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We demonstrate photophoretic trapping of micron-sized absorbing particles in air using pulsed and continuous-wave (CW) ultraviolet laser illumination at wavelengths of 351 nm and 244 nm. We compared the particle trapping dynamics in two trapping geometries consisting of a hollow optical cone formed by light propagating either with or against gravity. This comparison allowed us to isolate the influence of the photophoretic force from the radiative pressure and the convective forces. We found that the absorbing spherical particles tested experienced a positive photophoretic force, whereas the spatially irregular, non-spherical particles tested experienced a negative photophoretic force. By using two trapping geometries, both spherical and non-spherical absorbing particles could be trapped and held securely in place. The position of the trapped particles exhibited a standard deviation of less than 1 µm over 20 seconds. Moreover, by operating in the UV and deep-UV where the majority of airborne materials are absorptive, the system was able to trap a wide range of particle types. Such a general purpose optical trap could enable on-line characterization of airborne particles when coupled with interrogation techniques such as Raman spectroscopy.
© 2015 Optical Society of America
1. Introduction
The ability to optically trap and manipulate micron-sized particles has had an enormous influence in the fields of biology, chemistry, and physics [1,2]. Recently, there has been a great deal of interest developing optical techniques for trapping airborne particles for applications ranging from particle transport and delivery to remote sensing and on-line characterization [3–10]. For example, optical trapping enables the use of characterization techniques such as Raman spectroscopy [7,10], which provides excellent specificity, but requires long measurement times, limiting its use on freely moving airborne particles.
Optical trapping and manipulation is normally achieved using either radiation pressure forces (e.g. in optical tweezers) or photophoretic forces. Radiation forces result from the transfer of momentum from a photon to a particle and are typically divided into a scattering and a gradient force. While radiation forces have found widespread use in trapping and manipulating particles in liquid or on a substrate, the force is less well-suited for applications in air. Specifically, the high index contrast in air results in a strong scattering force and very high NA (e.g. NA~0.9) is thus required to obtain a stable trap [11,12]. However, such high NA objectives are impractical in many aerosol characterization techniques due to the very short working distance which limits compatibility with aerosol particle delivery nozzles and makes it difficult to integrate additional characterization optics [10].
In comparison, optical trapping using the photophoretic force has several advantages. Photophoretic forces act on a particle which has been heated above the ambient temperature by absorbing light. The photophoretic forces are a result of the interaction between a heated particle and the surrounding gas molecules. For example, if a temperature gradient exists across the particle surface, then gas molecules on the higher temperature side of the particle will tend to collide with the particle at higher velocity, imparting a net force pushing the particle in the direction of its colder side. A non-uniform surface temperature could result from non-uniform illumination of the particle, or from non-uniformities in the particle morphology and absorptivity. In addition, a non-uniform distribution of the thermal accommodation coefficient (which describes the ability of the particle to transfer heat to the surrounding gas), can also impart a force, even if the particle is uniformly heated [13]. Finally, heating the surrounding gas also causes nearby gas molecules to rise, resulting in an upward convective drag force on the particle (although this convective force is not generally included in estimations of the photophoretic force [14]).
The photophoretic force can be 4 to 5 orders of magnitude stronger than the gradient force typically used in optical tweezers [15]. These large forces are particularly crucial when trapping airborne particles, since the trap must be strong enough to overcome air turbulence. In addition, since photophoretic traps hold the particle in a low-intensity region, the particle is less susceptible to damage from ablation or heating, as compared to radiative pressure based trapping (i.e. optical tweezers) in which the particle is trapped at the high-intensity region. Finally, photophoretic trapping does not require the high NA optics required by gradient based trapping techniques, enabling integration with particle delivery equipment [10] as well as additional optics to record, for example, the Raman spectra of a trapped particle [7]. However, photophoretic trapping is only possible for absorbing particles. As a result, previous demonstrations of photophoretic trapping, which operated at visible wavelengths (e.g. 488 and 532 nm) could only trap particles with strong absorption coefficients in the visible spectrum. Since many relevant biological aerosol particles (e.g. bacteria, bacterial spores, and proteins) are transparent in the visible spectrum, previously demonstrated photophoretic traps were limited to a subset of common airborne particle types.
In this work, we show that by operating in the UV (at 351 nm) and the deep UV (at 244 nm), photophoretic trapping of a wide range of bio-aerosol particles is possible. In addition, we attempt to clarify the role of the photophoretic force in the optical trap. Using analytic expressions, we first estimate the relative strength of the photophoretic, radiative pressure, and convective forces acting on a particle with varying size, absorption length, and thermal conductivity. We found that the photophoretic force calculated with these approximate solutions is more than two orders of magnitude stronger than the remaining forces for ~10 µm diameter particles with moderate to high absorption. We then compare the particle trapping dynamics in two optical trapping geometries to isolate the influence of the photophoretic force from the radiative pressure and the convective forces. Finally, we demonstrate photophoretic trapping of 14 varieties of airborne particles including biological molecules, proteins, allergens, and fungal spores. The ability to trap such a wide range of particle types could enable a new generation of on-line aerosol characterization tools when coupled with interrogation techniques such as Raman spectroscopy.
2. Modeling the trapping force
In this section, we estimate the relative strength of the optical forces acting on a spherical airborne particle. In addition to the radiative force and photophoretic force discussed above, absorbing particles can also experience a convective force. A convective force results when an absorbing particle is at a higher temperature than the surrounding gas, thereby heating the nearby gas which becomes less dense and rises. This imparts a drag force on the particle which pushes the particle against gravity. The relative strengths of these forces depend on the particle type (e.g. size, morphology, absorptivity, thermal conductivity, and accommodation coefficient), the surrounding environment (e.g. the pressure and thermal conductivity of the surrounding gas), and on the strength and geometry of the optical field. Precisely calculating the relative strength of these fields would require a detailed knowledge of the particle morphology, particle orientation, and material properties, which are often unknown even for some common biological aerosols. Nonetheless, by estimating the strengths of these forces for a range of particle parameters, we can identify the operating regimes where a photophoretic force is dominant and photophoretic trapping should be possible. Below, we rely on analytic expressions derived for the forces acting on spherical particles with varying size, absorptivity, and thermal conductivity illuminated by a plane wave.
The radiative pressure force, Frp, attributable to light absorbed from an illuminating plane wave, can be expressed as [4]:
where c is the speed of light and Pabs is the power absorbed by the particle. The absorbed power can then be expressed in terms of the particle radius, a, the illumination intensity, I0, and the absorption efficiency, Qabs. The absorption efficiency was calculated via Mie theory at a wavelength of λ = 351 nm as a function of particle radius and the absorption length, labs = λ/(4πmi) in the particle where λ is the wavelength and mi is the imaginary part of the complex refractive index [16,17]. The free convective force, Ffc, can be expressed as [14]:where d is the diameter of the particle and Afc = 2.9 × 10−3 was fit to experimental observations of the convective force acting on particles in the 65-150 µm diameter range [14]. The photophoretic force Fpp, can be expressed as [4,18,19]:where µa = 1.73 × 10−5 Ns/m2 is the air viscosity, ρa = 1.29 mg/cm3 is the density of air, T is the temperature, kf is the thermal conductivity of the particle, and ka = 0.0262 W/m/K is the thermal conductivity of air. In addition, the sign and magnitude of the photophoretic force depends on the coefficient J1, which describes the asymmetric heating of the particle. For plane-wave illumination of an ideal black-body, J1 = −0.5, which results in a positive photophoretic force, pushing the particle in the same direction the light propagates [18]. For a partially absorbing particle, the asymmetry factor can be expressed in terms of the optical field distribution within the particle (see Eq. (3) in [19]), and it can be shown that for strongly absorbing spherical particles, the photophoretic force is always positive [13,20]. However, spatially non-uniform particles are known to exhibit more complex photophoretic forces, including negative forces and forces which result in complex motions such as elliptical or helical orbits [8,20–23]. These forces are explained by variations in the thermal accommodation coefficient across the particle surface. The thermal accommodation coefficient of a material describes the ability of the material to transfer heat to a surrounding gas. If the accommodation coefficient varies across a particle surface, then even if it is uniformly heated, it will experience a force pushing the particle toward the side with the lower accommodation coefficient. In practice it is very challenging to estimate the asymmetry coefficient, J1, and the accommodation coefficient for partially absorbing, non-spherical particles. As a result, the magnitude and even the direction of the photophoretic force can be difficult to predict. In the experimental sections below, we compare two optical trapping geometries to isolate the effect of the photophoretic force.Nonetheless, we can use Eqs. (1-3) to provide an estimate of the relative strength of the photophoretic force, radiative pressure force, and free convective forces acting on spherical particles. The illumination intensity was set to I0 = 100 W/cm2, the particle diameter was set at 5 µm, the absorption length at 0.1 µm, and the thermal conductivity at 0.159 W/m/K, based on the value reported for leather which is in the range of thermal conductivities of biological materials ranging from wood to animal tissues [24]. In Fig. 1, we show the relative strength of the forces acting on an airborne particle as a function of the Fig. 1(a) diameter, Fig. 1(b) thermal conductivity, and Fig. 1(c) absorption length of the particle while the remaining parameters are fixed. The asymmetry coefficient was set at the black-body limit of J1 = −0.5 in Figs. 1(a)-1(b). In Fig. 1(c), the asymmetry coefficient was set as J1 = −2a/3labs following the convention used in [4] for the moderate to low absorption regime, whereas in the high absorption regime, we applied the black-body limit of J1 = −0.5. Thus, in practice we set J1 = max{−0.5, −2a/3labs}. The forces in each case are normalized to the gravitational force on the particle. As shown in Fig. 1(a), the photophoretic force acting on particles with a diameter of ~5 µm is ~2 orders of magnitude stronger than either the free convective force, the radiative pressure force, or the gravitational force. Indeed, for these parameters, neither the radiative pressure, nor the convective forces are strong enough to counter-act the gravitational force and thus would not be sufficient to trap the particles. Even so, the free convective force might be over-estimated with the available formula since Afc was fit to measurements of particles with diameters greater than 65 μm [14]. The photophoretic force is also strongly dependent on the thermal conductivity of the particle, since a low thermal conductivity helps to maintain a strong temperature gradient across the particle. As shown in Fig. 1(b), for 5 µm diameter particles with thermal conductivity below ~10 W/m/K, the photophoretic force remains stronger than the gravitational force. Finally, the absorption length is also a crucial parameter, since strongly absorbing particles will experience a stronger photophoretic force, as shown in Fig. 1(c). This highlights the importance of operating in the UV and deep UV, where the vast majority of airborne particles experience strong absorption.
Fig. 1 Estimation of the relative strength of the photophoretic, convective, and radiative forces acting on a spherical airborne particle as a function of the (a) diameter, (b) thermal conductivity, and (c) absorption length of the particle while the remaining parameters are fixed at I0 = 100 W/cm2, d = 5 µm, labs = 0.1 µm, and kf = 0.159 W/m/K. The forces are normalized to the gravitational force acting on the particle.
Based on these calculations, we expect that the photophoretic force will be the dominant effect in the optical trapping technique presented below. However, the sign and magnitude of the photophoretic force are expected to depend on the accommodation coefficient of the particle and the asymmetry coefficient, both of which depend strongly on the particle morphology. In the next section, we present trapping experiments designed to isolate the effect of the photophoretic force on different particle types.
3. Comparison of photophoretic trapping geometries
Photophoretic trapping of airborne particles has been demonstrated using a variety of optical illumination patterns. These illumination geometries include vortex beams [4,5], hollow cones [6,10,22], tapered rings [23], optical lattices [25], and even speckle fields [26]. In each of these geometries, absorbing particles are trapped in the low optical intensity region(s). However, aside from that similarity, researchers have observed a significant variation in the types of particles (e.g. size, absorptivity, and morphology) which can be trapped in a given geometry, as well as the relative position of different particle types within a trap [22,26]. Moreover, the observation of complex particle motions such as rotational orbits in a variety of trapping geometries [8,22,23] highlights the need for a clearer understanding of the photophoretic force acting on spatially non-uniform particles. In an effort to separate the effect of the photophoretic force from the radiative and convective forces, we consider two optical trapping geometries, consisting of a hollow optical cone formed by light either propagating with or against gravity. The experimental apparatus is shown schematically in Fig. 2.
Fig. 2 (a) Experimental apparatus. We used either a CW laser operating at λ = 244 nm or a pulsed laser operating at λ = 351 nm (10 kHz, 70 ns). The laser was passed through two axicon lenses to form a collimated hollow beam which then entered one of the trapping geometries. In the “Downward” trapping geometry, the beam was reflected by a curved mirror to form a hollow cone with the light propagation in the direction of gravity. In the “Upward” trapping geometry, a lens was used to focus the hollow beam, forming a hollow cone with light propagating against gravity. (b, c) The bottom row shows the direction of the convective, gravitational, and radiative forces acting on the particle along the optical axis in the two geometries. The photophoretic force depends both on the particle properties and the position in the trap and we assume it is not known a priori. Instead, by comparing the net force acting on the particles in these two geometries, we are able to deduce the direction of the photophoretic force for different particle types. Note that the lengths of the vectors are not indicative of the relative strength of the forces but only their direction.
We used three laser systems operating in the visible, UV, and deep UV. The visible laser was a CW argon ion laser (Coherent Innova) operating λ = 488 nm at ~400 mW. The UV laser was a frequency tripled Nd:YLF laser operating in pulsed mode at 10 kHz with 70 ns pulse width at a wavelength of λ = 351 nm with ~550 mW average power (Photonics Industries DC50-351). Due to the relatively high repetition rate, we did not observe significantly different behavior using the pulsed laser compared with the CW laser and both lasers enabled stable trapping. The deep UV laser was a CW frequency doubled argon ion laser operating at λ = 244 nm at ~100 mW (Lexal 95-SHG). Note that optical trapping using the visible laser was presented previously [10]. In this work, we will primarily discuss particle trapping using the UV and deep UV lasers.
As shown in Fig. 2(a), the laser emission was first expanded and then passed through two axicon lenses (each with a cone angle of 175°), to form a collimated hollow beam. The hollow beam was then reflected off a mirror at 45 degrees into either a “downward illumination” trap, or an “upward illumination” trap. In the downward trap, the collimated hollow beam was focused to form a hollow cone by a concave mirror (25.4 mm diameter, 19 mm focal length). In the upward trap an aspheric lens (25.4 mm diameter, 20 mm focal length) was used to form a hollow cone. In both cases, a glass cylinder and quartz window were used to minimize air turbulence in the trapping region. The particles were then introduced into the chamber from above, through either a hole in the curved mirror, or an adjustable iris. A Nikon D7000 camera with a 65 mm, f/2.8 macro-lens was used to obtain high-resolution images of the trapped particles and record videos of the particle motion in the traps.
In both geometries, a hollow cone provided a low-light-intensity region for trapping. At the bottom of Fig. 2, we show the directions of the expected forces in each geometry. In the downward illumination geometry, we expect the radiative pressure force to be in the direction of gravity and opposite the free convective force. In the upward illumination geometry, the radiative pressure force is expected to change directions and be directed against gravity. In general, we expect the direction of the photophoretic force to depend on the particle type. Specifically, the particle morphology and material properties will dictate the asymmetry factor, J1, as well any accommodation-induced force. However, by comparing the particle motion in the two trapping geometries, we hope to separate the photophoretic and radiative effects (which depend on the illumination direction) from the convective and gravitational forces acting on the particle. Note that the particles will also be subject to radiative pressure and photophoretic forces in the lateral dimension. We expect these forces to balance along the optical axis and are not shown in Fig. 2 for simplicity.
Initially, we consider two particle types: spatially irregular fungal spores (Johnson smut grass spores) and spatially uniform, polymer spheres doped with fluorescent dye (Duke Scientific Catolog #2408, 6 µm diameter). Figure 3 and the associated supplementary movies depict the behavior of the two particle types in the two trapping geometries.
Fig. 3 Comparison of particle motion in the trapping geometries. The top row shows the behavior using a downward oriented cone whereas the bottom row shows the behavior using an upward oriented cone. Two particle types are considered, fungal spores (Johnson smut grass spores), which are highly absorbing, spatially irregular particles, and fluorescent polymer spheres. The supplementary videos show the flow of each particle type in the two trapping geometries (See Media 1, Media 2, Media 3 and Media 4). The right column summarizes the observations from the videos. The fungal spores experienced a negative photophoretic force and travelled against the illumination direction. Nonetheless, the fungal spore particles were trapped near the focal point of the cone using either illumination geometry. The fluorescent polymer spheres, on the other hand, experienced a positive photophoretic force, travelled along the illumination direction, and were only consistently trapped using the upward illumination geometry.
First, we will discuss the fungal spores. In the downward illumination trap [Fig. 3(a) and Media 1)], the fungal spores were found to move in the upward direction within the walls of the cone. That is, the particles moved against the direction of illumination. Based on the force diagram in Fig. 2(b), this indicated that the particles either experienced a negative photophoretic force, or the convective force was dominant. To differentiate between the two scenarios, we considered Fig. 3(b), in which the fungal spores are in the upward illumination trap (see Media 2). In that case, the fungal spores were observed to move downward. This indicated that the sum of the convective force and radiative pressure force was weaker than the photophoretic force, since the convective force should not depend on the illumination direction. In addition, since the particle motion was opposite to the radiative pressure force in both cases, this indicates that the fungal spores experienced a strong negative photophoretic force, and that this force was significantly stronger than the radiative force and convective force. We repeated this experiment with additional spatially irregular, strongly absorbing particles (e.g. nigrosin, carbon black, and carbon nanotubes) and observed similar behavior. Due to the relatively large size and strong absorption of these particles, we expect that the so-called “longitudinal” photophoretic force would be in the “positive” direction [20,21]. Thus, the observation of a negative photophoretic force indicates that the accommodation-based photophoretic force is dominant in these spatially irregular particles. A similar observation of a negative photophoretic force was recently reported [27] in which particles were illuminated by an optical beam in the horizontal direction and also found to move against the direction of light propagation. The mechanism that arranges these highly absorbing, spatially irregular particles into an orientation where they experience a negative photophoretic force is unclear, although several groups have reported observations of particles moving in helical or rotational trajectories [8, 20–23]. Nonetheless, fungal spore particles were trapped using either illumination geometry, as shown in Figs. 3(a)-3(b). The positions of trapped particles are indicated schematically in Figs. 3(e)-3(f). We also observed that the downward illumination trap tended to capture many more particles, resulting in the formation of a “particle cone” [22]. However, this may be a byproduct of the fact that in the downward illumination geometry, the photophoretic force counteracted gravity in the cone walls, and thereby kept the particles in motion much longer, providing additional opportunities for a particle to fall back into a trapping position.
Second, we discuss the fluorescent sphere behavior. In contrast to the smut spores, the fluorescent spheres were observed to move in the direction of light propagation in both geometries [Figs. 3(c)-3(d) see Media 3 and Media 4]. Since the particle motion depended on the direction of light propagation, we can rule out the convective force as a dominant mechanism. Our observation could then be explained either as a positive photophoretic force or by a dominant radiative pressure force, since these would both act in the same direction. However, based on our estimations in Fig. 1, we expect that the radiative pressure force is not strong enough to overcome the gravitational force using our illumination conditions. Moreover, we repeated the experiment with non-absorbing polystyrene spheres of similar size and found that the radiative pressure was not sufficient to overcome gravity. The observed motion in the fluorescent polymer spheres is therefore mainly attributed to a positive photophoretic force—the opposite of the force observed to act on the fungal spores. We expect that since the fluorescent particles were highly uniform spheres, the accommodation force was minimized and the longitudinal photophoretic force dominated. However, this meant that in the downward illumination geometry, the only force remaining in the upward direction was the free convective force which was not strong enough to overcome the remaining forces. As a result, stable trapping was not observed for the fluorescent spheres in the downward illumination geometry. On the other hand, in the upward geometry, the photophoretic, radiative, and free convective forces all worked against gravity and stable trapping of the fluorescent spheres was obtained near the focal spot, as indicated in Fig. 3(f).
4. Summary of trapped particles
We repeated the optical trapping experiment on fourteen varieties of airborne particles. Table 1 provides a summary of the trapped particles including the wavelength used to achieve optical trapping, the absorption length, labs, where available, and the primary particle size for each particle type. Aside from the fluorescent polymer spheres, the remaining particle types tested were each trapped in the downward illumination geometry. As a result of this observation, we expect that these spatially irregular particles (summarized in Table 1) tended to experience a negative photophoretic force. However, not all of the particles could be trapped at all wavelengths, since absorption is a pre-requisite for photophoretic trapping. For example, we found that a representative biological molecule, β-NADH, which could not be trapped in the visible was consistently trapped in the UV at 351 nm. In addition, Bacillus subtilis spores (a bacterial spore) and bovine serum albumin (a type of protein) particles did not exhibit sufficient absorption at 351 nm to be trapped consistently. However, these particles were readily trapped in the deep UV at 244 nm. This illustrates the advantage of shifting to shorter wavelengths where the vast majority of airborne biological particles exhibit sufficient absorption. For example, all living cells contain proteins and nucleic acids, which both exhibit strong absorption below approximately 300 nm. Trapping in the deep UV may also enable trapping of aerosolized chemical hazards. In addition, this work demonstrates that a pulsed laser could capture a moving particle and provide a stable optical trap. We found that for either the pulsed or CW laser, approximately 50 mW average power was required to capture moving particles and trap them in air.
Table 1. Summary of trapped particle typesa
5. Stability of a trapped particle
Finally, we consider the stability of particles held in the photophoretic trap. We found that the particles could be held in the optical trap for hours. To quantitatively characterize the stability of a trapped particle, we recorded a video of a Johnson grass smut spore trapped in the downward illumination geometry, as shown in Fig. 4. The 5 µm particle is clearly resolved using the macro lens (which provides ~2 µm resolution), as shown in Fig. 4(b). A supplementary video shows the particle motion during a period of 20 seconds (see Media 5). In addition, we plot the vertical and horizontal cross sections over 400 frames captured during the 20 second period in Figs. 4(c)-4(d). The standard deviation of the particle position was found to be less than 1 µm (limited by the resolution of our camera). Such a stable optical trap is well-suited for integration with particle characterization techniques such as Raman spectroscopy.
Fig. 4 (a) Trapped Johnson grass particle in the glass chamber using the downward hollow cone. (b) Close-up off the trapped particle, the supplementary video shows that the particle is held securely in place (see Media 5). (c, d) Cross sections of the image of the particle over 20 seconds along the horizontal (c) and vertical (d) directions. The particle position remains well defined throughout the observation period, with a standard deviation of less than 1 µm.
6. Conclusion
In summary, we compared two trapping geometries and found that the majority of the spatially irregular particles we could trap experienced a negative photophoretic effect, which we attribute to accommodation-based effects. On the other hand, highly uniform, spherical fluorescent particles experienced a positive photophoretic effect. We demonstrated, using a hollow cone geometry, photophoretic trapping of 14 different particle types, including fungal spores, bacterial spores, allergens, and particles made from biological molecules, proteins, and black carbon. By operating in the UV and deep UV, where the vast majority of the airborne biological particles exhibit strong absorption, such a photophoretic trap is able to capture a wide range of particle types. Moreover, the trap was extremely stable, and the position of a trapped particle was found to deviate by less than 1µm. Such a versatile, stable optical trap could enable on-line characterization of a wide-range of particle types.
Acknowledgments
This research was supported by the Defense Threat Reduction Agency (DTRA) under contract number HDTRA136477 and HDTRA1310184, US Army Research Laboratory mission funds, and under Cooperative Agreement Number W911NF-12-2-0019.
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