Flow and phase transitions in micellar systems

Transcription

1 Flow and phase transitions in micellar systems Shear thickening & time-dependent rheology Outline Basic shear thickening phenomena Wormlike micelles Experimental approach light scattering microscopy Rheological & structural behavior Collaborators Chu-heng Liu (Xerox) light scattering microscopy Philippe Boltenhagen (Strasbourg) rheo-scattering Yuntao Hu (Unilever ) rheo-scattering Jacqueline Goveas (Rice Univ. ) theory Worm-like micellar solutions (in water) surfactant ~- 4 Å random coil ~4 Å Polar head hydrophilic (often charged) H O H-C tail hydrophobic H O Concentration regimes critical micelle concentration (cmc) c < cmc only free surfactant molecules c > cmc micelles + free surfactant molecules overlap concentration (c*) micelles overlap each other above this concentration experimentally often defined by the concentration where solution viscosity = twice solvent viscosity Dr. David Pine, UCSB (ITP Complex Fluids Program 4//)

2 Shear-thickening worm-like micellar solutions CTAB + NaSal + H O (cetyltrimethylamonium bromide + sodium salicylate + water) Br O Na + OH O Sal ions are incorporated into micelles ~: (NMR) (this is unusual most charged micelles are not neutral!) c* ~ a few millimolar Most micellar systems which exhibit shear thickening are like this 3 Shear thickening in dilute worm-like micellar solutions (basic rheology) Response after turning on steady shear rate greater than some threshold η p η η Note long latency time prior to shear thickening p i η i ~ critical shear rate γ 4 6 Shear thinning Shear thickened state Shear thinning (after shear thickening) 8 Concentrations for shear thickening: ~cmc < c < ~3-5 c* 4 Dr. David Pine, UCSB (ITP Complex Fluids Program 4//)

3 Experimental Approach simultaneous mechanical & structural measurements stress Rheometer time Laser Cylindrical lens ~9 transparent shear cell CCD Illuminate sample in transparent shear cell with sheet of light Video monitor light scattering microscopy Collect light scattered by fluid structures with video camera (dark-field microscopy) 5 Time-dependent rheology & microscopy above the critical shear rate inner outer Arrows show growth of shear-induced structure light scattering microscopy images 4 3 constant shear rate, γ = 8 sec inner cylinder at left outer cylinder at right a white structure starts growing from inner cylinder (left) after about minute growth of white structure associated with increase in apparent viscosity white structure is very viscous or even gel-like 6 Dr. David Pine, UCSB (ITP Complex Fluids Program 4//) 3

4 Measuring velocity profile motor Laser Cylindrical lens transparent shear cell CCD orient sheet of light horizontally Video monitor track movement of tracer particles move through illuminated sheet 7 Velocity profiles during shear thickening experiment 3 velocity (a) 6 s..5.. (b) 438 s.5.. (c) 735 s.5.. (e) 37 s.5. gel There is little or no flow within the white phase we can call it a ~gel Note: shearing a gel causes it to scatter more light 8 Dr. David Pine, UCSB (ITP Complex Fluids Program 4//) 4

5 Why the steady state gel thickness & apparent viscosity increase discontinuously critical shear rate γ () When γ γ, gel begins to grow from inner cylinder > c () Since the gel doesn t flow, the shear rate and stress in the remaining fluid increase (3) This causes more gel to form and further increases the shear rate & stress (4) This process continues until there are no more micelles available to attach to gel 9 Time-dependent rheology & microscopy for controlled stress s 363 s 46 s 666 s constant stress, σ =. 6 Pa inner cylinder at left outer cylinder at right a white structure starts growing from inner cylinder after about s growth of white structure associated with increase in apparent viscosity white structure is very viscous or even gel-like gel-like structure continues to grow outward until reaching a steady-state thickness of about 5/8 of the cell gap width Dr. David Pine, UCSB (ITP Complex Fluids Program 4//) 5

6 Increase the stress at higher level of stress, gel seems to nearly fill the gap 4 3 constant stress, σ =. Pa there continues to be a close correspondence between the apparent viscosity and the thickness of the gel-like structure 3 4 BUT... sometimes the correspondence between gel thickness & apparent viscosity breaks down... s 3 s 34 s 5 s 86 s at s, there is no gel at 3 s, the gel is barely visible but can be seen nearly throughout the gap 4 constant stress, σ =.6 Pa at 34 s, the gel is clearly visible & fills gap BUT... the apparent viscosity does not rise significantly until ~ s later What is happening? HOMOGENEOUS NUCLEATION Dr. David Pine, UCSB (ITP Complex Fluids Program 4//) 6

7 Rheology of Shear-Thickening Micellar Solutions Nucleation and stability of gel phase Mechanical equilibrium (balance torque at r) σ r r Couette cell (top view) Stress is greatest near inner cylinder when stress or shear rate is increased incrementally, gel nucleates only at the inner cylinder when stress or shear rate is increased such that σ >> σ c, gel nucleates everywhere in the cell where σ >> σ c HOMOGENEOUS NUCLEATION 3 Role of stress and shear rate σ f σ s σ c III In region II, γ < γ, but gel remains under controlled stress } c II Shear rate in gel phase is nearly zero but gel phase persists for σ > σ c I Gel phase is observed iff σ > σ c. γ c stress (and not shear rate) controls the formation of gel 4 Dr. David Pine, UCSB (ITP Complex Fluids Program 4//) 7

8 Rheology of Shear-Thickening Micellar Solutions Apparent viscosity (same data as in previous plot) Homogeneous shear thinning solution Growing gel Constant viscosity due to slip layer apparent viscosity (Pa-s) (b) I II III Shear thinning.. σ c σ s σ f Shear thickening under constant stress and constant shear rate a rheological phase diagram (summary of data) σ f σ s σ c III II Stress plateau stress linear in shear rate re-entrant data controlled stress only gel/solution coexistence heterogeneous nucleation homogeneous nucleation I constant stress γ c c constant shear rate Dr. David Pine, UCSB (ITP Complex Fluids Program 4//) 8

9 Quenching from gelled state to lower shear rate Quench experiment () prepare system in high shear fully gelled state () reduce shear rate & follow evolution of stress shear (d) (c) (b) (a) (d) (b) (a) (c) Apparent Melting of gel 3 4 s 369 s 438 s 499 s Dr. David Pine, UCSB (ITP Complex Fluids Program 4//) 9

10 Shear thickening (II) and shear thinning (IV) (a) controlled stress controlled shear rate.7 mm IV III II I σ f σ s apparent viscosity (Pa-s) - - σ c -3 (b) I II controlled stress controlled shear rate III IV 9 Behavior in shear thinning region.6 apparent viscosity (Pa-s) Pa.7 Pa.85 Pa controlled stress controlled shear rate regime III regime IV Dr. David Pine, UCSB (ITP Complex Fluids Program 4//)

11 Effect of counterion valence on critical shear rate & stress critical 5 5 NaSal MgSal σ c, critical stress (mpa) 5 5 NaSal MgSal concentration of CTAB (mm) concentration of CTAB (mm) More rheology apparent viscosity (Pa-s) Na Mg mm shear rate ( s - ) apparent viscosity (Pa-s) mm Dr. David Pine, UCSB (ITP Complex Fluids Program 4//)