Accelerated self assembly of particles at the air-water interface with optically assisted heating due to an upconverting particle

Abstract

Particles can be assembled at the air-water interface due to optically induced local heating. This induces convection currents in the water which brings particles to the surface. We improve the technique by employing an upconverting particle (UCP), which, when illuminated with 975 nm light, not only emits visible emission but also generates heat owing to the poor efficiency of the upconversion process. This induces strong convection currents which makes particles dispersed in the suspension assemble at the interface and immediately under the UCP. We show assembly of polystyrene particles of 1 μm diameter and diamonds of 500 nm diameter bearing Nitrogen-Vacancy (NV) centers around the UCP. We also show, for the first time, that the microdiamonds are assembled within about 30 nm at the bottom of the UCP by utilizing non-radiative energy transfer that reduces the lifetime of the 550 nm emission from about 90 μs to about 50 μs.

Formation of Two-Dimensional Magnetically Responsive Clusters Using Hematite Particles Self-Assembled via Particle-Induced Heating at an Interface

“…Similarly, realizing the self-assembly of hematite particles at an air−water interface via direct laser illumination is challenging. Thus, we used an alternate strategy in which a UPC is first trapped at the interface to create a local hotspot that in turn generates local convection currents, which attract the hematite particles in the vicinity, thereby inducing selfassembly of the hematite particles 52 (Figure 6). This phenomenon is consistent with the theoretical framework.…”

“…It is linked to surface tension (σ) by the relation . Let us denote the width of the enclosure as d w = 0.01 m. The Cahn number is taken as 0.15 similar to our previous study …”

“…Similar to our previous study, we consider β = 0.01 m/s. 51,52 L is the chemical potential 52 given by = L F . F is the free energy given by 52,53…”

“…It is linked to surface tension (σ) by the relation = is taken as 0.15 similar to our previous study. 52 To reduce the computational cost, the Cahn−Hilliard equation is separated into two second-order PDE as follows 51,52,55 Here, f is the external free energy. In this study, we consider = 0 f similar to our previous study.…”

“…χ 1 indicates the diffusion time scale and is calculated as χ 1 = βϵ p 2 , where β is a constant term. Similar to our previous study, we consider β = 0.01 m/s. , L is the chemical potential given by . F is the free energy given by , Here, is the interfacial free-energy density and is the bulk free-energy density.…”