4 min using a Branson Sonifier 450A with a frequency of 20 kHz at 35% amplitude. Then, 2200 mg of fluorinated acrylic copolymer (FAC) binder (Dupont) (23% w/w in DI water) was dispersed in the solution and sonicated for 4 min. After mixture, the nanoparticle to binder ratio was 0.6 by weight, and the nanoparticle and binder concentration in the solution was 8% by weight. Next, a sieve (070054233, Giada de laurentiis) was used to filter the solution. After filtering, the solution was sprayed on the glass substrate at a distance of about 15 cm using a spray gun (VL-SET, Paasche Airbrush Company) at a pressure of 207 kPa. Finally, the coated sample was dried at 70 ± 3 °C and 45−55% RH for 30 min. To prepare the oleophobic sample, the same materials and process as that of the superoleophobic sample were used, and the concentration of the particles and binder in the solution was kept at approximately 8% by weight, but the ratio of the nanoparticles to the binder was reduced from 0.6 to 0.3. 2.2. Characterization of the Samples. 2.2.1. CA and CAH Measurements. The CA and CAH measurements were carried out based on the methods by Ebert and Bhushan.22 A liquid droplet of 5 μL volume was first deposited onto the samples using a microsyringe. Then, the image of the droplet was captured using a model 290-F4 Ramé-Hart goniometer (Ramé-Hart Inc., Succasunna, NJ), and the DROP image software was used to analyze the image of the droplet and measure the CA. CAH is the difference between the advancing contact angle and the receding contact angle. The tilting plate method was used to measure the advancing contact angle and receding contact, which was obtained from the leading and trailing edges of the droplet on the tilt base of the goniometer, respectively. Reproducibility of all CA/CAH data was reported as (±σ), obtained from measurements. 2.2.2. Morphology of the Samples in Air. The morphology of the five samples in air was measured using a D3000 AFM with a Nanoscope IV controller (Bruker Instruments, Santa Barbara, CA) in tapping mode. The AFM tip used was a rectangular N-type Si tip (Appnano) with a stiffness of 3 N/m and a resonance frequency of 76 kHz in air, and the backside of the tip was coated with Al. To image the samples, the AFM tip was driven to oscillate with a frequency close to the natural resonance frequency. A scan rate of 0.5 Hz and a set point of 90% of the free amplitude were chosen. 2.2.3. Electrostatic Force and Hydrodynamic Force Measurements. The colloidal probe AFM technique in contact mode was used for electrostatic force and hydrodynamic force measurements.8 The colloidal AFM probe was prepared by gluing a borosilicate sphere obtained from MO-Sci Corporation (GL018B/45-33) with a diameter of 58.9 μm to the end of an ORC8 rectangular AFM cantilever (Bruker) using epoxy (Araldite, Bostik, Coubert). The advantage of using a sphere with a large diameter was that the separation distance between the AFM cantilever and sample surface was large so the hydrodynamic force, which was related to the separation distance, exerted on the cantilever was small and could be neglected.23 The force obtained from the deflection of the AFM probe consists of van der Waals force (which was assumed to be small and neglected in this paper), electrostatic force, and hydrodynamic force. According to the theoretical work of Vinogradova,23 the hydrodynamic force exerted on the sphere is proportional to the approaching velocity between the sphere and the sample. When the velocity is low, the hydrodynamic force is small enough to be neglected and the measured force was assumed to be the electrostatic force. The lower sphere velocity used in our experiment to obtain the electrostatic force data was 0.22 μm/s. To obtain the hydrodynamic force, a higher sphere velocity of 38.5 μm/s was chosen. It should be noted that the force data obtained at the higher velocity included both the electrostatic part and the hydrodynamic part,

Table 3. Properties of Materials and Parameters Used in This Paper properties

symbol

value

permittivity of vacuum (F/m)19 dielectric constant of DI water19 elementary charge (C)19 Boltzmann constant (J/K)19 chemical valence of ions in DI water parameters used ionic concentration of DI water (M) radius of borosilicate sphere (μm) temperature (K)

ε0 ε e kb z

8.85 × 10−12 78 1.6 × 10−19 1.38 × 10−23 1

n0 R T

1 × 10−6 29.45 298

surface tension and hexadecane has the smallest, and DI water has the smallest dynamic viscosity and ethylene glycol has the largest. The contact angle (CA) and contact angle hysteresis (CAH) were measured on the samples with a droplet of DI water, of hexadecane, and of ethylene glycol. An AFM was used to measure the morphology of the five samples in air and the electrostatic and hydrodynamic forces of the samples immersed in DI water, hexadecane, and ethylene glycol. The surface charge densities of the five samples immersed in DI water were quantified, and the slip lengths on the samples immersed in the DI water, hexadecane, and ethylene glycol were obtained. 2.1. Preparation of Samples. 2.1.1. Hydrophobic Samples. A substrate of silicon wafer obtained from Silicon Quest International was used to prepare the hydrophobic samples. The wafer surface consisted of a SiO2 coating with a thickness of 300 nm, thermally grown. Before the preparation of the samples, the substrate was first immersed in piranha solution (1:3 mixture of 30% H2O2 and 98% H2SO4) for 30 min. Caution: Piranha solution reacts violently with organic matter and should be handled with extreme care! The substrate was then rinsed using DI water and ethanol for three times, and finally dried using compressed air. After being cleaned, PS coating and OTS coating were applied to the Si wafer. The methods used to prepare the PS and OTS samples were the same as previously reported by Bhushan and Pan20 and Bhushan et al.10 To prepare the PS sample, PS pellets with a molecular weight of 35 000 obtained from Sigma Aldrich, were completely dissolved in toluene (Mallinckrodt Chemical) to make PS solution with a concentration of 1% (w/w). Then, the PS solution was coated on the substrate at a speed of 2000 rpm using spin coating method to produce a thin coating. Then, the sample was put in an oven to heat at 53 ± 2 °C for 4 h.20 For the OTS sample, the OTS obtained from Gelest (SIO6640.1) was added into anhydrous toluene to mix and form OTS solution with a concentration of 1% (v/v). Then, the cleaned Si wafer substrate was immersed in the OTS solution for 24 h. After the substrate was coated, the OTS sample was rinsed using toluene for three times to remove remaining molecules.10 2.1.2. Superhydrophobic Samples with Different Degrees of Oleophobicity. The soda-lime glass substrate (7101, Pearl, China) with a 1.0−1.2 mm thickness was used for the preparation of superoleophilic, oleophobic, and superoleophobic samples, which were also superhydrophobic.21 Silane-modified, hydrophobic SiO2 nanoparticles with a diameter of 55 ± 15 nm (AEROSIL RX 50, Evonik Industries) were used to prepare the samples with different degrees of oleophobicity. To prepare the superoleophilic sample, tetrahydrofuran (THF) was first mixed into isopropyl alcohol (IPA) to form a 40%/60% (v/v) solvent. Then, 300 mg of nanoparticles was dispersed in the solvent to form a mixture with a concentration of 10 mg/mL. It was found that the concentration of 10 mg/mL was needed to obtain good superhydrophobicity and prevent the agglomeration of nanoparticles on the substrate. With the use of a Branson Sonifier 450A, the solvent was then sonicated at a frequency of 20 kHz and amplitude of 35% for 4 min. After sonication, 150 mg of methylphenyl silicone resin binder obtained from Momentive Performance Materials (SR355S) was dispersed in the mixture and sonicated for 4 min. It should be noted that this concentration of methylphenyl was needed to keep the good 14693


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Figure 1. AFM images, RMS roughness, and P−V distance of (a) hydrophobic and (b) superhydrophobic surfaces. so the electrostatic force obtained at the velocity of 0.22 μm/s should be subtracted from the measured force at the velocity of 38.5 μm/s.8 The measured deflection data of the probe was used to calculate the force data using Hooke’s law (F = k × Def, k is the stiffness of the probe and Def is the deflection of the probe). To obtain the stiffness value k of the probe, measurement on a hydrophilic Si sample immersed in DI water was first carried out. For a sample with zero slip length, the relationship between Def and hydrodynamic force, Fhydro, can be expressed as23

Def =

Fhydro k

=

6πηR2 V kD

(simplified as a sphere-flat sample system) used in this paper, the electrostatic force per unit area between a ball and a flat surface was integrated over the surface area to obtain the total electrostatic force between the sphere and the sample. The resulting relationship between the surface charge density and the electrostatic force of the sphere-flat sample system is obtained as11 Felectro(D) =

(1)

where η is the dynamic viscosity of the liquid, V and R are the velocity and radius of the sphere, and D is the separation distance between the sample and bottom of colloid probe. By fitting the Def−D curve obtained on the Si sample in a long-range distance of 800−3500 nm using eq 1, k/6πμR2 = 18.9 nm−1 s−1 for the probe. Then, the result of k = 0.3 N/m was obtained using the known parameters R = 29.45 μm and η = 9.8 × 10−4 Pa s at room temperature. 2.2.4. Quantification of the Surface Charge Density. For two charged surfaces at a fixed distance, the electrostatic force between the surfaces is related to the surface charge density. For the AFM system

⎤ 2π(σ12 + σ2 2) ⎡ R R + 2κD ⎢ 2κ(D + 2R) ⎥ ⎣e ⎦ ε0εκ − e 1 −1 π(σ12 + σ2 2) ⎡ e−2κ(D + 2R) − 1 ⎤ ⎥ − ln⎢ −κD ε0εκ −1 ⎦ ⎣ e 4πσ1σ2 ⎡ R + ⎢ ε0εκ ⎣ e κ(D + 2R) − e−κ(D + 2R) 2πσ1σ2 ⎡ 1 − e−κ(D + 2R) e κD + − · ln⎢ ε0εκ 2 ⎣ 1 + e−κ(D + 2R) e κD − +

e

κD

1⎤ ⎥ 1⎦

⎤ R −κD ⎥ −e ⎦

(2)

where ε0 and ε are the permittivity of the vacuum and the dielectric constant of the liquid, σ1 and σ2 are the surface charge densities of the sphere and the sample, respectively, κ−1 = (ε0εkbT/2n0z2e2)1/2 is the 14694


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40 ± 10 200 ± 50

120 ± 20 600 ± 100 1800 ± 200

9±3 16 ± 3

9±3 24 ± 4 7±2

54 ± 3 84 ± 2

69 ± 3 106 ± 4 159 ± 6

15 ± 4 140 ± 50

80 ± 20 350 ± 70 1350 ± 180

18 ± 3

2±1 38 ± 6 11 ± 2

0 32 ± 2

8±5 100 ± 4 150 ± 5 65 ± 20 150 ± 30 300 ± 50 700 ± 150 700 ± 150 700 ± 150 SiO2 and methylphenyl silicone resin SiO2 and fluorinated acrylic copolymer (0.3 by weight) SiO2 and fluorinated acrylic copolymer (0.6 by weight)

CAH (deg) CA (deg)

CAH (deg)

slip length (nm)

CA (deg)

ethylene glycol hexadecane

slip length (nm) CAH (deg)

Hydrophobic 94 ± 2 28 ± 3 40 ± 4 106 ± 2 Superhydrophobic 7±2 162 ± 5 163 ± 3 12 ± 2 3±1 165 ± 3 30 ± 10 5±3

superoleophilic oleophobic superoleophobic

where γlg, γsg, and γsl are liquid−gas, solid−gas, and solid−liquid surface tension, respectively, and θ is the contact angle. From eq 4, larger liquid−gas surface tension leads to a larger contact angle. On the basis of the surface tension data presented in Table 2, the trend of the CA being the largest when a droplet of DI water is used can be explained. When using the same liquid, the CA of the PS surface is smaller than that of the OTS surface because of the higher roughness in the so-called Wenzel regime.26

polystyrene (PS) octadecyltrichlorosilane (OTS)

(4)

superoleophilic oleophilic

γlg cos θ = γsg − γsl

CA (deg)

3. RESULTS AND DISCUSSION 3.1. Morphology and Surface Roughness. Figure 1 shows the AFM images of samples in the air, root-mean-square (RMS) roughness and peak to valley (P−V) distance. The two hydrophobic samples, PS and OTS samples, have a roughness lower than that of the superhydrophobic surfaces with different degrees of oleophobicity with a 1 μm × 1 μm scan size. For the three superhydrophobic surfaces, a 5 μm × 5 μm scan size was also used to present data because of the higher roughness. The AFM images show that nanoparticles form roughness on the nanoscale. The superhydrophobic samples have larger RMS roughness and P−V distances, and this multiscale roughness is desirable for superoleophobicity.26 We note that the RMS roughness and P−V distance increased going from superoleophilic, oleophobic to superoleophobic surfaces with the same scan size. 3.2. CA and CAH. 3.2.1. Hydrophobic Surfaces. Table 4 shows the CA and CAH data for the two hydrophobic surfaces, PS and OTS samples. For the PS and OTS samples, the CA is largest when using a droplet of DI water and the CA is smallest when using a droplet of hexadecane with the lowest surface tension. This result can be explained by the surface tension of the liquids using Young’s equation:27

thickness (nm)

From eq 3, b is the linear interception of V/Fhydro at the axis of separation distance.

composition

(3)

type of coating

1 (D + b) 6πηR2

DI water

Fhydro

=

Table 4. Coating Thickness, Contact Angle (CA), Contact Angle Hysteresis (CAH), and Slip Length of Two Hydrophobic and Three Superhydrophobic Samples

V

slip length (nm)

Debye length, kb is the Boltzmann constant, T is the absolute temperature, n0 is the ionic concentration of the liquid, z is the chemical valence of the ions in the liquid, and e is the elementary charge. The surface charge density was obtained by fitting the measured electrostatic force at a sphere velocity of 0.22 μm/s based on the theoretical model shown in eq 2. It should be noted that there are two unknown parameters, σ1 and σ2, in eq 2. To obtain σ1 and σ2, two separate steps were carried out. First, the electrostatic force between the borosilicate glass sphere and borosilicate glass sample was measured. In this case, we can assume that σ1 = σ2 and the surface charge density of sphere σ1 could be obtained under this assumption. After known σ1 was obtained, the colloidal probe with known σ1 was used to obtain the electrostatic force between the sphere and sample, and then σ2 was obtained by fitting the measured electrostatic force using eq 2 and known σ1. On the basis of the above method, for the first step of σ1 = σ2, eq 2 included σ12 and it could not provide the sign of σ1; therefore, we could not obtain the sign of σ211. To obtain the sign of σ1 and σ2, the point of zero charge (PZC) of the sample, a parameter characterized by the pH value of the electrolyte, should be known. When the pH value of electrolyte is equal to PZC, the sample shows a zero net surface charge. In the case where the pH value of electrolyte is larger than the PZC, the sample is negatively charged; otherwise, the sample is positively charged.24 Table 3 lists the properties of the materials and the parameters used to quantify the surface charge density.19 2.2.5. Slip Length Measurement. In the limit of large separation (D ≫ b), the relationship between Fhydro for a sample with slip length b and D can be expressed as25

∼0 30 ± 5

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Figure 2. Electrostatic forces of (a) two hydrophobic surfaces and (b) three superhydrophobic surfaces immersed in DI water, hexadecane, and ethylene glycol obtained at a sphere velocity of 0.22 μm/s.

Table 5. Surface Charge Density of Sphere and Five Samples Immersed in DI Water samples surface charge density (mC/m2)

sphere

PS

OTS

superoleophilic

oleophobic

superoleophobic

−0.217 ± 0.042

−0.082 ± 0.031

−0.124 ± 0.045

−0.056 ± 0.034

−0.038 ± 0.032

−0.042 ± 0.029

3.2.2. Superhydrophobic Surfaces with Different Degrees of Oleophobicity. Table 4 shows the CA and CAH data for the superhydrophobic surfaces having different degrees of oleophobicity. For these surfaces, the CA of each surface is larger than 160° when using droplets of DI water, and this can be explained by the multiscale roughness shown in Figure 1. Because the larger liquid−gas surface tension leads to a larger contact angle based on eq 3, the CA of each sample is largest when using the droplet of DI water and the CA is smallest when using the droplet of hexadecane. 3.3. Electrostatic Force and Surface Charge Density. 3.3.1. Hydrophobic Surfaces. The electrostatic forces of two hydrophobic surfaces, PS and OTS samples, immersed in DI water,

hexadecane, and ethylene glycol as a function of separation distance are shown in Figure 2a. For DI water, the electrostatic force of the OTS surface is larger than that of the PS surface, which means a larger absolute value of surface charge density for the OTS surface. A similar result was reported in our previous work.10 To obtain the surface charge density of the PS and OTS samples, the surface charge density of the borosilicate sphere in DI water should be known first. By fitting the measured electrostatic force of the borosilicate glass surface in DI water using eq 2 and the parameters presented in Table 3, the surface charge density of the sphere is about −0.2 mC/m2, as shown in Table 5. Then, the surface charge density of the PS and OTS surfaces immersed in DI water can be obtained by fitting the 14696


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Figure 3. Hydrodynamic force Fhydro and V/Fhydro of a borosilicate sphere on PS and OTS surfaces immersed in (a) DI water, (b) hexadecane, and (c) ethylene glycol obtained at a sphere velocity of 38.5 μm/s.

relevant electrostatic force using eq 2, and the results are shown in Table 5. For hexadecane and ethylene glycol, a similar result can be found. The electrostatic force of the OTS surface is larger than that of the PS surface, which means the absolute value of the surface charge density for the OTS surface is larger than that of the PS surface when immersed in both hexadecane and ethylene glycol. 3.3.2. Superhydrophobic Surfaces with Different Degrees of Oleophobicity. The electrostatic forces of three superhydrophobic surfaces with different degrees of oleophobicity immersed in DI water, hexadecane, and ethylene glycol as a function of separation distance are shown in Figure 2b. For DI water, the electrostatic force of the superoleophilic surface is larger than that of the oleophobic and superoleophobic surfaces, which means a larger absolute value of the surface charge density for superoleophilic surfaces. For the oleophobic and superoleophobic surfaces, close values of electrostatic forces are found, which means close absolute values of surface charge density. Through use of the known surface charge density of the borosilicate sphere

in DI water, the surface charge density of the three superhydrophobic surfaces with different degrees of oleophobicity immersed in DI water is quantified by fitting the relevant electrostatic force using eq 2, and the results are presented in Table 5. From Table 5, the superoleophilic surface has a larger absolute value of surface charge density than that of the oleophobic and superoleophobic surfaces, and the oleophobic and superoleophobic surfaces have close absolute values of surface charge density. For hexadecane and ethylene glycol, similar results are found. The electrostatic force of the superoleophilic surface is larger than that of the oleophobic and superoleophobic surfaces, and the oleophobic and superoleophobic surfaces have close values of electrostatic forces. Regarding the sign of the surface charge in Table 5, the surface charge density of the five samples immersed in DI water is expected to be negative and the reasons follow. For the borosilicate glass sphere immersed in DI water with a pH value is about 6, the glass sphere surface is negatively charged because the pH value of DI water is larger than the reported PZC of silica 14697


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Figure 4. Hydrodynamic force Fhydro and V/Fhydro of a borosilicate sphere on superoleophilic, oleophobic, and superoleophobic surfaces immersed in (a) DI water, (b) hexadecane, and (c) ethylene glycol obtained at a sphere velocity of 38.5 μm/s.

(in the pH range 2−4),28,29 and the surface is charged because of the dissociation of the silanol groups:30,31 −SiOH ⇌ −SiO− + H+

3.4. Slip Length Measurements. 3.4.1. Hydrophobic Surfaces. Fhydro and V/Fhydro of two hydrophobic samples, PS and OTS, immersed in DI water, hexadecane, and ethylene glycol at a sphere velocity of 38.5 μm/s as a function of separation distance are shown in Figure 3. For DI water, the V/Fhydro of the OTS surface is larger than that of the PS surface, which means a larger slip length on the OTS surface according to eq 3. This is in agreement with our previous results.10 For hexadecane and ethylene glycol, a similar result can be found. The V/Fhydro of the OTS surface is larger than that of the PS surface, which means a larger slip length for the OTS surface. The slip lengths of the hydrophobic surfaces are shown in Table 4 and Figure 5. From the results shown in Table 4 and Figure 5, we can find that the slip length on the OTS surface is larger than that on the PS surface when immersed in the same liquid. The slip length of each sample immersed in DI water is smallest, and the slip length of each sample immersed in ethylene glycol is largest. The slip length on the OTS surface immersed in hexadecane agrees with the work of Pit et al.,18 and the relatively

(5)

Then, based on the measured repulsive electrostatic force between the sphere and the sample surface being positive, as shown in Figure 2, the samples immersed in DI water should be negatively charged. The mechanism of the negatively charged OTS is the adsorption of H+ and OH− at the interface, and OH− has higher adsorption energy and saturation coverage than the H+.32 For superoleophobic surfaces with different degrees of oleophobicity, the dissociation of the silanol group leads to the negatively charged sample because of the existence of the SiO2 nanoparticle coated on the samples. It should be noted that the mechanisms of the different surface charge densities on different solid−liquid interfaces are not understood, and that more studies are needed to figure out these mechanisms. 14698


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solid−liquid interface, the viscous shear stress η∂v/∂z (∂v/∂z is the velocity gradient in the z direction at the solid wall) exerted by the liquid on the wall should be equal to the friction stress σ = λvs (vs is the relative velocity between the solid and liquid at the interface, and λ is the interfacial friction coefficient relating) presented on the liquid from the wall. Thus, the slip length b can be expressed as33 b = η/λ

(6)

According to eq 6, slip length is related to the viscosity of a liquid. For the same solid surface, a larger viscosity of a liquid leads to a larger slip length. This can be used to explain the results of the slip length being the smallest for each sample when immersed in DI water and the slip length being the largest for each sample when immersed in ethylene glycol for both hydrophobic and superhydrophobic surfaces.

4. CONCLUSIONS In this paper, AFM was used to study the electrostatic force and boundary slip on superoleophilic, oleophobic, and superoleophobic surfaces. Two hydrophobic surfaces, PS and OTS, which are superoleophilic and oleophilic, respectively, and three superoleophobic surfaces, one each of superoleophilic, oleophobic, and superoleophobic were prepared. Three liquids, DI water, hexadecane, and ethylene glycol, were used. The electrostatic force at different solid−liquid interfaces was measured, and the surface charge densities of the five samples immersed in DI water were quantified. The results showed that the electrostatic force of the OTS surface immersed in DI water, hexadecane, and ethylene glycol is larger than that of the PS surface. Thus, the OTS surface has a larger absolute value of surface charge density than that of the PS surface. The electrostatic force of the superoleophilic surface immersed in DI water, hexadecane, and ethylene glycol is larger than that of the oleophobic and superoleophobic surfaces. The oleophobic and superoleophobic surfaces have close values of electrostatic force, which means the superoleophilic surface has a larger absolute value of surface charge density than that of both the oleophobic and superoleophobic surfaces. The oleophobic and superoleophobic surfaces have close absolute values of surface charge density. The slip lengths of the five samples immersed in DI water, hexadecane, and ethylene glycol were obtained. The results show that for the same liquid, the slip length of each sample is related to the contact angle and a larger contact angle leads to a larger slip length. For the same surfaces, the slip length of each sample immersed in DI water is the smallest, and the slip length of each sample immersed in ethylene glycol is the largest. These results are related to the viscosities of the liquids, and a larger viscosity leads to a larger slip length.

Figure 5. Bar chart of slip length on the five samples immersed in DI water, hexadecane, and ethylene glycol.

smaller slip length on OTS in our experiment when comparing with the result of Pit et al. can be explained by a slightly incomplete OTS coating layer. This result can be used as a reference for the validity of our experiment. 3.4.2. Superhydrophobic Surfaces with Different Degrees of Oleophobicity. Fhydro and V/Fhydro of the three superhydrophobic surfaces, one each of superoleophilic, oleophobic, and superoleophobic, immersed in DI water, hexadecane, and ethylene glycol at a velocity of 38.5 μm/s as a function of the separation distance are shown in Figure 4. For the DI water, the V/Fhydro of the superoleophilic surface is the smallest and the V/Fhydro of the superoleophobic surface is the largest. Thus, the superoleophilic surface has the smallest slip length and the superoleophobic surface has the largest slip length according to eq 3. For hexadecane and ethylene glycol, a similar result can be found. The V/Fhydro of the superoleophilic surface is the smallest and the V/Fhydro of the superoleophobic surface is the largest. Therefore, the superoleophilic surface has the smallest slip length and the superoleophobic surface has the largest slip length. The slip lengths of the superhydrophobic surfaces are shown in Table 4 and Figure 5. For the same surface, we can find that the slip length of each sample immersed in DI water is the smallest, and the slip length of each sample immersed in ethylene glycol is the largest. 3.4.3. Discussion. From the results shown in Table 4 and Figure 5, for hydrophobic surfaces immersed in the same liquid, the slip length on OTS is larger than that of PS, and for superhydrophobic surfaces with different degrees of oleophobicity immersed in the same liquid, the slip length on the superoleophilic sample is smallest and the slip length on superoleophobic sample is largest. The result can be explained by the contact angle of the samples, and a larger contact angle leads to a larger slip length. The possible mechanism is that larger contact angle means weaker interaction and smaller force between the solid surface and liquid molecules, which leads to a larger slip length. For both hydrophobic and superhydrophobic surfaces with different degrees of oleophobicity, the slip length of each sample immersed in DI water is the smallest, and the slip length of each sample immersed in ethylene glycol is the largest. The results are related to the viscosity of the liquid, as shown in Table 2. At the

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AUTHOR INFORMATION

Corresponding Author

*B. Bhushan. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D. Jing acknowledges financial support from the Chinese Scholarship Council. The authors also like to thank Dr. Palanikkumaran Muthiah for preparing three superhydrophobic surfaces. 14699


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Boundary Slip of Superoleophilic, Oleophobic, and Superoleophobic

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