1. Introduction

In recent years, there has been rapid development in non-contact and non-invasive micro control technology [1–3]. The utilization of light and sound to transfer momentum to particles allows for contactless manipulation, opening a multitude of possibilities across various applications [4,5]. Optical tweezers have proven to be a versatile tool for manipulating particles at the micro/nano scale [6,7], enabling precise control over a diverse range of materials including latex spheres [8], dielectric spheres, bacteria, and even viruses [9]. The subsequent holographic optical tweezers can manipulate a small number of particles with high precision [10,11]. However, all types of optical tweezers have limited compatibility with non-transparent particles [12] and may potentially cause thermal damage to biological samples [13]. In contrast, acoustic tweezers provide excellent biocompatibility and have demonstrated successful manipulation of a wide range of particles, including latex particles up to 270 µm in diameter and even complex structures like frog egg clusters [14]. Acoustic tweezers offer a wider range of particle sizes, spanning from micrometers to millimeters, and can generate greater forces compared to optical tweezers. However, it is important to acknowledge that acoustic tweezers do have limitations in terms of flexibility [15–19]. To address this, a spatial ultrasound modulation technique utilizing microbubble arrays was developed [20], enabling the construction of complex acoustic fields. However, it is worth noting that despite these advancements, the dynamic tunability and stability of acoustic tweezers still lag behind that of optical tweezers.

To combine the advantages of optical tweezers and acoustic tweezers, researchers have explored the utilization of the photoacoustic effect [21] for micromanipulation. Several studies have demonstrated the use of the photoacoustic effect to manipulate small objects in non-liquid environments [22,23]. It holds immense promise for extending this non-contact manipulation capability to microparticles within a water environment. While there have been report on the generation of arbitrary acoustic fields using laser excitation in water [24], research on utilizing these acoustic fields for the manipulation of objects remains limited [25]. Therefore, it is of great significance to simultaneously control and long-distance transport particles in a liquid environment through photoacoustic effects.

In this paper, we introduce a novel method called Programmable Photoacoustic Manipulation (PPAM) of microparticles in a liquid environment. Our approach enables the arbitrary patterning and efficient transport of large numbers of particles. This is achieved by directing a pulsed laser onto a stainless-steel membrane coated with candle soot nanoparticles (CSNP) [26–29]. This system shows promise for various applications, including cell manipulation [30], cell screening [31], targeted drug delivery [32], as well as microfluidic chips. [33].

2. Design and results

Figure 1(a) illustrates the schematic diagram of the PPAM setup in a water environment. The experimental configuration involves a laser with parameters as follows: a pulse width of 5.6 ns, repeat frequency of 10 Hz, and a wavelength of 532 nm. To achieve parallel light, the laser beam is expanded using a lens pair and directed towards a Digital Micromirror Device (DMD) [34]. The DMD is a binary spatial light modulator widely utilized in wavefront shaping applications. In our setup, the structured light produced by the DMD is directed onto the bottom surface of a stainless steel membrane with the thickness of 10µm. A layer of candle soot nanoparticles (CSNP) is coated beneath the membrane to reduce laser reflectance and thereby enhance photoacoustic efficiency. When the laser irradiates the membrane, the illuminated section absorbs optical energy, leading to thermal expansion and subsequent mechanical vibration. This vibration propagates as localized antisymmetric Lamb waves along the metal membrane. The energy from the membrane's vibration is then transferred to the above water layer, resulting in the excitation of acoustic waves in the liquid.

 

Fig. 1. (a) The pulsed laser is modulated by a digital micromirror device (DMD) to generate structured light, which is then directed onto a membrane with the thickness of 10 µm for particle manipulation. (b) Illustration of the forces acting on particles in the liquid when excited by a single-stripe laser. The red arrow represents the mechanical force F_M, while the purple arrow represents the acoustic radiation forces F_A. (c) Micrograph showing silica particles with average diameter of 25 µm.

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The particles dispersed on the metal membrane are lifted by the membrane's vibration and simultaneously propelled by the acoustic radiation forces in the water. Figure 1(b) illustrates the physical process occurring in the YZ plane under excitation by striped structured light. In this configuration, the Lamb waves induced by the laser stripe propagate from the center of the stripe towards both sides. The red arrows represent the mechanical force F_M exerted on the particles by the vibration of the metal membrane. The particles are driven by the membrane motion until the upward velocity of the particles exceeds that of the membrane. After the particles bounce up and detach from the metal membrane, the particles are primarily influenced by the acoustic field within the liquid. In Fig. 1(b), the color map represents the distribution of the acoustic pressure field in the liquid. The purple arrows indicate the acoustic radiation forces (F_A) exerted on the particles in the water. These forces cause the particles to be pushed apart. Near the edges of the laser stripe, the movement of particles leads to the development of black patterns. The generation of acoustic fields provides a convenient and flexible means for manipulating particles. In our experiment, we chose semi-coated aluminum silica microspheres as the particles, as shown in Fig. 1(c). The reflective property of the aluminum-coated surface is higher compared to the uncoated side, which results in some particles are in black and others in white.

The effectiveness of PPAM is attributed to the synchronized interaction between the membrane's vibration and the acoustic wave in the liquid. The mechanical force F_M exerted by the membrane on the particles can reach micro-Newton (µN) levels, surpassing the magnitude of the acoustic radiation force F_A. This substantial force is necessary to counteract the adhesive forces (approximately 200 nN) between the particles and the membrane, primarily van der Waals forces, which impede particle movement. Consequently, the spatiotemporal distribution of the membrane's vibration determines the positions of the manipulated particles.

To develop a comprehensive understanding of how elastic waves propagate under the photoacoustic effect, we conducted numerical analysis on the vibrations of the membrane. In our simulation, we utilized a single-stripe light with a width of 175 µm and a length of 3 mm as the excitation source. The light source used in the simulation had a laser power density of 2.1 MW/cm². Figure 2(a) illustrates the temporal variation of the membrane's deformation quantity at the X direction of the laser stripe. The horizontal axis represents the propagation time of the elastic waves, while the vertical axis represents the spatial position relative to the center of the excitation source. Specifically, X = 0 mm corresponds to the central axis of the stripe excitation source.

 

Fig. 2. (a) Simulation results of time-position distribution of elastic waves propagation. The yellow dotted line represents the position at X = 0.12 mm. The purple dash line corresponds to the time at t = 0.35 µs. (b) Simulation results show that the displacement of the membrane with time at the X = 0.12 mm. (c) Simulation results of the spatial distribution of membrane deformation in the Z direction on the XY plane at t = 0.35 µs. The green dotted box represents the laser radiation area. (d, e, f) Experimental results correspond to the simulated results shown in (a, b, c).

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After laser excitation, the membrane's vibration exhibits a symmetric distribution. Fig. S1 in Supplement 1 demonstrates the dispersion, where the low-frequency components of the vibration possess a higher proportion of energy and slower speed. These low-frequency components, characterized by stronger vibrations, play a more significant role in particle movement. Figure 2(b) displays the variation of membrane Z-directional vibration displacement over time at X = 0.12 mm (the yellow dash line in Fig. 2(a)). Figure 2(c) displays the XY two-dimensional spatial distribution of the membrane's Z-directional vibration at t = 0.35 µs, indicated by the purple dotted line in Fig. 2(a). Figure 2(d, e, f) represent the experimentally measured Z-directional vibration of the membrane under identical conditions as the simulations. The measurements align well with the simulated results, confirming their agreement.

In Fig. 3(a-d), we present the distribution of the acoustic pressure field in the XZ plane at four specific time points (In Visualization 3). These time points offer insights into the changes in the lateral motion state of the particles as they are influenced by the acoustic radiation force in the liquid after detaching from the membrane. The sound wave generated by the pulsed laser in water is also in the form of pulses, rapidly spreading outward from the excitation position. This spreading leads to the particles immersed in water experiencing acoustic radiation forces that generally align with the direction of sound wave propagation. Consequently, the particles detached from the metal membrane are propelled apart by the sound wave. It is worth noting that the speed of acoustic wave in water (1500 m/s) is significantly higher than the propagation velocity of Lamb waves in membrane (67 m/s). The difference in velocity indicates that acoustic wave in water and Lamb wave in membrane also have a discrepancy in the duration of their force on the particles.

 

Fig. 3. (a-d) Distribution of the transient sound pressure field in the XZ plane at four specific times: 50 ns, 100 ns, 150 ns, and 200 ns. The pink arrows in the illustration indicate the magnitude and direction of the acoustic radiation force in the X direction. (e, f, g) Images of patterned particles forming “B”, “I”, and “T”, respectively, with the target pattern located in the lower right corner.

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As evident from Fig. 3(a-d), the acoustic intensity in the water significantly weakens after 200 ns, resulting in decrease of the acoustic radiation force on the particles. In contrast, as can be seen from Fig. 2(a, d), 12 µs after the laser irradiation, the vibration of the metal membrane remains strong. This suggests that even after a period of 200 ns, the particles continue to be affected in the Z direction due to the vibrations of the membrane. Within 200 ns following laser irradiation, the movement of particles in the X direction necessitates the combined action of mechanical force and acoustic radiation force. Over a period of 200 ns, the upward vibrations of the metal membrane induce upward jumps in the particles. Subsequently, upon detachment from the metal membrane, the particles are transported by the acoustic waves present in the water. The particles situated within the region of the membrane that is vibrating downward are unable to detach from the membrane and remain stationary. As a result, within the central area of the laser stripe irradiation region, the particles exhibit no movement in the X direction. Along the edges of the laser stripe, the particles are transported by the acoustic waves, resulting in the formation of empty troughs. These empty troughs manifest as two distinct black lines, as observed in experiments. This capacity enables the generation of arbitrary patterns through the flexible modulation of the pulsed laser.

We modulate light to create the patterns “B,” “I,” and “T” with a stripe width of approximately 175 µm. When the structured light is projected onto the undersurface of the stainless-steel membrane, the membrane's vibration lifts the silica particles, and the acoustic radiation force pushes them to either side of the pattern stripe. As a result, the particles arrange themselves in the shape of “B,” “I,” and “T,” as depicted in Fig. 3(e, f, g). The process of particles patterning is detailed in Visualization 1. The width of the stripes formed through particle patterning is approximately 320 µm, while the overall pattern size spans approximately 1.2 mm × 1.6 mm. This system enables the rapid arrangement of about 5000 of silica particles into various predefined shapes, including straight stripes and curves. Consequently, it achieves the rapid and arbitrary patterning of a large number of particles in a liquid environment.

In the PPAM system, we not only achieve rapid arbitrary patterning using thousands of particles but also enable the transport of multiple particles. To accomplish this, we utilize a DMD to modulate a pulsed laser into moving concentric ring light, as depicted in the green pattern in Fig. 4(a). This pattern consists of a solid inner circle and three ring-shaped circles, with a duty cycle of 50% and a period size of 350 µm.

 

Fig. 4. (a) Geometry of the concentric circles. (b) Experimental results of XY spatial displacement excited by concentric circles at 0.5 µs. (c) Simulation results of XY spatial displacement excited by concentric circles at 0.5 µs. (d) Simulation results of the force in the XY plane at 500 ns are displayed. The colormap indicates that red (blue) corresponds to forces directed outward (inward) toward the center of the circle. The dashed circles represent zero force. (e) Distribution of silica particles at t = 0 s. (f, g, h, i) The position of particles transported by concentric circles at 2 s, 11 s, 20 s, and 29 s.

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Following the irradiation of concentric ring light, the vibration fields of the membrane and the resulting force fields gradually diminish in range over time (In Visualization 4 and Visualization 5). Through experiments and simulations, we obtain the distribution of the XY spatial vibration field that is excited by the concentric ring light on a 10 µm stainless steel membrane 500 ns after laser irradiation, as shown in Fig. 4(b, c). The focusing effect of the shrinking vibrations is primarily significant within a radius of 0.1 mm. The corresponding distribution of the acoustic pressure field in the water within that region is illustrated in Fig. 4(d). In Visualization 4 and Visualization 5, it can be observed that the circular acoustic field gradually contracts inward, meaning that the regions of low sound pressure are also shrinking. This results in the particles progressively moving towards the center of the ring. The detailed process is described in Note 4 of the Supplement 1. The use of concentric circles structured light enables the trapping effect on multiple particles.

In the experiment, the lower surface of the membrane is illuminated with concentric circles structured light. Figure 4(e) illustrates the initial distribution of silica particles before the laser pulse irradiation. The red circle in the figure represents the operational region of the concentric circles structured light on the membrane. The position of the concentric circles on the DMD can be changed every 3 seconds along a predefined path. This enables adjustments to the position where the concentric circles illuminate on the membrane. The structured light irradiation generates an acoustic field that attracts the surrounding silica particles towards the center.

As the irradiation area follows a predetermined path, more than ten particles on the membrane also move accordingly, ultimately being transported to the predetermined end position guided by the movement of the irradiation area. Figure 4(g, h, i) illustrate the areas reached by the particles during transport at t = 11s, 20s, and 29s, respectively. The red arrow indicates the direction of transport. These figures provide an overview of the entire process of transporting multiple silica particles using concentric circles, showcasing the movement and positioning of the particles at different time intervals.

After 29 seconds, the silica particles are transported 320 µm from the starting position (indicated by the red dashed circle) to the designated location. The transport process is demonstrated in Visualization 2 of Supplementary Material. By employing the PPAM system for transport, simultaneous manipulation of multiple particles becomes feasible. Moreover, the transport path and positions of the particles can be arbitrarily set, demonstrating a high degree of flexibility and transport efficiency.

3. Conclusion

In summary, our work demonstrates the PPAM system in a liquid environment. By incorporating a DMD spatial light modulator, we can manipulate structured light and create customizable vibration and acoustic fields. The combined mechanical forces from the vibration field and acoustic radiation forces from the acoustic field allow for effective patterning, and transport of micrometer-sized particles. The system operates in a liquid environment and is capable of manipulating particles with diameters in the range of tens of microns. The simultaneous patterning of thousands of microparticles in a liquid using PPAM demonstrates higher efficiency than optical tweezers. This system exhibits excellent biocompatibility, making it well-suited for applications involving biological substances. We envision the potential for large-scale manipulation and transport of blood cells, targeted drugs, and other entities. Additionally, the PPAM system holds application prospects in areas such as microfluidic chips.

4. Method

4.1 Experimental system

In our PPAM system, we utilize a solid-state laser (Nimma-900 laser from Beamtech optronics co., Ltd.) to generate pulsed laser beams with a wavelength of 532 nm, a frequency of 10 Hz, and a diameter of 1 cm. After the laser emission, it passes through a plano-convex lens (f = 50 mm) for beam expansion, then through a doublet convex lens (f = 400 mm) to modulate into parallel light with a diameter of 3 cm, and finally irradiates onto the DMD (TI S1410-9031). The DMD has a resolution of 1400 × 1050 and an overall size of 0.95 inches. We connect the DMD to the computer and use a binary digital matrix to generate the letters ‘B’, ‘I’, and ‘T’ on the computer. In the matrix, the letter parts are represented by ‘1’ and the background parts by ‘0’. As a result, the corresponding letters will also appear synchronously on the DMD screen. The collimated light, modulated by the DMD to produce letter-shaped structured light, is reflected by a mirror and imaged by a convex lens (f = 50 mm). It is then projected onto the underside of the stainless steel membrane. Eventually, a large number of silica particles arranged in the corresponding letter shapes are formed on the membrane's surface. The size of the generated structured light field is approximately 1 mm × 1.4 mm, and due to Lamb wave propagation, the resulting particle pattern size is slightly larger than the structured light, measuring approximately 1.2 mm × 1.6 mm. In the experiment, an industrial camera (MV-CE200-10UC, Hikvision) is used to capture photos and videos. We use a laser vibrometer (Polytech VFX-F-110) to measure the distribution of membrane vibrations induced by single-line and concentric-circle structured light excitation.

4.2 Theoretical simulation

We utilized the COMSOL Multiphysics solver package to simulate the generation of sound waves in solid and fluid mediums induced by pulsed laser excitation. The simulation involved four modules: radiation beam heat absorption, solid (fluid) heat transfer, solid mechanics, and pressure acoustics. Additionally, three multi-physics fields were considered, including radiation heat transfer in the absorbing medium, thermal expansion, and the acoustic-structural boundary. In the simulation, both the water layer and stainless-steel membrane have a width of 12 mm. The thickness of the water layer is 8 mm, while the membrane thickness is 10 µm. The stainless-steel membrane has a density of 7930 kg/m³, a Young's modulus of 194 GPa, a Poisson's ratio of 0.3, a specific heat capacity of 0.5 kJ/kg·K, a thermal conductivity of 45.0 W/m·K, and an optical absorption coefficient of 1 × 10⁸ m^-1. The water has a density of 1000 kg/m³ and a sound velocity of 1500 m/s.