Article – Page 13 – Soft Matter Lab

Simulation of a Particle in an Optical Trap published in Am. J. Phys.

Simulation of a Brownian particle in an optical trap
Giorgio Volpe & Giovanni Volpe
American Journal of Physics 81(3), 224—230 (2013)
DOI: 10.1119/1.4772632

An optically trapped Brownian particle is a sensitive probe of molecular and nanoscopic forces. An understanding of its motion, which is caused by the interplay of random and deterministic contributions, can lead to greater physical insight into the behavior of stochastic phenomena. The modeling of realistic stochastic processes typically requires advanced mathematical tools. We discuss a finite difference algorithm to compute the motion of an optically trapped particle and the numerical treatment of the white noise term. We then treat the transition from the ballistic to the diffusive regime due to the presence of inertial effects on short time scales and examine the effect of an optical trap on the motion of the particle. We also outline how to use simulations of optically trapped Brownian particles to gain understanding of nanoscale force and torque measurements, and of more complex phenomena, such as Kramers transitions, stochastic resonant damping, and stochastic resonance.

Thermophoresis Driven by Coloured Noise published in EPL

Thermophoresis of Brownian particles driven by coloured noise
Scott Hottovy, Giovanni Volpe & Jan Wehr
EPL (Europhysics Letters) 99(6), 60002 (2012)
DOI: 10.1209/0295-5075/99/60002
arXiv: 1205.1093

Brownian motion of microscopic particles is driven by collisions with surrounding fluid molecules. The resulting noise is not white, but coloured, due, e.g., to the presence of hydrodynamic memory. The noise characteristic time-scale is typically of the same order of magnitude as the inertial time-scale over which the particle’s kinetic energy is lost due to friction. We demonstrate theoretically that, in the presence of a temperature gradient, the interplay between these two characteristic time-scales can have measurable consequences on the particle’s long-time behaviour. Using homogenization theory, we analyse the infinitesimal generator of the stochastic differential equation describing the system in the limit where the two time-scales are taken to zero keeping their ratio constant and derive the thermophoretic transport coefficient, which, we find, can vary in both magnitude and sign, as observed in experiments. Studying the long-term stationary particle distribution, we show that particles accumulate towards the colder (positive thermophoresis) or the hotter (negative thermophoresis) regions depending on their physical parameters.

Active Brownian Motion Tunable by Light published in J. Phys. Condens. Matter

Active Brownian motion tunable by light
Ivo Buttinoni, Giovanni Volpe, Felix Kümmel, Giorgio Volpe & Clemens Bechinger
Journal of Physics: Condensed Matter 24(28), 284129 (2012)
DOI: 10.1088/0953-8984/24/28/284129
arXiv: 1110.2202

Active Brownian particles are capable of taking up energy from their environment and converting it into directed motion; examples range from chemotactic cells and bacteria to artificial micro-swimmers. We have recently demonstrated that Janus particles, i.e. gold-capped colloidal spheres, suspended in a critical binary liquid mixture perform active Brownian motion when illuminated by light. In this paper, we investigate in more detail their swimming mechanism, leading to active Brownian motion. We show that the illumination-borne heating induces a local asymmetric demixing of the binary mixture, generating a spatial chemical concentration gradient which is responsible for the particle’s self-diffusiophoretic motion. We study this effect as a function of the functionalization of the gold cap, the particle size and the illumination intensity: the functionalization determines what component of the binary mixture is preferentially adsorbed at the cap and the swimming direction (towards or away from the cap); the particle size determines the rotational diffusion and, therefore, the random reorientation of the particle; and the intensity tunes the strength of the heating and, therefore, of the motion. Finally, we harness this dependence of the swimming strength on the illumination intensity to investigate the behavior of a micro-swimmer in a spatial light gradient, where its swimming properties are space-dependent.

Noise-induced drift in SDEs published in J. Stat. Phys.

Noise-induced drift in stochastic differential equations with arbitrary friction and diffusion in the Smoluchowski-Kramers limit
Scott Hottovy, Giovanni Volpe & Jan Wehr
Journal of Statistical Physics 146(4), 762—773 (2012)
DOI: 10.1007/s10955-012-0418-9
arXiv: 1112.2607

We consider the dynamics of systems with arbitrary friction and diffusion. These include, as a special case, systems for which friction and diffusion are connected by Einstein fluctuation-dissipation relation, e.g. Brownian motion. We study the limit where friction effects dominate the inertia, i.e. where the mass goes to zero (Smoluchowski-Kramers limit). Using the Itô stochastic integral convention, we show that the limiting effective Langevin equations has different drift fields depending on the relation between friction and diffusion. Alternatively, our results can be cast as different interpretations of stochastic integration in the limiting equation, which can be parametrized by α∈ℝ. Interestingly, in addition to the classical Itô (α=0), Stratonovich (α=0.5) and anti-Itô (α=1) integrals, we show that position-dependent α=α(x), and even stochastic integrals with α∉[0,1] arise. Our findings are supported by numerical simulations.

Microswimmers in Patterned Environments published in Soft Matter

Microswimmers in patterned environments
Giovanni Volpe, Ivo Buttinoni, Dominik Vogt, Hans-Jürgen Kümmerer & Clemens Bechinger
Soft Matter 7(19), 8810—8815 (2011)
DOI: 10.1039/C1SM05960B
arXiv: 1104.3203

Tiny self-propelled swimmers capable of autonomous navigation through complex environments provide appealing opportunities for localization, pick-up and delivery of microscopic and nanoscopic objects. Inspired by motile cells and bacteria, man-made microswimmers have been created and their motion in homogeneous environments has been studied. As a first step towards more realistic conditions under which such microswimmers will be employed, here we study, experimentally and with numerical simulations, their behavior in patterned surroundings that present complex spatial features where frequent encounters with obstacles become important. To study the microswimmers as a function of their swimming behavior, we develop a novel species of microswimmers whose active motion is due to the local demixing of a critical binary liquid mixture and can be easily tuned by illumination. We show that, when microswimmers are confined to a single pore whose diameter is comparable with their swimming length, the probability of finding them at the confinement walls significantly increases compared to Brownian particles. Furthermore, in the presence of an array of periodically arranged obstacles, microswimmers can steer even perpendicularly to an applied force. Since such behavior is very sensitive to the details of their specific swimming style, it can be employed to develop advanced sorting, classification and dialysis techniques.

Reply to Comment on Influence of Noise on Force Measurements published in Phys. Rev. Lett.

Reply to comment on “Influence of noise on force measurements”
Giovanni Volpe, Laurent Helden, Thomas Brettschneider, Jan Wehr & Clemens Bechinger
Physical Review Letters 107(7), 078902 (2011)
DOI: 10.1103/PhysRevLett.107.078902
arXiv: 1101.3916

Comparison Between Force Measurement Methods published in Phys. Rev. E

Force measurement in the presence of Brownian noise: Equilibrium distribution method vs. drift method
Thomas Brettschneider, Giovanni Volpe, Laurent Helden, Jan Wehr & Clemens Bechinger
Physical Review E 83(4), 041113 (2011)
DOI: 10.1103/PhysRevE.83.041113
arXiv: 1009.2386

The study of microsystems and the development of nanotechnologies require alternative techniques to measure piconewton and femtonewton forces at microscopic and nanoscopic scales. Among the challenges is the need to deal with the ineluctable thermal noise, which, in the typical experimental situation of a spatial diffusion gradient, causes a spurious drift. This leads to a correction term when forces are estimated from drift measurements [G. Volpe, L. Helden, T. Brettschneider, J. Wehr, and C. Bechinger, Phys. Rev. Lett. 104, 170602 (2010)]. Here we provide a systematic study of such an effect by comparing the forces acting on various Brownian particles derived from equilibrium-distribution and drift measurements. We discuss the physical origin of the correction term, its dependence on wall distance and particle radius, and its relation to the convention used to solve the respective stochastic integrals. Such a correction term becomes more significant for smaller particles and is predicted to be on the order of several piconewtons for particles the size of a biomolecule.

Work Done by Rotational Force Fields published in J. Opt.

Influence of rotational force fields on the determination of the work done on a driven Brownian particle
Giuseppe Pesce, Giovanni Volpe, Alberto Imparato, Giulia Rusciano & Antonio Sasso
Journal of Optics 13(4), 044006 (2011)
DOI: 10.1088/2040-8978/13/4/044006
arXiv: 1006.4534

For a Brownian system the evolution of thermodynamic quantities is a stochastic process, in particular the work performed on a driven colloidal particle held in an optical trap, changes for each realization of the experimental manipulation, even though the manipulation protocol remains unchanged. Nevertheless, the work distribution is governed by established laws. Here, we show how the measurement of the work distribution is influenced by the presence of rotational, i.e. nonconservative, radiation forces. Experiments on particles of different materials show that the rotational radiation forces, and therefore their effect on the work distributions, increase with the particle’s refractive index.

Fractal Plasmonics published in Opt. Express

Fractal plasmonics: Subdiffraction focusing and broadband spectral response by a Sierpisky nanocarpet
Giorgio Volpe, Giovanni Volpe & Romain Quidant
Optics Express 19(4), 3612—3618 (2011)
DOI: 10.1364/OE.19.003612

Plasmonic nanostructures offer a great potential to enhance light-matter interaction at the nanometer scale. The response upon illumination at a given wavelength and polarization is governed by the characteristic lengths associated to the shape and size of the nanostructure. Here, we propose the use of engineered fractal plasmonic structures to extend the degrees of freedom and the parameters available for their design. In particular, we focus on a paradigmatic fractal geometry, namely the Sierpinski carpet. We explore the possibility of using it to achieve a controlled broadband spectral response by controlling the degree of its fractal complexity. Furthermore, we investigate some other arising properties, such as subdiffraction limited focusing and its potential use for optical trapping of nano-objects. An attractive advantage of the focusing over more standard geometries, such as gap antennas, is that it occurs away from the metal surface (≈ 80nm) at the center of the nanostructure, leaving an open space accessible to objects for enhanced light-matter interaction.

Influence of Noise on Force Measurements published in Phys. Rev. Lett.

Influence of noise on force measurements
Giovanni Volpe, Laurent Helden, Thomas Brettschneider, Jan Wehr & Clemens Bechinger
Physical Review Letters 104(17), 170602 (2010)
DOI: 10.1103/PhysRevLett.104.170602
arXiv: 1004.0874

We demonstrate how the ineluctable presence of thermal noise alters the measurement of forces acting on microscopic and nanoscopic objects. We quantify this effect exemplarily for a Brownian particle near a wall subjected to gravitational and electrostatic forces. Our results demonstrate that the force-measurement process is prone to artifacts if the noise is not correctly taken into account.