Microparticle Surfaces - Chemistry of

Article pubs.acs.org/cm

Controlled Cavitation at Nano/Microparticle Surfaces Lu Zhang,†,‡ Valentina Belova,‡ Hongqiang Wang,‡ Wenfei Dong,*,§ and Helmuth Möhwald*,‡ †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, Jilin Province, P. R. China ‡ Max Planck Institute of Colloids and Interfaces, Am Mühlenberg 1, 14476 Potsdam-Golm, Germany § Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, 88th Keling Road, 215163, Suzhou, Jiangsu Province, P.R. China S Supporting Information *

ABSTRACT: Here we proposed a model that describes the nucleation of cavitation bubbles on nano/ microparticle surfaces, which is of significance for the yet unsolved problem of particle-cavitation interaction. The model was verified through rational experimental design varying parameters including particle size, their shapes and additives. The surface morphology evolution of particles under high intensity ultrasonic irradiation in aqueous solution can be tailored by changing the nucleation energy barrier of cavitation bubbles on particle surfaces. Cavitation-induced breakage of particles under sonication has also been addressed, which predicts the critical effect of the initial size of solid particles in affecting ultrasound-driven intraparticle fracture. These results shed light on the effect of commonly used sonication treatments on nanostructured materials and sonochemical surface modification in particle science and technology.

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INTRODUCTION Cavitation is the process of formation, growth, and implosion of bubbles in a liquid as a result of high temperature and pressure gradients.1,2 During ultrasonic irradiation of solids in liquids, cavitation bubbles normally form on the solid surface by heterogeneous nucleation, which requires less energy when forming at preferential sites like phase boundaries.3 The collapse of a bubble can generate locally extreme pressures and temperatures in a short time period, as well as intense shock waves and microjets.4 The microjet induced by asymmetric bubble collapses generates a high-speed steam of liquid toward the solid surface, which can physicochemically change the surface of irradiated materials.5 The control of bubble nucleation on the surface and the following collapse is of vital importance for the development of a promising discipline serving biomedical and material science.6−8 The effect of ultrasonic cavitation at liquid/solid interfaces has been widely investigated primarily on metal and polymer surfaces.9−11 Nucleation can be facilitated and suppressed by changing surface energy, ultrasonic power, frequency, and the temperature of the liquid medium.12 Nevertheless, the studies have been focused on extended planar surfaces, which do not fully account for the different ultrasound-induced reactions and the geometry of the surfaces, as they neglect the contribution of feature shapes and size of particles. The typical shape is spheroid, and the effective apparent contact angle of gas bubbles on spherical surfaces (θ) in contact with a liquid is bigger than the intrinsic contact angle of bubbles on planar surfaces (θ0) (Figure 1a), which may influence the surface energy, the volume work, and nucleation energy barrier of the bubble. To present, little work has been addressed to investigate the effect of cavitation on suspended spherical © 2014 American Chemical Society

particle surfaces. Possible reasons may be as follows: 1) It is difficult to record the evolution of cavitation bubbles formed on suspended particle surfaces especially for nano/microsized particles because all particles move freely.13 2) Particle collision results in extreme heating at the point of impact, which can lead to effective local melting to fuse metal particles and dramatic increases in the rates of many solid−liquid reactions.14 3) The effect of particle solution under ultrasonic treatment is known for particle dispersions and size reduction due to high speed shear rate and associated high velocity interparticle collisions driven by ultrasound.15,16 Nevertheless, the understanding of ultrasonic cavitation on particle surfaces is crucial for particle surface modification and fabrication of novel composites using ultrasound techniques.17 Particle dimensions may be relevant for many aspects of ultrasonic impact. First, critical cavitation nuclei are expected to have dimensions between 10 and 100 nm (at ultrasonic frequency of 20 kHz to ∼1 MHz),18,19 hence the nucleation of bubbles is affected by the size/curvature of the particles (Figure 1b).20 Second, the bubble size before collapse is expected to be around 100 μm,21 hence collapse may be less affected by particle dimensions below 10 μm (Figure 1b). Third, the rupture strength depends on particle size, hence the impact of microjet on particle may also have a correlation with particle size. Therefore, the investigation of the cavitation effect on particle surface against particle size is an important issue. Here, a model that describes the nucleation of cavitation bubbles on nano/microparticle surfaces was proposed. The Received: December 22, 2013 Revised: March 7, 2014 Published: March 10, 2014 2244

dx.doi.org/10.1021/cm404194n | Chem. Mater. 2014, 26, 2244−2248

Chemistry of Materials

Article

were washed with ethanol by centrifugation. The hydrophobicity was changed by surface modification of such hydrophilic particles with 3GPS in a sol−gel method.25,26 1 mL of 3-GPS was added into a mixture of 100 mg of SiO2 particles and 50 mL anhydrous ethanol with stirring at 50 °C for 24 h. The hydrophobic particles were obtained after wash treatment. Synthesis of SiO2 Rods. The rods were synthesized by a one-pot consecutive hydrolysis of two silica precursors in n-pentanol in the presence of water/ethanol and polyvinylpyrrolidone (Figure 3b).27,28 PVP (1 g), deionized water (350 μL), sodium citrate solution (100 μL of 180 mM), and ammonium hydroxide (170 μL) were dissolved in npentanol (10 mL). Then, TEOS (50 μL) was added into the above mixture for 20 h. One mL of ethanol, 10 μL of TEOS, and 20 μL of HDTMOS were added to the reaction mixture and gently shaken. The hydrolysis of TEOS was preceded for another 20 h. The obtained particles were washed by ethanol for 4 times at 5000 rpm. Sonication Treatment. The ultrasonic processor UIP1000 hd (20 kHz, Hielscher Ultrasonics GmbH Teltow, Germany) was used inside the converter. During all experiments, a flat sonotrode with 3.14 cm2 area was placed in a small flask with 15 mL of particle solution (2 mg mL−1) within an ice bath. The ultrasonic power density was 51.3 W cm−2, and the temperature of the particle solution was about 10 °C. The result particles were washed with deionized water by centrifugation. Characterization. SEM was performed on a Gemini Leo 1550 microscope, applying an operating voltage of 3 kV to study the morphology of SiO2 particles. Samples were sputtered with gold. TEM images of SiO2 tubes were obtained by a Zeiss EM 912 Omega transmission electron microscope operated at 120 kV. Samples were carefully placed onto the copper grids. AFM analysis was performed on a Nanoscope III Multimode AFM (Digital Instruments Inc., USA) operating in tapping mode.

Figure 1. (a) Scheme for nucleation and growth of a cavitation bubble at a particle surface. θ the effective apparent contact angle, θo an intrinsic contact angle. (b) Scheme for cavitation nuclei (top row) and bubble before collapse (bottom row) on small (left) and large (right) particles. (c) Nucleation energy barrier as a function of contact angle θ derived from the maximum of the sum of surface energy and volume work (Es+Eb). See Figure S1 in the Supporting Information for details.

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RESULTS AND DISCUSSION It has been demonstrated that the nucleation of gas bubbles at planar surfaces can be controlled by ultrasonic frequency, ultrasonic pressure, reaction temperature, surface energy of solid surface, etc.12,29 It should be noted that particles can also serve as nucleation sites for cavitation bubbles.16 Bubble formation at a liquid/particle interface is sketched in Figure 1a, and the nucleation energy barrier is given by eq 129−31

model was verified by testing against the effects of varying initial particle size and shape and additives on particle morphology. Our results indicate that surface tension of solution has a significant effect on particle surface morphology evolution under ultrasonic irradiation, as it has an impact on the nucleation barrier of the cavitation bubble on the particle surface. Cavitation-induced breakage dynamics of particles under ultrasonic irradiation has also been addressed. It predicts the critical effect of the initial size of solid particles in affecting ultrasound-driven intraparticle fracture. Our results are of vital importance for the utilization of sonochemistry in particle science and technology.

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ΔE =

4πσ 3(2 + 3cos θ − cos3 θ ) 3P 2

(1)

where σ is the gas/liquid surface tension, P is the sum of the vapor pressure, the gas pressure, and the acoustic pressure.32,33 θ is the effective apparent contact angle of nucleation bubble on a curved surface in contact with a liquid (Figure 1a). When the particle is very small, e.g. 500 nm, θ is higher than that on a bigger particle surface (Figure 1a and b). If the particle diameter is big enough, e.g. 50 μm, the nucleation of a 100 nm bubble on its surface is similar to that on a planar surface (Figure 1b). According to eq 1, the θ drastically influences the nucleation energy. Figure 1c shows the nucleation energy barrier as a function of contact angle, which is derived from the maximum or the sum of the surface energy and the volume work (see detailed calculations in Figure S1). It is thus supposed that the nucleation energy barrier increases as the contact angle decreases, indicating that bubbles preferably nucleate on submicrometer particle surfaces. The result implies that the particles in solution not only influence but also promote the nucleation of bubble. To investigate the cavitation effect on spherical surfaces, silica (SiO2) particles were selected as substrates for measuring cavitation-induced erosion. To start with, 500 nm-sized SiO2 particles were subject to sonication treatment. It was found that

EXPERIMENTAL SECTION

Materials. Polyvinylpyrrolidone (PVP, molecular weight 40 kg mol−1), tetraethylorthosilicate (TEOS), 3-aminopropyl-triethoxysilane (APS), hexadecyltrimethoxysilane (HDTMOS, >85%), mercaptopropyltrimethyoxysilane (MPTMOS, 95%), n-pentanol, ammonium hydroxide (28−30 wt %), 3-glycidoxypropyltrimethoxysilane (3GPS), and anhydrous ethanol were purchased from Sigma-Aldrich. Sodium chloride (NaCl, analytical grade) was obtained from Merck. Five and 50 μm SiO2 particles were purchased from microParticles GmbH (Germany). Deionized water (Milli-Q grade) with resistivity of 18.0 MΩ cm−1 was used in all the experiments. All chemicals were used as received. Synthesis of Spherical SiO2 Particles. Hydrophilic SiO2 particles of various sizes were prepared by hydrolysis and condensation of TEOS in the presence of ethanol, Milli-Q water, and ammonia as catalyst.22−24 For 500 nm-sized particles, TEOS (3.56 mL) dissolved in ethanol (4.41 mL) was added rapidly to a mixture of ethanol (41.66 mL), Milli-Q water (8.64 mL), and ammonia (1.73 mL, 30%). For 1 μm SiO2 particles, TEOS (3.14 mL) dissolved in ethanol (3.89 mL) was added rapidly to a mixture of ethanol (33.84 mL), Milli-Q water (6.7 mL), and ammonia (12.43 mL, 30%). The mixture was stirred vigorously at ambient temperature for 24 h. The resultant particles 2245



Article

Figure 2. SEM images of 500 nm hydrophilic SiO2 particles (a-c), 500 nm hydrophobic SiO2 particles (d-f), 5 μm hydrophilic SiO2 particles (g-i), and 50 μm hydrophilic glass beads (j-l) under different sonication times.

the hydrophilic SiO2 particles were not affected by cavitation at an earlier stage of ultrasonic treatment (10 and 30 min, Figures 2a and 2b, Figure S2a). With increasing sonication time (up to 1 h), we observed that the surface becomes rough (Figure 2c). In contrast to hydrophilic particles, significant surface changes were observed for hydrophobic particles after 30 min ultrasonic irradiation, and numerous pores were found on the particle surfaces even after just 10 min of sonication (Figures 2d and 2e, Figure S2b). These pores can entrap the gas initiating the following growth of new cavitation bubbles, and then these pores will grow bigger as the sonication time increases. Upon another 30 min sonication, hedgehog-like particles were obtained revealing a violent cavitation effect driven by ultrasound, and the size of sonicated particles decreased (Figure 2f). The surface irregularities after ultrasonic treatment by AFM analysis revealed that the roughness of the hydrophobic particle surface (3.73 nm) is much larger than that of hydrophilic one (6.52 nm) as shown in Figure S3. This demonstrated that the hydrophobicity of the particle surfaces has significant influence on bubble nucleation, assigned to differences in the contact angle of bubbles on particle surfaces. Figure S4 shows that a cavitation bubble nucleates on a 500 nm hydrophobic particle surface and the contact angle of a gas bubble on a particle surface is bigger than that on a hydrophilic particle surface. For microparticles, the surface of 1 μm SiO2 particles also became rough after 1 h of sonication, but the cavitation-induced damage on the surface was less pronounced as that of 500 nmsized particles (Figure S5). These results confirmed the expectation described in eq 1 that cavitation preferentially

occurs at small particle surfaces due to a lower energy barrier. Nevertheless, the cavitation-induced damage is quite different for 5/50 μm-sized particles (Figure 2g-l). As shown in Figures 2h and 2k, 10 min of sonication was sufficient for the breakage of particles, and most particles were broken into pieces after 30 min of sonication. It seemed that the impact on micrometersized particles driven by ultrasound is much stronger. However, it must be pointed out that the breakage of micrometer-sized particle is not ascribed to preferential cavitation on its surface but to the fact that it has lower fracture energy than submicrometer-sized particle (see below). During bubble collapse, microjets, intense shock waves, and high speed interparticle collisions lead to solid−liquid reactions and effective local deformation of particle surface. These deformations primarily driven by particle-shock wave interactions lead to particle breakage.34 The deformation Δ is a function of the applied load P, particle size X, Poisson ratio ν, and the Young modulus of elasticity Y, as shown in eq 2. The deformation is proportional to the 2/3 power of the applied load. The area under the load-deformation curve corresponding to the fracture energy can be predicted from eq 2 as given below35 2 ⎤1/3 ⎡ 2⎛ 9P 1 − v 2 ⎞ ⎥ ⎢ Δ=2 ⎟ ⎜ ⎢⎣ 16X ⎝ Y ⎠ ⎥⎦

E= 2246

∫ PdΔ = 0.832X

(2)

⎛ 1 − v 2 ⎞2/3 ⎟ ⎜ ⎝ Y ⎠

−1/3 5/3

P

(3)



Article

became rough upon 30 min of sonication (Figure 4). From roughness analysis (Figures 4d and 4e), the surface

where the shock wave pressure is 1 MPa as measured for laserinduced cavitation, and ν and Y for silica are assumed to be 0.16 and 7.35 × 1010 Pa, respectively.35 The specific fracture energy of a particle is shown in Figure 3a, exhibiting a decrease with an

Figure 3. (a) Relationship between diameter X and fracture energy for SiO2 particles, with the dashed lines representing the size of the particles used. SEM images of SiO2 tubes (b), SiO2 tubes after sonication for 10 min (c) and 30 min (d).

increase in particle size. Nanosized particles have quite big fracture energies effectively avoiding the destructive damages of them driven by the bubble collapse. On the contrary, the fracture energy of micrometer-sized particles significantly decreases explaining the complete destruction during bubble collapse. Thus, the morphology of resulting particles under sonication is affected by the initial particle size, revealing that the size of a particle may be kept at a certain value due to the reduced fracture energy for SiO2 with ultrasound treatment. We have to note that eqs 2 and 3 were derived only for spherical particles. To further understand the effect of particle size, ultrasonic irradiation on an asymmetric silica rod with 500 nm width and 5 μm length was applied. Upon 10 min of sonication, some rods were broken, and the initial breakage could be at the weakest point of the rods (Figure 3c). As sonication time increased, the length of the rods decreased but not below 2 μm (Figure 3d). Due to this limiting length, the critical fracture energy for SiO2 particles in a sonication bath can be estimated to be about 36 J. It is worth mentioning that we did not consider the differential stress-induced bending fracture in this study. After breakage, the surface of the obtained rods became rough (Figure S6) and full of wrinkles (Figure 3d), and some of them were even beaten flat (AFM images in Figure S7). The surface damages of rods confirmed that the breakage of the above-described particles was not only due to interparticle collisions but also due to a synergistic effect of cavitation-induced microjets and intense shock waves. Apart from θ related to particle size, surface tension has a considerable influence on the nucleation of cavitation bubble on a particle surface under ultrasonic treatment, as seen from eq 1. Introducing surfactants, organic solvents, polyelectrolytes, or salts to the sonicated liquid will result in drastic change in surface tension.36 Decreasing the surface tension leads to a decrease of the nucleation barrier of cavitation bubbles on solid surfaces and, as a consequence, facilitates bubble nucleation. By adding cetyltrimethylammonium bromide (CTAB) into the system, the 5 μm SiO2 particles did not break, and their surface

Figure 4. SEM images of 5 μm SiO2 particles in 2 mM (a) and 20 mM (b) CTAB solution after sonication for 30 min. AFM images of 5 μm SiO2 particles (c), and these particles in 2 mM (d) and 20 mM (e) CTAB after sonication for 30 min.

irregularities of particles treated in 20 mM CTAB solution is higher than that in 2 mM CTAB solution due to the much lower nucleation energy barrier (see details in Figure S1). Finally, increasing the surface tension by adding sodium chloride, most of the 500 nm particles were broken under sonication, as shown in Figure S8. Hence, the ultrasonically induced fracture can be controlled. The mechanism proposed here is consistent with the experimental observations. Therefore, cavitation behavior on particle surfaces as well as fracture can be controlled, providing an effective approach to control cavitation processes on the nanometer scale for performing sonochemical reactions.

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CONCLUSIONS In conclusion, a simple model that describes the nucleation of cavitation bubbles on nano/microparticles is proposed. The surface tension has a significant influence on cavitation impact on particles by changing the nucleation barrier of the cavitation bubble on the particle surface. Hence, the cavitation behavior can be controlled by simply changing the surface tension of the solution. Furthermore, cavitation-induced breakage of particles under sonication has also been addressed, which predicts a critical influence of the initial size of solid particles in affecting ultrasound-driven particle fracture. Taking SiO2 as an example, particles smaller than 2 μm could not collide with sufficient energy to break. The results not only enrich the knowledge on cavitation at liquid/solid interfaces but also pave the way of utilization of sonochemical surface modification in particle science and technology. 2247



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ASSOCIATED CONTENT

S Supporting Information *

Detailed calculations of the nucleation energy barrier, a scheme for cavitation nuclei on a 500 nm hydrophobic particle surface, and TEM and AFM images of SiO2 particles after sonication. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank R. Pitschke, H. Runge, and A. Heilig for help with SEM and AFM measurements. The financial support from the National Natural Science Foundation of China (No. 91123029), International S&T Cooperation Program of China (No. 2013DFR70490) and 863 project (2012AA063302).

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Microparticle Surfaces - Chemistry of

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