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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 17 — Aug. 16, 2010
  • pp: 18483–18491
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Flow-assisted Single-beam Optothermal Manipulation of Microparticles

Yangyang Liu and Andrew W. Poon  »View Author Affiliations


Optics Express, Vol. 18, Issue 17, pp. 18483-18491 (2010)
http://dx.doi.org/10.1364/OE.18.018483


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Abstract

An optothermal tweezer was developed with a single-beam laser at 1550 nm for manipulation of colloidal microparticles. Strong absorption in water can thermally induce a localized flow, which exerts a Stokes’ drag on the particles that complements the gradient force. Long-range capturing of 6 µm polystyrene particles over ~ 176 µm was observed with a tweezing power of ~7 mW. Transportation and levitation, targeted deposition and selective levitation of particles were explored to experimentally demonstrate the versatility of the optothermal tweezer as a multipurpose particle manipulation tool.

© 2010 Optical Society of America

1. Introduction

Single-beam gradient force laser tweezers have been developed and widely used to control and actuate dielectric microparticles and other microscopic objects [1

1. A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm, and S. Chu, “Observation of a single-beam gradient force optical trap for dielectric particles,” Opt. Lett. 11, 288 – 290 (1986), http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-11-5-288. [CrossRef] [PubMed]

, 2

2. D. G. Grier, “A revolution in optical manipulation,” Nature 424, 810 – 816 (2003), http://www.nature.com/nature/journal/v424/n6950/full/nature01935.html. [CrossRef] [PubMed]

]. The momentum transfer that occurs due to scattering can result in a gradient force that confines the microparticles to the focal region of the tweezing beam. The typical trapping range of a conventional laser tweezer is on the order of only a few microns [3

3. A. Ashkin, “Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime,” Opt. Lett. 11, 288 – 290 (1986), http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-11-5-288. [CrossRef] [PubMed]

, 4

4. C. D’Helon, E. W. Dearden, H. Rubinsztein-Dunlop, and N. R. Heckenberg, “Measurement of the optical force and trapping range of a single-beam gradient optical trap for micron-sized latex spheres,” J. Mod. Opt. 41, 595 – 601 (1994). [CrossRef]

]. Trapping beyond 80 µm is possible below the focal plane with a tweezing power as high as 29 mW, but the range is nevertheless limited to the region illuminated by the tweezing laser [5

5. P. Schiro, C. DuBois, and A. Kwok, “Large capture-range of a single-beam gradient optical trap,” Opt. Express 11, 3485–3489 (2003), http://www.opticsinfobase.org/oe/abstract.cfm?id=78205. [CrossRef] [PubMed]

]. In addition, laser-induced heating in optical tweezers is a known issue and has been a matter of concern [6

6. E. J. G. Peterman, F. Gittes, and C. F. Schmidt, “Laser-induced heating in optical traps,” Biophys J. 84, 1308 – 1316 (2003), http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1302707/. [CrossRef] [PubMed]

, 7

7. H. Mao, J. R. Arias-Gonzalez, S. B. Smith, I. Tinoco, Jr., and C. Bustamante, “Temperature control methods in a laser tweezers system,” Biophys J. 89, 1308–1316 (2005), http://www.ncbi.nlm.nih.gov/pubmed/15923237. [CrossRef] [PubMed]

].

A different approach was recently developed for particle manipulation using thermal tweezers [8–12

8. D. Braun and A. Libchaber, “Trapping of DNA by thermophoretic depletion and convection,” Phys. Rev. Lett. 89, 188103 (2002), http://link.aps.org/doi/10.1103/PhysRevLett.89.188103. [CrossRef] [PubMed]

]. Particles in a temperature gradient experience thermophoretic force and move either toward or away from the heated region. With a single-beam laser optically imposing a temperature gradient inside a particle colloid chamber, thermophoresis and convection together can cause particles to aggregate in a layer close to the bottom of the chamber, thus efficiently redistributing the particles on a 2D plane [8

8. D. Braun and A. Libchaber, “Trapping of DNA by thermophoretic depletion and convection,” Phys. Rev. Lett. 89, 188103 (2002), http://link.aps.org/doi/10.1103/PhysRevLett.89.188103. [CrossRef] [PubMed]

, 9

9. S. Duhr and D. Braun, “Two-dimensional colloidal crystals formed by thermophoresis and convection,” Appl. Phys. Lett. 86, 131921 (2005), http://link.aip.org/link/APPLAB/v86/i13/p131921/s1. [CrossRef]

].

We previously proposed an optothermal tweezer for laser manipulation of microparticles in an aqueous medium [13

13. Y. Liu and A. W. Poon, “Optothermal manipulation of colloidal microparticles,” in Proceedings of the Conference on Lasers and Electro-Optics (IEEE and Optical Society of America, San Jose, CA, 2010), JWA76, http://www.opticsinfobase.org/abstract.cfm?URI=CLEO-2010-JWA76.

]. Built upon the basis of a conventional all-optical laser tweezer, the optothermal tweezer employs a laser source at 1550 nm, where the optical absorption in water is significantly higher than at shorter wavelengths that are more commonly used [14

14. G. M. Hale and M. R. Querry, “Optical constants of water in the 200-nm to 200-µm wavelength region”, Appl. Opt. 12, 555 – 563 (1973), http://www.opticsinfobase.org/abstract.cfm?URI=ao-12-3-555. [CrossRef] [PubMed]

]. The temperature increase due to laser absorption, however, does not act directly on the colloidal particles as in the case of thermal tweezers. Instead, we were able to use a laser-driven flow to bring particles from beyond the optical field towards a volume where the optical radiation force is dominant, thus extending the tweezing range.

In this study, we further explored the optical and thermal processes in flow-assisted optothermal manipulation of microparticles. Optical-driven thermal effects were observed in a water-based colloid of 6 µm polystyrene particles, including capturing over a long range of ~176 µm with a tweezing power of ~7 mW and levitation in a segregated particle colloid. In addition to aggregation and trapping alone, the rich dynamics in the vertical direction adds a new dimension to particle manipulation and opens up various possibilities for design of schemes. As a first attempt towards proving the versatility of the optothermal tweezer, we experimentally demonstrated three different modes of operation for potential applications, including 1) levitation and transportation, 2) targeted deposition and 3) selective levitation of microparticles in a water-based colloid.

2. Principles

To state the problem, the configuration of the optothermal tweezer is shown in Fig. 1a. A single-beam laser at 1550 nm is focused vertically into a thin chamber filled with a colloid of deionized water and 6 µm polystyrene particles. The chamber is 125 µm thick and sealed with candle wax. As in the case of conventional optical tweezers, the gradient force creates a potential minimum in both the vertical and the lateral direction. Particles that are very close to the focus of the tweezing beam can thus be trapped in the focal region.

By choosing the tweezing wavelength to be 1550 nm, the strong absorption of the tweezing laser power also excites a thermal process that provides a second tweezing mechanism. Optically induced temperature difference in the water medium results in a pressure gradient through thermal expansion, which, in turn, drives a localized convective flow in the vicinity of the beam focus (Fig. 1b). Particles in the colloid therefore experience a drag force as described by the Stokes’ law. In the lateral direction, the flow can be either radially inward or outward, which facilitates or disturbs capturing of the particles, respectively (Figs. 1c – 1d).

Fig. 1. (a): Schematic of the optothermal tweezer. CCD: charge-coupled device camera; LED: light-emitting diode; EDFA: erbium-doped fiber amplifier. Inset: zoomin view of the microparticle colloid chamber. (b) Illustration of the fluid flow inside the colloid chamber showing lateral plane c with inward flow, plane d with outward flowx and the beam axis e. (c) System potential in the lateral direction with inward flow. (d) System potential in the lateral direction with outward flow. (e) System potential along the beam axis. (Not to scale.)

Owing to the small length scale of the chamber in the vertical direction, structural confinement and surface interactions need to be taken into consideration to approximate the effective overall potential (Fig. 1e). With the focus of the single tweezing beam situating inside the chamber, up to three separate local potential minima could occur, among which only the gradient force trap has stable three-dimensional confinement near the beam focus. Gravity, surface attraction and upward Stokes’ drag could create two other minima in the vertical direction close to the bottom and the top of the chamber, which could potentially lead to segregation of the particles into two layers.

The different confinement and range scales of the optical gradient force and the Stokes’ drag offer more complexity than conventional optical tweezers to enable new possibilities in particle manipulation. By adjusting the position of the tweezing beam, it is therefore possible to control the spatial distribution and dynamics of microparticles through properly shaping the system potential.

3. Thermal-induced effects

3.1. Long-range capturing

Figures 2a – 2d shows flow-assisted long-range capturing of a 6 µm polystyrene particle. With a stationary beam of ~7 mW positioned at the upper right corner, the particle (circled in red) is captured from a distance of ~176 µm away from the tweezing beam (note that the maximum capture range under the specific configuration is larger than this recorded range). Figure 2e shows the velocity and acceleration of a different captured particle as function of its distance from the beam focus. The effect of the Stokes’ drag is evidenced by the facts that the capture range is much larger than the beam radius of ~5 µm, as shown in the inset, which limits the tweezing range of the optical gradient force, and that the particle maintains a much smaller but relatively constant velocity beyond ~10 µm away from the beam focus.

Fig. 2. (a) – (d) Long-range capturing of a 6 µm microparticle (circled in red) with a stationary laser beam of ~7 mW from over 176 µm away from the beam position. (e) Velocity and acceleration of a particle being trapped as a function of its distance to the beam focal axis. Inset: Beam radius near the focus. The image shows a beam spot captured by an infrared camera with its calculated center and 1/e radius.

In this case, the system configuration allows a radially inward water flow to bring the particle into the focal region, where the gradient force then takes effect to further accelerate it towards the beam focus. Effectively, particles that are originally too far from the focus to be captured by the gradient force alone can now fall along a potential curved tilted by the flow drag that is longer in range (cf. Fig. 1c). The defocusing of the captured particle in Fig. 2d is due to its levitation caused by a vertically upward flow near the beam focus, which will be discussed in the following section.

3.2. Segregation and levitation

With the pumping from the optothermal tweezer, particles in the colloid can segregate into two distinctive layers with a vertical separation of ~90 µm in between (Fig. 3a, cf. Fig. 1e) under thermophoretic repulsion from the most heated region close to the laser focus. In the lower layer, particles are attracted toward and disappear at the focal axis of the tweezing beam, while in the upper layer, particles emerge at and are repelled away from the focal axis (Fig. 3b, (Media1Media2), cf. Figs. 1c – 1d).

The disappearing and reemerging of particles is a direct result of levitation of particles from the lower layer to the upper layer, which is driven by a thermal-induced circulation that flows vertically upward between the two layers. Figure 3c shows the transient levitation rate in a particle colloid of a fixed concentration for a tweezing power of ~4.2 mW, ~5.6 mW, ~ 7 mW and ~8.4 mW. The levitation rate increases exponentially as a function of time elapsed after the tweezing laser beam is turned on during the first 1800 seconds, following r(Pop, t) = R(Pop) exp [t/τ (Pop)], where r(Pop, t) is the levitation rate at time instance t with a tweezing laser power of Pop, R(Pop) and τ (Pop) are the instantaneous levitation rate and stabilization time constants for a given tweezing power Pop. Experimentally, r(Pop, t) can be obtained by taking the inverse of the average time between two consecutive occurrences of particle levitation with a tweezing power of Pop at time t. The measured values of R and τ in Fig. 3c are summarized in Table 1, which shows the dependency of the levitation rate on the optical tweezing power.

Table 1. Instantaneous levitation rate R and flow stabilization time τ for some values of tweezing power Pop in optothermal levitation of 6 µm polystyrene particles.

table-icon
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4. Applications

Apart from varying the tweezing power, a different way to change the flow conditions and thus to modify the system potential for various particle manipulation applications is to position the tweezing beam focus at different vertical locations in the colloid chamber. Here we present the experimental configurations for three different modes of the optothermal tweezer.

4.1. Levitation and transportation

Figure 4 shows the levitation and transportation mode of the optothermal tweezer. With the tweezing beam focused at the upper layer in a segregated colloid (Fig. 4a), particles in the lower layer can follow the potential slope tilted by the upward fluid flow to reach the gradient-force trap, where its three-dimensional confinement then allows for transportation of the particle in the lateral direction (Fig. 4b).

Figure 4c (Media3) shows an experimental demonstration of this levitation and transportation process. Initially, there are two particles (circled in green) in the upper layer A and one particle (circled in red) in the lower layer B. After the laser is turned on and focused to layer A, the particle circled in red, being the closest one to the beam focal axis, gets attracted toward and levitated along the beam focal axis to appear in the upper layer A. The beam is then steered laterally relative to the colloid chamber (which is experimentally done by moving the substrate containing the chamber while keeping the tweezing laser beam stationary), and the particle circled in red, which is now trapped by gradient force, follows the movement of the beam and is transported past the two particles circled in green.

Fig. 3. (a) Levitation of 6 µm particles in a colloid segregated into two layers A and B. (b) First row: a particle (circled in red) appears at the center (beam focal axis) of the upper layer A (Media1); second row: a different particle moves towards the center and disappears from the lower layer B (Media2). (c) Transient levitation rate for a tweezing power of ~4.2 mW, ~5.6 mW, ~7 mW and ~8.4 mW.

Compared with conventional optical tweezers, the levitation and transportation mode of the optothermal tweezer provides a faster and more efficient way for particle manipulation applications where the selection of particles is not critical. The process of locating and trapping particles with the short-range gradient force is simplified by the flow-assisted long-range capturing and levitation of particles, which sustain a highly localized particle flux that fall into the gradient-force potential trap as they rise and stop at the beam focus, and only the subsequent transportation of the trapped particle alone would require steering of the tweezing beam.

Fig. 4. Levitation and transportation mode. (a) Tweezer configuration. Particles in the colloid are segregated into layers A and B. (b) System potential in the vertical direction. (c) First row: layer A; second row: layer B. The particle circled in red is attracted toward the beam focus (columns 1 – 2), levitated from B to A (column 3) and transported past the two particles circled in green (columns 4 – 6) (Media3).

4.2. Targeted deposition

Figure 5 shows a slight modification to the levitation and transportation mode. With the beam focus just below the upper layer in the colloid (Fig. 5a), the gradient-force trap can be positioned at a location away from the global potential minimum (Fig. 5b), thus levitating and depositing particles without trapping them. Particles roll along the potential slope past the gradient-force trap to reach the global potential minimum, where there is no longer confinement in the lateral direction, as they rise through the beam focus to stop at the upper layer. Therefore, subsequent movement of the tweezing laser beam does not affect the position of particles already levitated, and this effectively implements the targeted deposition mode. To create a pattern, we simply need to move the tweezing beam in the same lateral plane as if “writing” with particles. Figure 5c (Media4) shows an initial demonstration of the targeted deposition process. Methods need to be explored in order to hold the deposited particles in place on the upper plane.

4.3. Selective levitation

Figure 6 shows the selective levitation mode of the optothermal tweezer. In this mode, the tweezing beam is focused just slightly above the particle to be levitated in the lower layer of a segregated particle colloid (Fig. 6a). With a strong yet highly localized upward flow at the particle position, it is possible for the particle to gain sufficient energy to escape from the gradient-force trap and for levitation to occur (Fig. 6b). The beam focus needs to be some vertical distance away from the particle for the flow to transfer enough kinetic energy to the particle, yet, on the other hand, localization of the upward flow requires that the focus is positioned close to the particle.

Fig. 5. Targeted deposition mode. (a) Tweezer configuration. (b) System potential in the vertical direction. (c) The letter “T” is written in the upper layer of the segregated colloid (Media4).

Figure 6c (Media5) shows the experimental demonstration of the selective levitation mode. The tweezing beam is focused very close to the particle circled in red, which is levitated and released from the lower layer. Nearby particles that are ~10 µm away from the levitated particle are not much affected by this action. Ideally, it may be possible to hold a particle at the gradientforce trap as a marker of the beam focus position. This selective levitation process specifically makes use of the temporal short-term effect of the optothermal tweezer as described by the instantaneous levitation rate R(Pop) (cf. Section 4.1 and Fig. 3c) and can be potentially used as a reverse operation of the targeted deposition mode to create negative patterns in the lower layer of the colloid chamber.

5. Conclusion

We demonstrated flow-assisted single-beam optothermal manipulation of 6 µm polystyrene particles in a water-based colloid using a tweezing laser at 1550 nm. We observed thermal effects induced by the strong optical absorption in water at this wavelength, including 1) longrange capturing of particles over a distance of ~176 µm from the tweezing laser focal axis using a stationary beam of ~7mW, 2) segregation of particles in the colloid into two distinctive layers and 3) highly localized vertical levitation of the particles from the lower layer to the upper layer. The levitation rate can be controlled optically through varying the tweezing laser power.

We further implemented three modes of optothermal manipulation for various potential applications. With the tweezing beam focus positioned at the upper layer of a segregated colloid, particles can be captured and levitated from the lower layer and transported laterally in the upper layer. Lowering the position of the focus would remove the confinement in the lateral direction and particles can be deposited at desired locations by laterally steering the tweezing beam. Selective levitation of particles in the lower layer was also realized by directing the beam focus slightly above the particle to be levitated.

The optothermal tweezer has proven its potential as a particle manipulation tool, and we expect future studies to focus on 1) finite element simulation of the optical, thermal and flow processes, 2) experimental measurements and quantitative analysis of particle drift due to different effects and 3) applications of the optothermal tweezer in opthofluidic and biomedical applications.

Fig. 6. Selective levitation mode. (a) Tweezer configuration. (b) System potential in the vertical direction. (c) The particle circled in red is levitated from the lower layer, while other nearby particles are not much affected (Media5).

Acknowledgements

We thank Prof. Weijia Wen and his group for facilitating some of the experiments, Seng Fatt Liew (currently at Yale University) for early development of this project and the Amonics Ltd. for providing a demo module EDFA. This work was supported by the Undergraduate Research Opportunities Program and the Department of Electronic and Computer Engineering at the Hong Kong University of Science and Technology.

References and links

1.

A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm, and S. Chu, “Observation of a single-beam gradient force optical trap for dielectric particles,” Opt. Lett. 11, 288 – 290 (1986), http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-11-5-288. [CrossRef] [PubMed]

2.

D. G. Grier, “A revolution in optical manipulation,” Nature 424, 810 – 816 (2003), http://www.nature.com/nature/journal/v424/n6950/full/nature01935.html. [CrossRef] [PubMed]

3.

A. Ashkin, “Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime,” Opt. Lett. 11, 288 – 290 (1986), http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-11-5-288. [CrossRef] [PubMed]

4.

C. D’Helon, E. W. Dearden, H. Rubinsztein-Dunlop, and N. R. Heckenberg, “Measurement of the optical force and trapping range of a single-beam gradient optical trap for micron-sized latex spheres,” J. Mod. Opt. 41, 595 – 601 (1994). [CrossRef]

5.

P. Schiro, C. DuBois, and A. Kwok, “Large capture-range of a single-beam gradient optical trap,” Opt. Express 11, 3485–3489 (2003), http://www.opticsinfobase.org/oe/abstract.cfm?id=78205. [CrossRef] [PubMed]

6.

E. J. G. Peterman, F. Gittes, and C. F. Schmidt, “Laser-induced heating in optical traps,” Biophys J. 84, 1308 – 1316 (2003), http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1302707/. [CrossRef] [PubMed]

7.

H. Mao, J. R. Arias-Gonzalez, S. B. Smith, I. Tinoco, Jr., and C. Bustamante, “Temperature control methods in a laser tweezers system,” Biophys J. 89, 1308–1316 (2005), http://www.ncbi.nlm.nih.gov/pubmed/15923237. [CrossRef] [PubMed]

8.

D. Braun and A. Libchaber, “Trapping of DNA by thermophoretic depletion and convection,” Phys. Rev. Lett. 89, 188103 (2002), http://link.aps.org/doi/10.1103/PhysRevLett.89.188103. [CrossRef] [PubMed]

9.

S. Duhr and D. Braun, “Two-dimensional colloidal crystals formed by thermophoresis and convection,” Appl. Phys. Lett. 86, 131921 (2005), http://link.aip.org/link/APPLAB/v86/i13/p131921/s1. [CrossRef]

10.

D. R. Mason, D. K. Gramotnev, and G. Gramotnev, “Thermal tweezers for manipulation of adatoms and nanoparticles on surfaces heated by interfering laser pulses,” J. Appl. Phys. 104, 064320 (2008), http://link.aip.org/link/JAPIAU/v104/i6/p064320/s1. [CrossRef]

11.

R. D. Leonardo, F. Ianni, and G. Ruocco, “Colloidal attraction induced by a temperature gradient,” Langmuir 25, 4247 – 4250 (2009), http://pubs.acs.org/doi/abs/10.1021/la8038335. [CrossRef] [PubMed]

12.

C. B. Mast and D. Braun, “Thermal trap for DNA replication,” Phys. Rev. Lett. 104, 188102 (2010), http://link.aps.org/doi/10.1103/PhysRevLett.104.188102. [CrossRef] [PubMed]

13.

Y. Liu and A. W. Poon, “Optothermal manipulation of colloidal microparticles,” in Proceedings of the Conference on Lasers and Electro-Optics (IEEE and Optical Society of America, San Jose, CA, 2010), JWA76, http://www.opticsinfobase.org/abstract.cfm?URI=CLEO-2010-JWA76.

14.

G. M. Hale and M. R. Querry, “Optical constants of water in the 200-nm to 200-µm wavelength region”, Appl. Opt. 12, 555 – 563 (1973), http://www.opticsinfobase.org/abstract.cfm?URI=ao-12-3-555. [CrossRef] [PubMed]

OCIS Codes
(140.6810) Lasers and laser optics : Thermal effects
(140.7010) Lasers and laser optics : Laser trapping
(350.4855) Other areas of optics : Optical tweezers or optical manipulation

ToC Category:
Optical Trapping and Manipulation

History
Original Manuscript: June 1, 2010
Revised Manuscript: July 31, 2010
Manuscript Accepted: August 2, 2010
Published: August 13, 2010

Virtual Issues
Vol. 5, Iss. 13 Virtual Journal for Biomedical Optics

Citation
Yangyang Liu and Andrew W. Poon, "Flow-assisted Single-beam Optothermal Manipulation of Microparticles," Opt. Express 18, 18483-18491 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-17-18483


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References

  1. A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm, and S. Chu, “Observation of a single-beam gradient force optical trap for dielectric particles,” Opt. Lett. 11, 288 – 290 (1986), http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-11-5-288. [CrossRef] [PubMed]
  2. D. G. Grier, “A revolution in optical manipulation,” Nature 424, 810 – 816 (2003), http://www.nature.com/nature/journal/v424/n6950/full/nature01935.html. [CrossRef] [PubMed]
  3. A. Ashkin, “Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime,” Opt. Lett. 11, 288 – 290 (1986), http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-11-5-288. [CrossRef] [PubMed]
  4. C. D’Helon, E. W. Dearden, H. Rubinsztein-Dunlop, and N. R. Heckenberg, “Measurement of the optical force and trapping range of a single-beam gradient optical trap for micron-sized latex spheres,” J. Mod. Opt. 41, 595 – 601 (1994). [CrossRef]
  5. P. Schiro, C. DuBois, and A. Kwok, “Large capture-range of a single-beam gradient optical trap,” Opt. Express 11, 3485-3489 (2003), http://www.opticsinfobase.org/oe/abstract.cfm?id=78205. [CrossRef] [PubMed]
  6. E. J. G. Peterman, F. Gittes, and C. F. Schmidt, “Laser-induced heating in optical traps,” Biophys J. 84, 1308 –1316 (2003), http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1302707/. [CrossRef] [PubMed]
  7. H. Mao, J. R. Arias-Gonzalez, S. B. Smith, I. Tinoco, Jr., and C. Bustamante, “Temperature control methods in a laser tweezers system,” Biophys J. 89, 1308–1316 (2005), http://www.ncbi.nlm.nih.gov/pubmed/15923237. [CrossRef] [PubMed]
  8. D. Braun, and A. Libchaber, “Trapping of DNA by thermophoretic depletion and convection,” Phys. Rev. Lett. 89, 188103 (2002), http://link.aps.org/doi/10.1103/PhysRevLett.89.188103. [CrossRef] [PubMed]
  9. S. Duhr, and D. Braun, “Two-dimensional colloidal crystals formed by thermophoresis and convection,” Appl. Phys. Lett. 86, 131921 (2005), http://link.aip.org/link/APPLAB/v86/i13/p131921/s1. [CrossRef]
  10. D. R. Mason, D. K. Gramotnev, and G. Gramotnev,“Thermal tweezers for manipulation of adatoms and nanoparticles on surfaces heated by interfering laser pulses,” J. Appl. Phys. 104, 064320 (2008), http://link.aip.org/link/JAPIAU/v104/i6/p064320/s1. [CrossRef]
  11. R. D. Leonardo, F. Ianni, and G. Ruocco, “Colloidal attraction induced by a temperature gradient,” Langmuir 25, 4247 - 4250 (2009), http://pubs.acs.org/doi/abs/10.1021/la8038335. [CrossRef] [PubMed]
  12. C. B. Mast, and D. Braun, “Thermal trap for DNA replication,” Phys. Rev. Lett. 104, 188102 (2010), http://link.aps.org/doi/10.1103/PhysRevLett.104.188102. [CrossRef] [PubMed]
  13. Y. Liu and A. W. Poon, “Optothermal manipulation of colloidal microparticles,” in Proceedings of the Conference on Lasers and Electro-Optics (IEEE and Optical Society of America, San Jose, CA, 2010), JWA76, http://www.opticsinfobase.org/abstract.cfm?URI=CLEO-2010-JWA76.
  14. G. M. Hale and M. R. Querry, “Optical constants of water in the 200-nm to 200-m wavelength region,” Appl. Opt. 12, 555 – 563 (1973), http://www.opticsinfobase.org/abstract.cfm?URI=ao-12-3-555. [CrossRef] [PubMed]

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