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Controlling dispersion forces between small particles with artificially created random light fields.

Brügger G, Froufe-Pérez LS, Scheffold F, José Sáenz J - Nat Commun (2015)

Bottom Line: These interaction forces are usually strongly anisotropic depending on the interference landscape of the external fields.Here we show, both theoretically and experimentally, that dispersion forces between small colloidal particles can also be induced and controlled using artificially created fluctuating light fields.Using optical tweezers as a gauge, we present experimental evidence for the predicted isotropic attractive interactions between dielectric microspheres induced by laser-generated, random light fields.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Fribourg, Chemin du Musée 3, Fribourg CH-1700, Switzerland.

ABSTRACT
Appropriate combinations of laser beams can be used to trap and manipulate small particles with optical tweezers as well as to induce significant optical binding forces between particles. These interaction forces are usually strongly anisotropic depending on the interference landscape of the external fields. This is in contrast with the familiar isotropic, translationally invariant, van der Waals and, in general, Casimir-Lifshitz interactions between neutral bodies arising from random electromagnetic waves generated by equilibrium quantum and thermal fluctuations. Here we show, both theoretically and experimentally, that dispersion forces between small colloidal particles can also be induced and controlled using artificially created fluctuating light fields. Using optical tweezers as a gauge, we present experimental evidence for the predicted isotropic attractive interactions between dielectric microspheres induced by laser-generated, random light fields. These light-induced interactions open a path towards the control of translationally invariant interactions with tuneable strength and range in colloidal systems.

No MeSH data available.


Related in: MedlinePlus

Optical binding between dielectric microspheres in random light fields.From the analysis of the thermal motion in the two adjacent optical traps, the particle interaction potential U(D) is obtained experimentally. The melamine (n=1.68) microspheres have sizes 2R=2 μm (a), 2R=3 μm (b) and 2R=4 μm (c). All particles are suspended in a 2.7 mM KCl aqueous solution. In the figure, the measured interaction potential is shown for three different laser power settings P in each panel. The vertical lines show the estimated contact positions and the experimental uncertainty ΔD∼±5 nm. By numerical evaluation of equation (6), we can fit the data and then extrapolate U(D; P) to obtain values for the contact potential U0 as a function of particle size 2R and laser power P. The inset shows an enlarged plot for 2R=4 μm and P=5 W. Notice that the depicted experimental graphs U(D) reflect an average of 16 independent measurements including s.d.
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f3: Optical binding between dielectric microspheres in random light fields.From the analysis of the thermal motion in the two adjacent optical traps, the particle interaction potential U(D) is obtained experimentally. The melamine (n=1.68) microspheres have sizes 2R=2 μm (a), 2R=3 μm (b) and 2R=4 μm (c). All particles are suspended in a 2.7 mM KCl aqueous solution. In the figure, the measured interaction potential is shown for three different laser power settings P in each panel. The vertical lines show the estimated contact positions and the experimental uncertainty ΔD∼±5 nm. By numerical evaluation of equation (6), we can fit the data and then extrapolate U(D; P) to obtain values for the contact potential U0 as a function of particle size 2R and laser power P. The inset shows an enlarged plot for 2R=4 μm and P=5 W. Notice that the depicted experimental graphs U(D) reflect an average of 16 independent measurements including s.d.

Mentions: The final results for U(D=r−2R), shown in Fig. 3, are obtained by averaging over 16 independent experimental runs carried out under the same conditions. We find clear evidence for OB in a random light field. Moreover, we are able to quantify the potential U(D=r−2R) as well as its dependence on the incident light power. Varying the laser power from 5.0 to 1.0 W weakens the attractive potential. For an estimate of the contact potential U0≡U(D=0), we adjust the prefactor in equation (6) for a best fit to the data as shown in Fig. 3. The overall agreement between experiments and theory is remarkable, except for the predicted oscillation of the interaction potential at large interparticle distances. We believe the lack of clear oscillations can be explained by the approximations made when deriving equation (6) in the first Born (Rayleigh–Gans) approximation. The predicted oscillations are due to constructive and destructive interferences when summing over equidistant pairs of elementary dipolar scatters in the particle. However, for higher refractive indexes these interferences are smeared out, as equidistant pairs of dipoles positioned at different distances from the particle surface do not contribute in the same way anymore. An additional contribution that might affect the comparison between theory and experiment is the absence of a full 4π isotropic illumination in the experiment due to the absence of incoming photons with momentum near parallel to the mirror.


Controlling dispersion forces between small particles with artificially created random light fields.

Brügger G, Froufe-Pérez LS, Scheffold F, José Sáenz J - Nat Commun (2015)

Optical binding between dielectric microspheres in random light fields.From the analysis of the thermal motion in the two adjacent optical traps, the particle interaction potential U(D) is obtained experimentally. The melamine (n=1.68) microspheres have sizes 2R=2 μm (a), 2R=3 μm (b) and 2R=4 μm (c). All particles are suspended in a 2.7 mM KCl aqueous solution. In the figure, the measured interaction potential is shown for three different laser power settings P in each panel. The vertical lines show the estimated contact positions and the experimental uncertainty ΔD∼±5 nm. By numerical evaluation of equation (6), we can fit the data and then extrapolate U(D; P) to obtain values for the contact potential U0 as a function of particle size 2R and laser power P. The inset shows an enlarged plot for 2R=4 μm and P=5 W. Notice that the depicted experimental graphs U(D) reflect an average of 16 independent measurements including s.d.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4557368&req=5

f3: Optical binding between dielectric microspheres in random light fields.From the analysis of the thermal motion in the two adjacent optical traps, the particle interaction potential U(D) is obtained experimentally. The melamine (n=1.68) microspheres have sizes 2R=2 μm (a), 2R=3 μm (b) and 2R=4 μm (c). All particles are suspended in a 2.7 mM KCl aqueous solution. In the figure, the measured interaction potential is shown for three different laser power settings P in each panel. The vertical lines show the estimated contact positions and the experimental uncertainty ΔD∼±5 nm. By numerical evaluation of equation (6), we can fit the data and then extrapolate U(D; P) to obtain values for the contact potential U0 as a function of particle size 2R and laser power P. The inset shows an enlarged plot for 2R=4 μm and P=5 W. Notice that the depicted experimental graphs U(D) reflect an average of 16 independent measurements including s.d.
Mentions: The final results for U(D=r−2R), shown in Fig. 3, are obtained by averaging over 16 independent experimental runs carried out under the same conditions. We find clear evidence for OB in a random light field. Moreover, we are able to quantify the potential U(D=r−2R) as well as its dependence on the incident light power. Varying the laser power from 5.0 to 1.0 W weakens the attractive potential. For an estimate of the contact potential U0≡U(D=0), we adjust the prefactor in equation (6) for a best fit to the data as shown in Fig. 3. The overall agreement between experiments and theory is remarkable, except for the predicted oscillation of the interaction potential at large interparticle distances. We believe the lack of clear oscillations can be explained by the approximations made when deriving equation (6) in the first Born (Rayleigh–Gans) approximation. The predicted oscillations are due to constructive and destructive interferences when summing over equidistant pairs of elementary dipolar scatters in the particle. However, for higher refractive indexes these interferences are smeared out, as equidistant pairs of dipoles positioned at different distances from the particle surface do not contribute in the same way anymore. An additional contribution that might affect the comparison between theory and experiment is the absence of a full 4π isotropic illumination in the experiment due to the absence of incoming photons with momentum near parallel to the mirror.

Bottom Line: These interaction forces are usually strongly anisotropic depending on the interference landscape of the external fields.Here we show, both theoretically and experimentally, that dispersion forces between small colloidal particles can also be induced and controlled using artificially created fluctuating light fields.Using optical tweezers as a gauge, we present experimental evidence for the predicted isotropic attractive interactions between dielectric microspheres induced by laser-generated, random light fields.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Fribourg, Chemin du Musée 3, Fribourg CH-1700, Switzerland.

ABSTRACT
Appropriate combinations of laser beams can be used to trap and manipulate small particles with optical tweezers as well as to induce significant optical binding forces between particles. These interaction forces are usually strongly anisotropic depending on the interference landscape of the external fields. This is in contrast with the familiar isotropic, translationally invariant, van der Waals and, in general, Casimir-Lifshitz interactions between neutral bodies arising from random electromagnetic waves generated by equilibrium quantum and thermal fluctuations. Here we show, both theoretically and experimentally, that dispersion forces between small colloidal particles can also be induced and controlled using artificially created fluctuating light fields. Using optical tweezers as a gauge, we present experimental evidence for the predicted isotropic attractive interactions between dielectric microspheres induced by laser-generated, random light fields. These light-induced interactions open a path towards the control of translationally invariant interactions with tuneable strength and range in colloidal systems.

No MeSH data available.


Related in: MedlinePlus