Yeast cells of diameter 5-6 ^m were used as target biological particles for manipulation using a focused LP21 mode beam of approximately 10 mW at 650 nm. Figure3.17a-d shows the capture of cell A by the LP21 beam (denoting by dashed concentric circles) and moved toward cell B to form a dimer, followed by the rotation of the dimer by 30 (Fig. 3.17c); Fig.3.17d-f display dimer separation: theLP21 beam was turned off, and turned back on after adjusting the beam angle to the position of cell B. Cell B was subsequently captured in the beam and moved toward left with increasing speed, while cell A remained stationary. As the LP21 beam spot accelerated up, the trapped cell eventually slipped out of the beam chuck due to Stokes resistance force (media 1). The process was recorded at 12 frames/s, and the frame counter between Fig.3.17e, f, (f) and (g), (g) and (h) were at 3, 2 and 6, corresponding to time intervals from the start of 0.25 s, 0.16 s, and 0.50 s respectively. Using the coordinates of cell A as a stationary reference, the beam speed of motion was
Fig. 3.17 a, b shows the pairing of a two-cell dimer, with dashed concentric circles denoting the beam spot of theLP2i trap, followed by Fig. 3.16c, with the rotation of dimer by 30°. Figure3.16d-f display the separation process of the dimer: the LP21 trap beam was turned off and turned on after being moved to the position of cell B. The LP21 trap beam is then acceleration toward field left. Trapped cell B eventually slipped out of beam capture due to Stokes force (media 1)
calculated, and the lateral trapping force was estimated to be f = 0.84pN using the speed measured at the critical trapping condition (v & 20 ^ms-1) in the Stokes equation f = 6nqav, with n = 0.8937 x 10-3 s(Pa-s) at 25 °C. This measured value is less than the calculated 1.2 pN, suggesting that the intensity distribution of each lobe deviated from the Gaussian assumption after being focused with an axicon, which is confirmed by the profile image Fig. 3.15b. Considering a very low laser power (<10mW at cell) is used in this manipulation process, the LP21 mode exhibits a high efficiency in optical trapping and manipulation.
Figure3.18 shows an example of particle rotation. Yeast cells A, B, C, and D were immersed in water on a cover glass, with cell D behind the plane where cell A, B and C located. Figure3.18a-i displays the rotation of cell dimer A-B: both cell A and B were trapped by the LP21 beam, while cell C was outside the four-lobed trap and drifted freely toward field right. The beam was rotated by twisting a fiber segment, and dimer A-B followed the rotation counter-clockwise through 107° of rotation, as shown in Fig.3.18b-h. Then the beam was turned off and cells A and B immediately started Brownian free motion, along with neighbor cells C and D that was located under dimer A-B. Throughout dimer rotation, cell C remained stationary in its position and cell orientation, indicating that the focused LP21 beam was highly localized and only affected cell A and B.
The rotational torque, as modeled in next section, depends on the length of the arm lever (&7 ц-m) and the tangential force applied to the particle or cluster of particles. Based on the intensity distribution of the focused LP21 beam (Fig. 3.15b), the dissociation energy needed to separate a dimer tangentially was approximately twice of the energy needed for radial separation, and the rotational torque for yeast dimers was estimated to be 11.8pN-^m. It was also observed that the LP21 beam was capable of rotating clusters of 3, 4, and 5 cells.
Fig. 3.18 a-i display rotation of cell dimer A-B: both cell A and B were trapped by LP21 beam, while cell C was outside four-lobed trap and moves freely toward right. As LP21 beam rotated, dimer A-B followed the rotation by 107° counter clockwise, as shown in Fig. 3.18b-h. Then LP21 beam was powered off and optical trapping disappeared, cell A and B exhibited free moving, along with neighbor cells C and D, which was located under dimer A-B when laser bean was on