Optical trapping and Raman spectroscopy of solid particles

Abstract

The heterogeneous interactions of gas molecules on solid particles are crucial in many areas of science, engineering and technology. Such interactions play a critical role in atmospheric chemistry and in heterogeneous catalysis, a key technology in the energy and chemical industries. Investigating heterogeneous interactions upon single levitated particles can provide significant insight into these important processes. Various methodologies exist for levitating micron sized particles including: optical, electrical and acoustic techniques. Prior to this study, the optical levitation of solid micron scale particles has proved difficult to achieve over timescales relevant to the above applications. In this work, a new vertically configured counter propagating dual beam optical trap was optimized to levitate a range of solid particles in air. Silica (SiO2), α-alumina (Al2O3), titania (TiO2) and polystyrene were stably trapped with a high trapping efficiency (Q = 0.42). The longest stable trapping experiment was conducted continuously for 24 hours, and there are no obvious constraints on trapping time beyond this period. Therefore, the methodology described in this paper should be of major benefit to various research communities. The strength of the new technique is demonstrated by the simultaneous levitation and spectroscopic interrogation of silica particles by Raman spectroscopy. In particular, the adsorption of water upon silica was investigated under controlled relative humidity environments. Furthermore, the collision and coagulation behaviour of silica particles with microdroplets of sulphuric acid was followed using both optical imaging and Raman spectroscopy.

Measurement of the Raman spectra and hygroscopicity of four pharmaceutical aerosols as they travel from pressurised metered dose inhalers (pMDI) to a model lung

“…The optical trap uses a counter propagating dual beam (CPDB) trap 33 configuration first described by Rkiouak et al (Rkiouak, et al, 2014) and deployed in several 34 subsequent experiments (Tang, et al, 2014) (Jones, et al, 2015) (Hunt, et al, 2015). This trapping 35 setup is remarkable because it is capable of stably trapping micron-sized solid particles of non-36 spherical geometry for periods of time up to several hours.…”

“…The 3 laser power at output was 15 mW from the top objective lens and 10 mW through the bottom 4 objective lens (figure 2). The asymmetry in power ensured that trapped particles were driven closer 5 to the optical focus plane of the bottom objective through which the Raman laser is passed, ensuring 6 better focus on the resulting images (Rkiouak, et al, 2014). The foci of the lasers were positioned ~10 7 µm apart, which created a trapping volume large enough to stably hold 1-5µm particles for long 8 periods.…”

Dynamic viscosity mapping of the oxidation of squalene aerosol particles

“…The Raman signal was directed to a spectrograph (Spectrapro 2500i, 1200 groove blazed at 500 nm) and imaged onto a CCD camera (Princeton Instruments, Spec10). For greater details on the optical strategy see Rkiouak et al 2014 12 . Squalene particles were introduced into the optical trap and subsequently subjected to ozone (11 ppm) generated by passing synthetic air through a penray lamp.…”

“…44 A Nd:Yag CW IR laser (Ventus 1064, Laser Quantum) was used as a laser source. The trapping beam was split using a beam splitter and the two resulting beams were passed into optical fibres.…”

Rapid interrogation of the physical and chemical characteristics of salbutamol sulphate aerosol from a pressurised metered-dose inhaler (pMDI)

“…10,11 The trap is optimized to trap solid micron sized particles over long timescales. This optical trapping configuration can readily capture non-spherical aerosol particles.…”