Optical Trapping - MIT

1 Optical TrappingMIT Department of Physics(Dated: August 11, 2017)An Optical trap or Optical tweezers is a device which can apply and measure piconewton sizedforces on micron sized dielectric objects under a microscope using a highly focused light beam. Itallows very detailed manipulations and measurements of several interesting systems in the fields ofmolecular and cell biology and thus acts as a major tool in biophysics. They are used in biologicalexperiments ranging from cell sorting to the unzipping of DNA.

2 Similar principles are also usedin physical applications such as atom cooling. In this experiment, you will measure the Brownianmotion of a trapped silica microsphere in aqueous solution, both testing the theory of statisticalmechanics and calibrating the spring constant of the trap. Then, using the calibrated trap, you willmeasure forces in biological systems, such as the actin-myosin molecular motors of vesicle transportin onion cells, theE. coliflagellar motor, or the restoring force of a stretched DNA its present form, large portions of this lab guide are derived from the literature for MITB ioenhineering subject [1] and UC Berkeley Physics subject Physics 111 Lab [2].

3 PREPARATORY QUESTIONS1. In the limit of ray optics, the Trapping force ona dielectric sphere can be understood as arisingas a reaction force to the change in linear mo-mentum experienced by refracted light rays. Tobetter understand how the scattering and gradientforces and the trap s stability vary withdisplacement from the trap center both verticallyand horizontally, spend some time exploring thisJava applet simulator developed by the lab ofRoberto DiLeonardo, CNR-IPCF Dipartimento diFiscica, Universita di Roma Sapienza in Italy [3] andqualitatively sketch how a dielectric sphere slightlydisplaced from the center of a stable trap experi-ences a restoring force.

4 Is the center of the trap atthe same location as the focus of the light? Explainwhy high numerical aperature optics are used inthe experiment. Finally, given the wavelength ofthe laser and the sizes of objects to be trappedin this experiment, do you trust the ray opticssimulation to be quantitatively accurate?2. Estimate the time and distance required for a mo-bile bacteria of typical bacterial speed in an aque-ous environment to come to a halt under viscousdrag. See the seminal work of Purcell (1976) [4].

5 How do these time and length scales compare tobiologically relevant scales? How doesmacompareto the force needed to keep the bacteria moving atits initial constant speed (before it stopped), whereais the average deceleration of the bacteria, andmis its mass?3. What are the principle safety hazards you couldencounter in this experiment? How do you avoiddanger from these hazards?SUGGESTED SCHEDULEDay 1:Familiarize yourself with the apparatus. Makedetailed notes on the effects of each control an appropriate sample and trap a 2:Calibrate the QPD voltage to stage position us-ing a fixed bead sample.

6 Measure Brownian noiseon a floating bead to obtain data for equipartitionand PSD analysis. Obtain a first estimate of Boltz-mann s constant and trap 3:Make an onion cell sample and trap a 4:Finish onion cell experiment. Optionally, doStokes drag measurement to refine Boltzmann sconstant or further biological experiments. Notethat biological samples may take days to prepare,so you must plan ahead and communicate with experimental goals are:1. Measure Boltzmann s constant using equipartitiontheorem and Brownian PSD2.

7 Calibrate Optical trap stiffness versus laser supplycurrent3. Estimate force and speed of molecular motorstransporting vesicles in onion cellsI. INTRODUCTIONL ight can impart a force, due to the fact that photonscarry momentum. These forces are very small comparedwith those typical in the macroscopic world, but they canbe very large relative to typical forces on single atoms,molecules, and small biological organisms, at the microm-eter and nanometer scale. Focused laser beams can se-lectively impart force to atoms, to cool them from roomId: ,v 2012/02/06 23:45:01 spatrick Exp spatrickId: ,v 2012/02/06 23:45:01 spatrick Exp spatrick2temperature to a few micro-Kelvin and below.

8 They canalso be used to push or trap microscopic dielectric spheres or even entire, living, cellular organisms, inside bio-logical method of Optical Trapping was discovered byArthur Ashkin in 1970 [5] [6]. He calculated that the ra-diation pressure from a high power laser, focused entirelyonto a micron-sized bead (or microsphere ), would ac-celerate the bead forward at nearly 106m/s2. Whenhe performed the experiment to test this prediction, hefound that while the target bead was indeed accelerateddownstream, other beads in the solution were attractedlaterally into the beam-path from other parts of the sam-ple.

9 He then created the first working Optical trap by us-ing two opposing laser beams. At one point a bacteriumthat had contaminated a sample became trapped in thebeam, thus instigating the trap s revolutionary use in cellbiology. Today Optical traps are used extensively in bothatom- Trapping experiments and in biophysics labs this laboratory experiment, you will explore theuse of Optical forces to trap dielectric microspheres heldwithin a thin layer of water and vesicles in onion typical mechanical forces involved are on the scale ofpiconewtons (10 12N).

10 Relative to this scale, hydrody-namical forces (drag and diffusion) on the microspheresand vesicles are substantial. Thus, the Optical trap pro-vides an excellent opportunity to study the physics ofBrownian motion, which you will use to obtain a quan-titative measurement of Boltzmann s constant. In theprocess, you will calibrate the dependence of trap stiff-ness (force/distance) on laser supply current. Biologi-cal motors, which are vital to intracellular transport andbacterial locomotion, also act with forces on this may thus employ the Optical trap to quantify thespeed and force of a molecular motor moving a vesiclealong an actin fiber in an onion The Physics of Optical TrappingThe following material in this subsection is takennearly verbatim from UC Berkeley s Junior Lab guideon their Optical trap experiment [2].