Self-thermophoresis of laser-heated spherical Janus particles

Abstract

An analytic framework is presented for calculating the self-induced thermophoretic velocity of a laser-heated Janus metamaterial micro-particle, consisting of two conducting hemispheres of different thermal and electric conductivities. The spherical Janus is embedded in a quiescent fluid of infinite expanse and is exposed to a continuous light irradiation by a defocused laser beam. The analysis is carried under the electrostatic (Rayleigh) approximation (radius small compared to wavelength). The linear scheme for evaluating the temperature field in the three phases is based on employing a Fourier–Legendre approach, which renders rather simple semi-analytic expressions in terms of the relevant physical parameters of the titled symmetry-breaking problem. In addition to an explicit solution for the self-thermophoretic mobility of the heated Janus, we also provide analytic expressions for the slip-induced Joule heating streamlines and vorticity field in the surrounding fluid, for a non-uniform (surface dependent) Soret coefficient. For a ‘symmetric’ (homogeneous) spherical particle, the surface temperature gradient vanishes and thus there is no self-induced thermophoretic velocity field. The ‘inner’ temperature field in this case reduces to the well-known solution for a laser-heated spherical conducting colloid. In the case of a constant Soret phoretic mobility, the analysis is compared against numerical simulations, based on a tailored collocation method for some selected values of the physical parameters. Also presented are some typical temperature field contours and heat flux vectors prevailing in the two-phase Janus as well as light-induced velocity and vorticity fields in the ambient solute and a new practical estimate for the self-propelling velocity. Graphic abstract

Optical trapping in micro- and nanoconfinement systems: Role of thermo-fluid dynamics and applications

“…Thermoosmotic flows are observed experimentally near a solid-liquid interface heated by a photothermal effect of Au nanoparticle [44] and are also investigated numerically for both light-absorbing particles and solvents [45]. This flow is considered as the origin of self-thermophoresis of Janus particles [46][47][48] or spheroidal nanoparticles [49], which have temperature distribution around their In addition to optical forces, suspended particles are subject to a drag force and a thermophoretic force (see, Sec. 4.2.2) in such a situation.…”

Dynamics of Active SiO2–Pt Janus Colloids in Dilute Poly(ethylene oxide) Solutions

“…Most commonly used active JCs are made of a silica or polystyrene (PS) core whose one-half is coated with platinum (Pt), which catalyzes the decomposition of hydrogen peroxide (H 2 O 2 ) in an aqueous medium . The resultant asymmetric concentration gradient of the reaction products causes the JCs to propel in the direction away from the metal through a mechanism known as self-diffusiophoresis . − JCs have also been known to propel under a self-generated electric potential ( self-electrophoresis ) , or a temperature gradient ( self-thermophoresis ). − The other widely recognized class of artificial swimmers consist of dispersed droplets self-propelling in the continuous phase in the presence of surfactants. Here, spontaneously generated Marangoni stresses at the droplet interface lead to their propulsion .…”

Self-Diffusiophoresis and Symmetry-Breaking of a Janus Dimer: Analytic Solution

“…There is a vast body of literature on self-propulsion of capped Janus spherical colloids arising due to material/chemical symmetry-breaking which can be modelled by applying disparate boundary conditions over different parts of the surface [9][10][11][12][13][14][15][16], typically with a symmetric Janus composed of two hemispheres. However, other three-dimensional (3D) geometries that can be analytically handled are, for example, convex separable shapes, such as ellipsoidal and spheroidal particles (slender rods and elliptic discs as limiting cases) [17][18][19][20][21][22][23].…”