As the demand for energy soars, the introduction of algae biofuels as a renewable source of energy is receiving much attention. Suspensions of these actively swimming microorganisms exhibit an effective viscosity that may depend on volume fraction, cell shape, and the nature of locomotion (e.g. ``pushers\'\' vs. ``pullers\'\'). Here we report experimental measurements of shear viscosity for suspensions of unicellular green algae (Dunaliella primolecta, a biflagellated ``puller\'\'). We use a cone-and-plate rheometer to measure the dynamic shear viscosity for both motile and non-motile suspensions of D. primolecta at concentrations ranging from 0.1% to 10% of volume fraction. Viscosity increases with concentration for both cases, but the active suspensions of ``pullers\'\' have a comparatively higher effective viscosity than passive suspensions. This observation confirms recently proposed theories that predict higher effective viscosity for ``puller\'\' suspensions compared to non-motile suspensions. Our locomotion study reveals that motile algal cells prefer to align and migrate in the direction of positive shear flow vorticity. It is our belief that such a shear-induced response of the algal cells impacts the resulting effective shear viscosity.

1. 1 caret004@umn.edu University of Minnesota Effective Viscosity of Actively Swimming Algae Suspensions Randy H Ewoldt1,2, Lucas Caretta2 , Anwar Chengala3, Jian Sheng4 1. Institute for Mathematics and its Applications 2. Department of Chemical Engineering and Materials Science3. St. Anthony Falls Laboratory, Department of Civil Engineering4. Department of Aerospace Engineering & Mechanics 4/5/2011

3. Background of Suspension Viscosity

4. Actively Swimming Suspensions

5. Experimental Procedure and Difficulties

6. Results

7. Future Work4/5/2011

9. Motile

10. Bi-flagellated

11. Cell-wall-less

12. Unicellular green algae

13. Slightly negatively buoyant {23% oil content (Chisti Y, 2007)}

14. Biofuels!*

15. Exxon: $600 million

16. No Competition with Agriculture

17. Less Competition with water

18. H2O, CO2, Sunlight

19. Algae = 2000 gallons of fuel / Acre / Year

20. Ethanol = 250 gallons of fuel/ Acre/ Year

21. Gasoline, diesel, jet fuelScience (Perspective), 13 August 2010 “Fuel source of the future?” Nature (News), 24 September 2009 *exxonmobil.com/algae 4/5/2011

22. Suspension Viscosity caret004@umn.edu 4 Passive Suspensions Objective Intrinsic viscosity measurement Einstein 1906, 1911; Frankel and AcrivosJ. Fluid Mech. 1970 … Active Suspensions “pullers”increase viscosity “pushers”decrease viscosity [Ramaswamy 2010] [Hatwalne 2004] [Saintillan 2009] 4/5/2011

24. Bacillus Bacteria

25. Vortex decay in a 2D film

26. PRL 2010

27. ChlamydomonasReinhardtiimicroalgae

28. Cone and Plate RheometerIntrinsic viscosity not resolved [1] SalimaRafäı, LevanJibuti, and Philippe Peyla, Physical Review Letters (2010) [2] AndreySokolov and Igor S. Aranson, Physcial Review Letters (2009) 4/5/2011

29. Experimental Difficulties caret004@umn.edu 6 Torque, Displacement Concentration gradient rotational rheometer Issues: Settling (non-motile organisms) Flow-induced locomotion (Chengala et al. 2010) Low absolute torque signal Small torque increase Inertia and secondary flow Solutions: t < 75sec to avoid transients Micropipettes (precision) Shear-rate within experimental window 4/5/2011

31. Migration would be unknown without visualizationInward vorticity t = 0min 5min 4min 13min 2min 1min 6min 3min Outward vorticity 13min t = 0min 5min 4min 2min 1min 6min 3min 4/5/2011

33. Measurement time t < 2 min OKLive Primolecta Inward vorticity 5min 4min t = 0min 2min 1min 6min 3min 13min Outward vorticity 5min 4min t = 0min 2min 1min 6min 3min 13min

34. Low Viscosity caret004@umn.edu 9 Experimental Limitations for Low Viscosity Measurements Torque too low Inertia too high R=20mm, =2° cone 4/5/2011

35. Low Viscosity caret004@umn.edu 10 Experimental Limitations for Low Viscosity Measurements Torque too low Inertia too high accessible experimental range R=20mm, =2° cone 4/5/2011

36. Low Viscosity caret004@umn.edu 11 Experimental Limitations for Low Viscosity Measurements Torque too low Inertia too high accessible experimental range R=20mm, =2° cone 4/5/2011

37. Low Viscosity caret004@umn.edu 12 Experimental Limitations for Low Viscosity Measurements Torque too low Inertia too high accessible experimental range R=20mm, =2° cone 4/5/2011

39. 40mm 2° SST Cone

40. Peak Hold Flow at 51s-1

41. 24 degrees Celsius

42. Data average over 15-75 secondscaret004@umn.edu 13 4/5/2011

44. 40mm 2° SST Cone

45. Peak Hold Flow at 51s-1

46. 24 degrees Celsius

47. Data average over 15-75 secondsPhotograph taken by Dr. PawelKonieczny caret004@umn.edu 14 4/5/2011

49. 40mm 2° SST Cone

50. Peak Hold Flow at 51s-1

51. 24 degrees Celsius

52. Data average over 15-75 seconds“pullers”increase viscosity Photograph taken by Dr. PawelKonieczny caret004@umn.edu 15 4/5/2011

54. 40mm 2° SST Cone

55. Peak Hold Flow at 51s-1

56. 24 degrees Celsius

57. Data average over 15-75 seconds“pullers”increase viscosity Photograph taken by Dr. PawelKonieczny caret004@umn.edu 16 4/5/2011

59. 40mm 2° SST Cone

60. Peak Hold Flow at 51s-1

61. 24 degrees Celsius

62. Data average over 15-75 seconds“pullers”increase viscosity Photograph taken by Dr. PawelKonieczny caret004@umn.edu 17 4/5/2011

65. Modeling opportunitiesD. Primolecta “puller” 4/5/2011

67. Anwar Chengala; Civil Engineering, St. Anthony Falls Laboratory

68. Prof. JianSheng; Department of Aerospace and Mechanics

69. Prof. Miki Hondzo; St. Anthony Falls Laboratories

70. Dr. PawelKonieczny; Institute for Mathematics and its Applications

71. Prof. Chris Macosko, Dr. David Giles, and the Polymer GroupQuestions? 4/5/2011

72. University of Minnesota Appendix Slides caret004@umn.edu 20 4/5/2011

73. Appendix A: Preparing Concentrated and Dead Algae Centrifuge 12 x 50 mL of “standard” algae culture (10min, 1000 rpm) Aspirate ~45 mL of supernatant from each tube Remaining 50mL is divided in half (25mL Motile, 25mL non-motile) Fixed (non-motile) Living (motile) Kill algae by addition of 2mL of 4% weight by volume formaldehyde in PBS Wash out formaldehyde - Add media (575mL), re-suspend (mix), Centrifuge, aspirate supernatant (575mL) - Centrifuge, aspirate supernatant (22mL), Add media (22mL), re-suspend (mix) Deposit varying volumes into smaller vials to be diluted to desired concentrations for rheological experiments 4/5/2011 caret004@umn.edu XVI

74. 4/5/2011 caret004@umn.edu XVII Appendix B: Assumptions for Einstein Analysis Surrounding fluid or solvent is incompressible and Newtonian and can be treated as a continuum. Creeping flow (i.e., negligible body forces, torques, and in- ertia). Neutral density, p, = p , (i.e., no settling). No slip between the particle and the fluid. Rigid, spherical particles. Dilute (noninteracting) particles. No influence of walls. No particle migration. Velocity perturbations due to a particle are local; the average velocity field in the surrounding fluid is the same as if the particles were not present. Macosko, Rheology, 1994

76. 40mm 2° SST Cone

77. Algae Conc̴ 18.74mil cells/mL

78. 24 degrees Celsius

79. Data average over 15-75 seconds4/5/2011 caret004@umn.edu XVIII

81. 40mm 2° SST Cone

82. Algae Conc̴ 18.01mil. cells/mL

83. 24 degrees Celsius

84. Data average over 15-75 seconds4/5/2011 caret004@umn.edu XIX

86. Occur only above a critical shear rate (S >10s-1)

87. Below critical shear, quasi-random bias towards flow directionSlide notes and video courtesy of Anwar Chengala (Civil Engineering) 4/5/2011

89. Ratio of the torque and speed can be used to calculateviscosity. Cone and Plate geometry Image from C.W. Macosko XXI 4/5/2011

90. Appendix F: Rheology Instrumentation caret004@umn.edu XXII DSR Visualization Setup SST Cone Upper Geometry Plexiglas Lower Geometry Optical Access from Below Photograph from underside of sample 4/5/2011