Atomistic modeling and rational design of optothermal tweezers for targeted applications

Abstract

Optical manipulation of micro/nanoscale objects is of importance in life sciences, colloidal science, and nanotechnology. Optothermal tweezers exhibit superior manipulation capability at low optical intensity. However, our implicit understanding of the working mechanism has limited the further applications and innovations of optothermal tweezers. Herein, we present an atomistic view of opto-thermo-electro-mechanic coupling in optothermal tweezers, which enables us to rationally design the tweezers for optimum performance in targeted applications. Specifically, we have revealed that the non-uniform temperature distribution induces water polarization and charge separation, which creates the thermoelectric field dominating the optothermal trapping. We further design experiments to systematically verify our atomistic simulations. Guided by our new model, we develop new types of optothermal tweezers of high performance using low-concentrated electrolytes. Moreover, we demonstrate the use of new tweezers in opto-thermophoretic separation of colloidal particles of the same size based on the difference in their surface charge, which has been challenging for conventional optical tweezers. With the atomistic understanding that enables the performance optimization and function expansion, optothermal tweezers will further their impacts.

Keywords: molecular dynamics simulation; optical manipulation; optical tweezers; optothermal tweezers; thermophoresis.

Figure 1

Opto-thermal manipulation of nanoparticles. (a) Schematic illustration of particle trapping. (b) Successive optical images of a representative trapping process (the cross marks the position of laser beam). (c) Position of PS beads versus time and drift velocity versus position profiles. The corresponding SD is indicated by shaded areas (the average values are obtained by processing eight individual videos). (d) Trapping or repelling of charged PS beads in water, NaCl and CTAC solution. Positively and negatively charged beads are repelled and trapped in water, respectively. A totally opposite phenomenon can be observed in NaCl solution. In CTAC solution, both beads are trapped. (e) Mean drift velocity in three liquids (negative velocity corresponds to repelling velocity).

Figure 2

Atomistic view of optothermal tweezers. (a) Water molecule shows strong polarity: The charges of an oxygen atom and a hydrogen atom are roughly −0.8e and 0.4e, respectively. Water can be polarized by thermal gradient: Water molecules rotate and transport until oxygen and hydrogen atoms are aligned towards the cold and hot region, respectively. (b) The mechanism of opto-thermophoretic tweezers: Under a temperature gradient, water molecules are in a specific arrangement and generate an electric field toward cold region for negative particle trapping. In the region far away from the hot substrate, water molecules are randomly oriented. (c) Temperature gradient (blue dots) and ionic distribution (NaCl and CTAC solutions) at steady state obtained in MD simulations. Green, purple, cyan, pink and yellow spheres are chlorine, sodium, carbon, carbon (tail) and nitrogen (head) atoms, respectively. (d) Difference in the effective S_T between cations and anions (pink squares: CTAC, green dots: NaCl). (e) The mechanism of opto-thermoelectric tweezers: Under temperature gradient, cations diffuse father than anions. Such separation leads to an electric field toward the hot region for positive particle trapping. In the region far away from the hot substrate, water molecules and ions are in random order. Additionally, ions stay close with their counterions. (f) TE fields of water, NaCl (1 M) and CTAC (1 M). The highlighted areas represent the hot (red) and cold (blue) regions.

Figure 3

Effects of solutions on trapping ability. (a) Thermoelectric fields/thermoelectric forces of water (blue solid/open triangles), NaCl (green solid/open pentagons) and CTAC (pink solid/open squares) solutions at different concentrations. (b) Charge separation of 1 M and 75 mM CTAC solutions (top panel) and 1 M NaCl solution (bottom panel). The separation of oxygen and hydrogen atoms (water polarization) in pure water (bottom). n₊ (n₋) is local number fraction of cations (anions). Note that the vertical axis of the bottom panel is much smaller than the top one.

Figure 4

Effects of particles on trapping ability. (a) Snapshot of CTAC adsorption on the surface of Si particle. (b) Snapshot of CTAC adsorption on the surface of SiO₂ particle. Green, cyan, pink and yellow spheres are chlorine, carbon, carbon (tail) and nitrogen (head) atoms, respectively. In order to show a better comparison, water molecules are not plotted here (simulation setup is shown in Fig. S12 in the ESM). (c) Relative density profiles of adsorbed ions on the surface of Si particle in 75 and 131 mM CTAC solution. (d) Relative density profiles of adsorbed ions on the surface of SiO₂ particle in 75 and 131 mM CTAC solution. (e) Measured and simulated zeta potential of Si and SiO₂ particles in DI water and CTAC solution at various concentrations. (f) Measured and simulated normalized trapping force of Si and SiO₂ particles in DI water and CTAC solution at various concentrations. The sizes of Si and SiO₂ particles are 1 μm. Measured (simulated) trapping forces are normalized by dividing by the measured (simulated) trapping force of Si in DI water.

Figure 5

The drift velocities of a 2 μm positively charged PS particle in different solutions along with the corresponding electrolyte concentrations.