Reversible optical assembly of composites

Abstract

A method to reversibly assembly micro and nanostructures with a force provided by light inside an embedding medium which behaves as solid during steady state and as a fluid during assembly state. The embedding medium is a material capable to change from solid to fluid state during assembly and from fluid to solid state during fixation. The change in state at the embedding medium is controlled with the temperature and/or shear strain.

Claims

1. We claim a light induced mechanism to arrange composites containing at least one micro or nanostructured material and one embedding medium. The embedding material can behave as a fluid or as a solid depending on temperature and shear strain applied. The micro or nanostructured material can be arranged with the momentum provided by light while the embedding medium behaves as a fluid, but fixated when the embedding medium behaves as a solid.

2. The mechanism of claim 1 where the embedding medium has a well-defined transition of phases (melting and freezing points).

3. The mechanism of claim 1 where the embedding medium has a complex rheology and the viscoelasticity has a dependence on temperature and shear strain.

4. The mechanism of claim 1 where the embedded material corresponds to particles of arbitrary sizes, shapes and materials.

5. The mechanism of claim 1 where the embedded material are displaced with light between different steady positions.

6. The mechanism of claim 1 where the embedded material are rotated with light between different steady positions.

7. The mechanism of claim 1 where the micro or nanostructures are arranged from light by optical pressure or optical tweezers.

8. The mechanism of claim 1 where the micro or nanostructures are arranged from light by thermophoresis.

9. The mechanism of claim 1 where the micro or nanostructures are arranged from light by acoustophoresis.

10. The mechanism of claim 1 where the transition of phase in the embedding medium is obtained by heating the composite externally.

11. The mechanism of claim 1 where the embedding medium becomes fluid by heating the structure-medium interface with the absorption of the same light used for the assembly.

12. The mechanism of claim 1 where the embedding medium becomes fluid because of the reduction in the viscosity due the shear strain produced by the force applied to the embedding element.

13. The mechanism of claim 1 where the micro or nanostructures are assembled collectively with a spread light beam.

14. The mechanism of claim 1 where the micro or nanostructures are assembled locally with a focused light beam.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 is a graph of the rheology of a typical thermoplastic (e.g. the complex shear modulus (G′+i G″) of pHEMA changes from 1.4×109+i 2.0×107 Pa in the glassy regime to 2.9×104+i 2.0×104 Pa well above its glass transition temperature (Tg) of 300° C.).

[0032] FIG. 2 is a representation of the dynamics of nanoparticle displacement in a standing wave. The force is proportional to the gradient of the intensity. Ag nanoparticles in the negative regime are pushed towards the minimum intensity regions.

[0033] FIG. 3 is a measure of a relative maximum force acting on Ag nanoparticles of different diameters with a standing wave of 532 nm.

DETAILED DESCRIPTION OF THE INVENTION

[0034] The invention refers in broad terms to assemblies of micro or nanostructures in solid or semi-solid media with the action of light. The reduction of the adhesion that fixated structures is induced by temperature or by the force applied to the embedding element. This includes composites containing a transparent or semi-transparent material and a second micro or nanostructured material. For example, nanoparticles fixated by frozen water. If nanoparticles absorb light, they can increase locally the temperature to produce a transition of phase. Furthermore, when heat is transferred with pulsed light before dissipation, it is possible to locate the transition of phase just at the boundary of nanoparticles while they are displaced or rotated. A similar effect can be induced in gelatines, but with the additional effect of reduction in viscosity due the shear strain applied at the boundaries of nanoparticles. A composite has demonstrated to work with silver nanoparticles embedded hydrogel. An expanded example is described in the following section, however the invention is not limited to the embodiment.

Working Example

[0035] An assembly of nanoparticles embedded in a solid is reconfigurable with optical forces when the viscoelasticity of the medium permits the migration and the stabilization in a reversible manner. When the optical force passes a threshold, nanoparticles overcome surface adhesion, elastic forces, and the static friction induced by the medium. This phenomenon is analogous to “the knife in the butter”, where the medium changes its stiffness according to the temperature of the metal. The example is described in three sections: overview, and experimental demonstration.

Overview

[0036] Ag nanoparticles are arranged dynamically in a three-dimensional (3D) space with a poly(2-hydroxyethyl methacrylate) (pHEMA) as the embedding medium. Ag nanoparticles were considered due its high optical scattering and absorption. PHEMA was chosen as embedding medium due its unique rheological characteristics. PHEMA matrix can reversibly transform from its glass state to its rubber state by increasing the temperature at the nanoparticle boundaries. The glass and the rubber behaviours correspond to the fixated and fluid states respectively. When the pHEMA matrix increases in temperature the viscoelasticity reduces allowing the migration of the nanoparticles. Notice that the phase transition temperature of Ag nanoparticles is lower than bulk Ag but still higher than the degradation temperature of pHEMA, both of which are slightly above 300° C. (S. A. Little, et al. Appl. Phys. Lett., vol. 100, no. 5, p. 51107, January 2012; M. Cokun, et al. Polym. Degrad. Stab., vol. 61, no. 3, pp. 493-497, 1998). The temperature at the boundary of the nanoparticles dictates the mechanical properties of the surrounding medium. Since pHEMA has low heat conduction, the high temperature at the nanoparticle-pHEMA boundary allows the pHEMA matrix to behave like viscoelastic rubber. Furthermore, this effect is present as long as the heat of the metal diffuses in the pHEMA matrix. A pHEMA matrix that transforms from its glass state to its rubber state by increasing the temperature at the nanoparticle boundaries was rationally designed FIG. 1.

[0037] When nanoparticle boundaries temperature is increased, nanoparticles move their position. Depending on the size, nanoparticles settle at the maximum intensity or minimum intensity regions of the interference fringe (P. Zemánek, et al. J. Opt. Soc. Am. A, vol. 19, no. 5, pp. 1025-1034, May 2002). We use optical standing waves to control heat and optical force to arrange nanoparticles in different 3D configurations. Dielectric and metal nanoparticles in viscoelastic media have a complex behavior in the presence of radiation gradients. An optical force (tractor force) results from the momentum transfer associated with the spatially asymmetric light scattering and absorption of a nanostructure (O. Brzobohaý, et al. Nat. Photonics, vol. 7, no. 2, pp. 123-127, February 2013). Electromagnetic forces in gradients can push particles toward regions of maximum intensity (positive force) or minimum intensity (negative forces) (M. {hacek over (S)}iler, et al. J. Quant. Spectrosc. Radiat. Transf., vol. 126, pp. 84-90, September 2013). In dielectrics, the force can be positive or negative when the nanoparticle has higher or lower refractive indexes than the medium, respectively (K. C. Neuman and S. M. Block, Rev. Sci. Instrum., vol. 75, no. 9, pp. 2787-2809, September 2004). The phase shift of the scattering dictates the direction of the force. In metal nanoparticles, the phase and intensity of the scattering depends on the Surface Plasmon Resonance (SPR) produced by the free electron cloud. Hence, the direction of the optical force is dictated by different factors including geometry, size and material of the nanoparticle, the surrounding medium, and the wavelength of the applied field (A. Dogariu, et al. Nat. Photonics, vol. 7, no. 1, pp. 24-27, January 2013; K.-S. Lee and M. A. EI-Sayed, J. Phys. Chem. B, vol. 109, no. 43, pp. 20331-20338, November 2005).

[0038] Arbitrary standing waves were defined with the interference of two counter-propagating. The phase of the standing wave was controlled with the relative phase difference between the beams. Hence, the force exerted by the standing wave of two counter-propagating beams is proportional to the gradient of the intensity of the beam FIG. 2 showed the displacement of nanoparticles from bright regions to dark regions in a standing wave produced by a negative force.

[0039] In order to calculate the migration, it was applied the generalized Stokes' law for a nanoparticle of radius r embedded in a complex viscoelastic medium with a shear modulus G=2.9×10.sup.4+i2.0×10.sup.4. FIG. 3 shows the relative maximum force of Ag nanoparticles using a standing wave from a 532 nm light source.

Experimental Demonstration

[0040] In order to demonstrate a 3D nanoassembly, a Nd:YAG (532 nm, 5 ns) pulsed laser was used to form a standing wave. The nanoparticle displacement was increased by repeating the number of pulses. Ag nanoparticles were arranged in a slanted 3D structure with a periodicity of ˜λ/2. We recorded multilayer structures by titling the sample at different angles with respect to the standing wave. The fabricated nanostructure served as a narrow-band wavelength-selective filter to diffract an intense color at 8° away from the sample normal.

[0041] To demonstrate reversibility, we recorded a grating at 5° from the surface plane and erased it (recorded at 0°) iteratively several times. The holographic patterning technique can be used to configure different crystal plane orientations. Bragg planes were superposed at 5°, 10°, 15°, 20°, and 25° to form photonic crystals. To erase the pattern, pHEMA matrix was aligned parallel to the surface plane of the object (i.e. front-surface mirror) at 0°. This configuration aligned the multilayer structure with the specular reflection (zero order). Crystal structures in binary configurations were also recorded and erased to demonstrate volumetric data storage. The composite was also utilized to fabricate dynamic lenses and holographic reconstruction of coins and other objects.

[0042] Although some embodiments are shown to include certain features, the applicant(s) specifically contemplate that any feature disclosed herein may be used together or in combination with any other feature on any embodiment of the invention. It is also contemplated that any feature may be specifically excluded from any embodiment of an invention.