Interactive Virtual Reality Simulation for Nanoparticle Manipulation and Nanoassembly using Optical Tweezers

Krishna C Bhavaraju University of Louisiana at Lafayette

ABSTRACT Nanotechnology is one of the most promising technologies

for future development. This paper proposes virtual reality (VR) as a tool to simulate nano particle manipulation using optical tweezers towards achieving nano- assembly and to handle effectively issues such as difficulty in viewing, perceiving and controlling the nano-scale objects. The simulation modeled using virtual reality displays all the forces acting on nanoparticle during the manipulation. The simulation is developed for particles that belong to the Rayleigh region and represents interactions of OT (a laser beam) with the nanoparticle. The laser beam aimed on to the nanoparticle traps the particle by applying optical forces. The trapped particle is then moved by moving the laser beam. The proposed VR based simulation tool with it capabilities can be easily extended and used for creating and open system framework by connecting it to a real OT setup to control nanoparticles manipulation. In addition, a feedback system can be build to increase of precision of movement. Keywords: Virtual Reality, Optical Tweezers

INDEX TERMS: I 6 [Simulation and Modeling]: Applications, Types of Simulation-Visual.

1 Introduction

The disadvantages can be overcome by using OT as a manipulation technique. OT is a non contact method which can be used to grab, move and release particles in three dimensional (3-D) space. In addition, the extreme small magnitudes of the forces produced from OT benefit OT as a most suitable technique to deal with different types of nanoparticles. The transformation of OT from manipulation of single particle to several particles and finally towards nanoassembly has progressed significantly. Some of the new techniques being investigated to achieve nanoassembly using OT are Chemical assembly [1] and Holographic Optical Tweezers [2]. The applications of OT range from bio-medical sciences to chemistry to technology.

Role of Virtual Reality Nanotechnology deals with processes at nanolevel. This

makes it difficult to visualize or perceive the processes in real

time. Virtual Reality (VR) forms an exact tool by providing

access from macro-world to nano-world. The ability of VR to visualize real time processes and provide control to the use by acting as an interface make it the most desirable method to interact with the nano-world. This paper proposes a novel technique of understanding the manipulation of Rayleigh regime nanoparticles using VR simulation techniques.

2 Theory of Optical Tweezers

OT consists of a laser beam focused on the nanoparticle to trap or grab the nanoparticle and place it at a required point or destination. Laser beam has photons which carry momentum. When the beam hits the particle, the photons tend to lose their momentum which is converted as force (radiation force) according to the Law of Conservation of Momentum. This force has two components namely gradient and scattering force. The gradient force attracts the particle towards the laser focus (pulling force) while the scattering force pushes the particles along the direction of the laser beam. Since the beam has number of photons, the resultant force (radiation force) from each photon traps the particle inside the laser beam [3].This trapped particle is then moved or manipulated by moving the laser beam. In order to manipulate any particle, it is necessary to understand all the forces that determine the dynamic state of the particle.

3 Methodology

Methodology involves developing a mathematical model of all the force equations and designing VR simulations. The VR software used in this work is the “EON Professional 5.5 Professional Suite”. Various Forces considered for the manipulation The forces arising from the laser beam are called optical forces. Scattering and gradient forces are called the optical forces. In order to lift the particle and move it, it is important that the pulling force be more than other forces. Apart from optical forces, particle also experiences inter molecular forces due to the interaction between particle and substrate on which it is placed. Therefore, to understand the amount of force required to levitate or lift the particle, it is important to consider all possible forces that a particle might experience during the manipulation process. Various forces that a particle might experience are as follows: Gradient Force, Scattering Force, Van der Waals Force, Capillary Force, Electrostatic Force, Brownian Force, Gravitational Force, and Buoyancy Force.

The force equations are fed into mathematical software (Mathematica) and a mathematical model is developed. The mathematical model is then imported into the VR simulations. When the simulations are run, the equations in the

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mathematical model are triggered and forces are calculated. These force values are applied on nanoparticle through the VR modules. In this way particles are manipulated by applying forces dynamically. Figure 1 shows sample OT model.

Figure 1 Optical Tweezer setup

4 Results

Various stages of the manipulation process and forces active during each stage are shown in this section. Resting Stage:

Figure 2 shows particle initially resting on substrate. The particle is moved from the existing location to another location (in this figure towards group of nanoparticles present at one of the corners of the substrate). The forces active in this stage are intermolecular forces shown as electrostatic (blue vector), capillary (pink vector), van der waals (orange vector) and gravitational force (green vector).

Figure 2 Resting Stage Trapping Stage:

In the trapping stage (Figure 5b) the particle is trapped. The above figure shows the laser beam activated on to the resting nanoparticle. The optical force equations are triggered and are applied onto a particle trapping it towards the focus of the laser beam. As the forces are applied the particle is lifted from the substrate. As the distance between particle and substrate increases, the intermolecular forces between particle and substrate decrease and disappear. The forces that hold the

particle during manipulation are scattering force (red vector), gradient force (grey vector) and the gravitational force (green vector). The gradient force points towards the focus (pyramid in the laser beam) while the scattering force points downwards along the direction of propagation of light. Among the three vectors, the gradient vector exceeds in magnitude.

Figure 3 Trapping Stage Manipulating and Release Stage:

Once the particle is lifted it is then transported to the destination position. Once the particle has reached its destination position the laser beam is turned off and the particle is released from the laser beam. As the particle comes close to the substrate the intermolecular forces arise which can be seen in the below figure in the forms vectors pointing downward direction.

Figure 4 Manipulation and Release Stage

References

[1] Homlin, R., Schiavoni, M., and Whitesides, G., “Assembly of multicomponent, three dimensional structures using optical tweezers”, Chem. Int. Ed., vol.39, no.19, pp 3503-3506, 2000

[2] Korda, P., Spalding, G., Dufresne, E., and Grier, D., “Nanofabrication with holographic optical tweezers”, Rev.Sci.Instr, vol.73, pp.1956-1957, 2002

[3] Zemanek, P., Liska, M., Jonas, A., and Sramek, L., 1999, “Optical trapping of nanoparticles and micro particles by a Gaussian standing wave,” Optics Letters, Vol 24, No 21, pp.1448-1450, 1999

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