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Controlled Mechanical Motions of Microparticles in Optical Tweezers micromachines Review Controlled Mechanical Motions of Microparticles in Optical Tweezers Jing Liu 1 and Zhiyuan Li 2,* 1 Institute of Laser and Intelligent Manufacturing Technology, South-Central University for Nationalities, Wuhan 430074, China; [email protected] 2 School of Physics and Optoelectronics, South China University of Technology, Guangzhou 510640, China * Correspondence: [email protected]; Tel.: +86-020-8711-4175 Received: 15 April 2018; Accepted: 9 May 2018; Published: 12 May 2018 Abstract: Optical tweezers, formed by a highly focused laser beam, have intriguing applications in biology and physics. Inspired by molecular rotors, numerous optical beams and artificial particles have been proposed to build optical tweezers trapping microparticles, and extensive experiences have been learned towards constructing precise, stable, flexible and controllable micromachines. The mechanism of interaction between particles and localized light fields is quite different for different types of particles, such as metal particles, dielectric particles and Janus particles. In this article, we present a comprehensive overview of the latest development on the fundamental and application of optical trapping. The emphasis is placed on controllable mechanical motions of particles, including rotation, translation and their mutual coupling under the optical forces and torques created by a wide variety of optical tweezers operating on different particles. Finally, we conclude by proposing promising directions for future research. Keywords: optical tweezers; mechanical motions; optical force; micromachine; Janus particles 1. Introduction Light can drive the mechanical motions of micro- and nano-objects because light carries both energy and momentum that can exchange with these objects. However, this is not an intuitive and obvious concept because the force is extremely weak in usual situations. Thus, in our daily life, optical forces are invisible. Despite this, the light pressure had been hypothesized in the explanation of the nature of comet tails by Kepler (1696). In fact, the electromagnetic theory developed by Maxwell has proposed the optical force, which was described by the term “radiation pressure”. Until the 1960s, the advent of lasers greatly broadened the knowledge on light and made it possible to clearly demonstrate the concept of optical force. In 1986, Ashkin, the pioneer of optical tweezers, creatively used a highly focused laser beam to implement three-dimensional (3D) trapping of dielectric particles around the focus spot [1], as shown in Figure1a. To understand the principle of this trap (see Figure1b), the total optical force acting on a particle can be decomposed into two contributions: (1) the radiation pressure, known as the scattering force, which is proportional to the Poynting vector of optical field and points along the direction of the incident beam, tends to destabilize the trap; and (2) the gradient force, which is proportional to the gradient of the light intensity and points toward the tarp focus, confines the particle near the focal spot. The gradient force represents the attraction of particles to the highest intensity region. Thus, the condition for 3D optical trapping is that the gradient force is larger than the sum of the scattering force and stochastic force due to Brownian motion. Micromachines 2018, 9, 232; doi:10.3390/mi9050232 www.mdpi.com/journal/micromachines Micromachines 2018, 9, 232 2 of 28 Micromachines 2018, 9, x FOR PEER REVIEW 2 of 28 Figure 1. (a) A sketch of optical tweezers generated by a strongly focused laser beam to trap objects; Figure 1. (a) A sketch of optical tweezers generated by a strongly focused laser beam to trap objects; and (b) a schematic illustration of optical forces exerted on a colloidal particle around the focus spot. and (b) a schematic illustration of optical forces exerted on a colloidal particle around the focus spot. (Reprinted with permission from Springer Nature [2].) (Reprinted with permission from Springer Nature [2].) Since then, research on optical tweezers has begun to spread out and expand. Essentially, the basicSince principle then, of research optical ontweezers optical is tweezers the interaction has begun between to spread light outand and matter, expand. through Essentially, momentum the basicand energy principle transfer of optical and tweezersexchange, is to the achieve interaction manipulation between light of objects. and matter, As is throughwell known, momentum optical andtweezers energy can transfer limit objects and exchange, bound by tooptical achieve potential manipulation well in a of small objects. space As range. is well It can known, trap various optical tweezersobjects as can small limit as objectsnanometer bound-size by particles optical potentialand can exert well inoptical a small forces space with range. controllable It can trap amount various at objectsthe femtonewton as small as resolution, nanometer-size which particles is the ideal and range can exert for the optical study forces of molecule with controllable mechanica amountl property. at theTherefore, femtonewton optical resolution, tweezers have which become is the a ideal promising range fortool the to studyinvestigate of molecule the mechanical mechanical properties property. of Therefore,biological specimens, optical tweezers and examples have become include a promisingthe force measurements tool to investigate of DNA the- and mechanical RNA-based properties motors of[3– biological6]. This tool specimens, has been and widely examples applied include to inve thestigate force measurements cell cytometry, of artificial DNA- and fertilization RNA-based of motorsmammalian [3–6]. cells, This tooland haseven been micro widely-surgery applied [7,8].to investigate cell cytometry, artificial fertilization of mammalianIn the early cells, years, and even the micro-surgerytechnique of optical [7,8]. tweezers were successfully applied to capture and manipulateIn the early micrometer years, the particles technique [9], even of optical atoms tweezers[10]. However, were successfullyoptical forces applied on the captured to capture objects and manipulatedepend crucially micrometer on the particles interaction [9], evenbetween atoms the [10 laser]. However, field and optical the objects forces onthemselves. the captured Therefore, objects dependmultiple crucially degrees onand the freedoms interaction of control between over the optical laser force field can and be the greatly objects aroused themselves. by shaping Therefore, either multiplethe beam degrees or the particles. and freedoms To achieve of control various over motions optical forceof trapped can be micro greatly-objects, aroused two by typical shaping schemes either thehave beam been or successfully the particles. adopted To achieve in an variousoptical tweezers motions ofsystem. trapped One micro-objects, scheme is to modulate two typical laser schemes beams, havefor instance been successfully using optical adopted vortex beams in an optical[11], Bessel tweezers beams system. [12], Laguer Onere scheme–Gaussian is to beams modulate (LG beam) laser beams,[13], and for novel instance beams using [14] optical, to build vortex optical beams tweezers. [11], Bessel The beams other [12 scheme], Laguerre–Gaussian makes use of beamsthe special (LG beam)properties [13], andof particles, novel beams for instance [14], to build introducing optical tweezers.precisely Thefabricated other scheme asymmetric makes micro use of-objects the special [15] propertiesrather than of usual particles, homogeneous for instance spherical introducing particles, precisely such fabricatedas micro-turbines asymmetric or gammadion micro-objects shaped [15] rathermicro- thanrotors, usual birefringent homogeneous particles spherical [16,17] particles,and Janus such particles as micro-turbines [18] into an optical or gammadion tweezers shapedsystem. micro-rotors,These asymmetric birefringent particles, particles when [embedded16,17] and within Janus particlesusual Gaussian [18] into or an other optical special tweezers optical system. traps, Thesewill exhibit asymmetric interesting particles, mechanical when embedded motion behaviors. within usual Applications Gaussian or have other been special explored optical in traps, the fields will exhibitof Janus interesting motors [19,20] mechanical, switchable motion devices behaviors. [21], Applicationsand optical probes have been [22] explored. Henceforward, in the fields optical of Janustweezers motors have [19 opened,20], switchable research deviceson micro [21-objects], and opticaldynamic probes processes [22]. Henceforward, with the aid of optical high-resolution tweezers havemicroscope opened and research laser ontechnologies. micro-objects dynamic processes with the aid of high-resolution microscope and laserInspired technologies. by the complex organization of biological organisms, the design of artificial microInspiredmachines by that the exhibit complex controlled organization mechanical of biological motion and organisms, perform thesophisticated design of tasks artificial is an micromachinesultimate pursuit that of exhibitmicro-scale controlled engineering. mechanical A micro motionmachine and allows perform flexible sophisticated control of tasks both is the an ultimatedirection pursuit and speed of micro-scaleof motion. A engineering.t the same time, Amicromachine expanding the allowsscope of flexible their utility control in ofpractice both thehas led to the large-scale production of micromachines. Along
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