Vibrationally directed assembly of micro- and nanoparticle-polymer composites
Graphical abstract
Introduction
Directed assembly of micro- and nanoparticles provides a pathway for the fabrication of materials that support a variety of emerging applications for functional surfaces (Joung and Buie, 2015), smart material actuator elements (Velders et al., 2017), electrically conductive networks (Yilmaz et al., 2017), and sensing of external stimuli (Cho et al., 2017). In order to meet the demand for emerging applications, it is necessary to have a wide variety of techniques available to fabricate structured functional materials. For example, tuning the alignment of particles in a polymer solution during assembly can influence the underlying material properties (Tsai et al., 2016).
Directed assembly of particles within a polymer solution results in the fabrication of composite structures that improve the mechanical strength (Fu et al., 2008) or enhance responsiveness to external magnetic (Filipcsei et al., 2007), electric (Gupta et al., 2016) or thermal (Huang et al., 2005) stimuli. Electric field alignment of particles within a polymer solution has been demonstrated experimentally using induced dipole interactions (Xie et al., 2005) or dielectrophoresis (Bowen et al., 1998), although mechanisms that influence the assembly process, such as electrophoresis or electro-osmosis, are not as well characterized in particle-polymer systems. Particles that are ferromagnetic (Stepanov et al., 2007) or dispersed in a paramagnetic medium (Liu et al., 2005) can be aligned in the presence of a magnetic field to produce aligned composite structures.
An alternative to electric or magnetic fields is the use of physical substrate vibration to drive particle assembly. Chladni was the first to describe the assembly of small particles on a vibrating substrate in 1787 (Chladni, 1787). The mechanical strain placed on the substrate by localized flexing generate pressure waves that propagate through a medium as a surface standing acoustic wave (SSAW) (Kerboua et al., 2008). The acoustic radiation produced by these pressure waves interact with particles in a medium to transport them to regions of minimum or maximum surface displacement (nodes or anti-nodes, respectively) depending on particle-medium properties (Dorrestijn et al., 2007).
The effect of transporting particles in external acoustic fields, acoustophoresis, has been used to assemble a variety of microparticles (Owens et al., 2016), nanoparticles (Dean et al., 2015) and biological samples (Xu et al., 2011). Acoustic fields can act on a variety of material combinations as long as the particles and medium in which they are dispersed have different density and compressibility properties. Acoustic fields can be generated using off-the-shelf components (Austin Suthanthiraraj et al., 2012) with minimal use of microfabrication and can act on areas (∼1000 mm2) that are larger than other directed assembly approaches available for composite fabrication (Llewellyn-Jones et al., 2016).
In this study, we demonstrate the vibration-driven assembly of micro- and nanoparticles over an area that is three times larger than previously described by other acoustic field works. Unlike other directed assembly approaches that require microfabrication (Martinez-Duarte, 2012) to create electrode structures to generate the external fields necessary to transport colloidal particles, the approach described here relies on a simply supported substrate driven by a mechanical wave generator. Our reliance on off-the-shelf equipment means that this approach represents a low-cost method for fabricating heterogeneously structured particle-polymer composite structures.
We observe that particles in solution migrate when the substrate is mechanically driven to form two-dimensional SSAWs as the substrate flexes due to the vibrational input. The SSAWs form displacement nodes and anti-nodes across the vibrating substrate. Glass microparticles migrate to displacement anti-nodes, forming well-defined clusters within 40–50 s. When we introduce iron oxide nanoparticles to the solution, we observe migration towards displacement nodes. The patterns formed through particle migration depends on input frequency and particle size.
Substrate vibration links different vibrational modes with natural frequencies that depend on substrate material properties (Tufoi et al., 2014). Since assembly is conducted in solution, we expect that the natural frequencies to occur at lower values than in the absence of an incompressible fluid on top of the substrate based on published results from the literature (Tariverdilo et al., 2013). We compare our experimental results to a numerical solution to two-dimensional wave equation in MATLAB to identify the vibrational modes in our sample. The distribution of glass microparticles is also compared to a suspension balance model to compare the characteristic force acting on the particles in solution at low and high frequencies.
To demonstrate that this approach can be used as a materials fabrication platform, we immobilize glass microparticles and iron oxide nanoparticles using a photocurable hydrogel. A solution consisting of polyethylene glycol diacrylate (PEGDA) is prepared with an initiator. Micro- and nanoparticles are dispersed in the medium and a vibrating substrate assembles Micro- and nanoparticles along displacement antinodes and nodes, respectively. Exposure to a UV light source initiates PEGDA crosslinking, which immobilizes the pattern formed by the different types of particles during vibration. The resulting particle-polymer composite is robust enough to be held by hand after curing.
Section snippets
Experimental set-up
Fig. 1A diagrammatically shows the experimental platform used to vibrate substrates and assemble particles. A function generator (Vizatek, model number 01VZMFG2120) is connected to a mechanical wave drive (Pasco, part number SF-9324) to introduce an oscillatory vibration to the substrate. A 60-mm diameter glass petri dish is used as a substrate to assemble both glass microparticles and iron oxide nanoparticles. After securing the substrate to the moving arm of the wave driver, a 1.5 g
Mechanics of vibrating substrates
The equation of motion for a vibrating plate with a time-varying displacement of w that depends on position is expressed as (Kwak, 1991, Leissa, 1969),where ρ is the density of the plate and the flexural rigidity of the plate, D, is defined by,with E as Young’s modulus for the plate material, h as plate thickness, and ν is the plate Poisson ratio. The solution to the equation of motion (Eq. (1)) can be used to identify nodal lines for thin, vibrating flat plates based
Results and discussion
Glass microparticles placed in a solution composed of 30 wt% PEGDA in DMF were added to the vibration stage to form the patterns shown in Fig. 2. These patterns were immobilized by polymer cross-linked through exposure to a UV light. The modes were identified by adjusting m and n in Eq. (3) and visually comparing the resulting displacement distribution to the observed pattern. The glass microparticles appear to migrate towards displacement anti-nodes where the degree of displacement in our
Conclusion
We examined the use of physical substrate vibration as a means to assemble heterogeneously structured particle-polymer composites. Where other directed assembly techniques are limited by device dimensions, we illustrate that vibration can drive the assembly of micro- and nanoparticles over an area of 3000 mm2. The mechanisms for assembly vary with particle size and frequency. Microparticles are dominated by the pressure gradients induced by the vibration of the substrate, as demonstrated by our
Acknowledgments
This work was supported by funding from the Department of Mechanical Engineering at Iowa State University. S.S. and J.J.J. acknowledge the Iowa State University Center for Nondestructive Evaluation for providing access to μ-CT equipment. We also thank Zhan Zhang from the Center for Nondestructive Evaluation, Iowa State University for help with capturing the radiograph images.
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