LOGO

Visualization of Acoustic Cavitation Effects on Existing Crystals

报告人：李洁琼导师：王静康 教授2013/03/23

Click to add your text

References

Contents

Introduction1

Experimental2

Results and discussion3

Conclusions4

Introduction

Introduction

acoustic cavitation

buble growth buble collapse shockwave emerge shockwave impact

Inertial Cavitation (high energetic)

Non-inertial Cavitation (low energetic)

high energetic shockwaves

microjets

Acoustic Effects Seed Crystals & Agglomerates

Alteration of CrystalHabit & Size

vibration&

implosion

attrition&

breakagecrystal

growth rateAgglomerates

An assemblage of particles which are loosely coherent.

Distinction

Aggregates

An assemblage of particles rigidly joined together.

Introduction

HS

OBD

SEM

methods

Effect of Cavitation

high speed imaging

optical beam deflection measurements

Scanning Electron Microscopy analysis

创新点

Materials

Experimental

AThe first batch was synthesized, using the method described by Nancollas and Reddy: slowly adding 2dm3 of a 0.20M CaCl2 (VWR, 100%) solution to 2dm3

0.20M Na2CO3 (Boom, >99.5%) solution at 25 ℃ and dried at 105 ℃.

BThe second batch was synthesized by adding both solutions simultaneously at 25 ℃.

B1The freshly precipitated seed crystal batch A and part of batch B, were aged overnight in mother liquor and were subsequently washed with Milli-Q water each day for one week. Afterwards, the washed seed crystals were aged for three weeks before filtering.

B2The other part of batch B wasalso washed with Milli-Q by carefully replacing the mother liquid by Milli-Q water. After washing the seeds each day for one week, the seeds were stored in Milli-Q water.

Fig. 1. SEM pictures of seed crystals: (A) seed crystals A and (B) seed crystals B.

Experimental set-up and procedures

ultrasonic irradiation

small thermostatted square glass box

0.05 gcalcite seeds

95 mM KCl 1.6 mM NaHCO3

1.6 mM CaCl2

Experimental

Fig. 2. Scheme of the experimental set-up consisting of: (1) ultrasonic transducer, (2) high power light, (3) high-speed camera with zoom lens, and (4) glass box.

100mL saturated CaCO3 solution

Results and discussion

Bubble structures

Fig. 3. Streamer (A) and large clusters (B). Image A is recorded without seed crystals in the liquid, for better distinction of the bubbles from the seeds. Cluster formation always started on the surface of a particle. Part of the ultrasonic source is visible (dark silhouette) at the top of both images. Scale bar is 500 lm (both images).

Results and discussion

Disruption of aggregates and deagglomeration

Fig. 4. Images of an oscillating cavitation bubble that nucleated on the surface of a type A calcite crystal. The single crystal fragmentizes as a result of the violent collapse of the bubble. Frame rate: 125,000 fps. Fig. 5. Images of streamer cavitation causing deagglomeration of type B1 calcite crystal (arrow 1). The agglomerate (length: ~49μm) splits up (arrow 3) as a result of the bubble collapse (arrow 2). Frame rate: 125,000 fps; shutter time: 1μs, scale bar is 50μm.

Fig. 6. Rupture of type B1 calcite crystal (length: ~41μm), shot at 300,000 fps, scale bar is 53μm.

Results and discussion

Seed acceleration by bubble expansion and collapse

Fig. 7. An example of crystal (B1, length: ~ 36μm, arrow 2) acceleration by bubble (arrow 1) expansion and collapse. Frame rate: 100,000 fps. Scale bar length is 100μm. The cavitation cycle is 23μs which is slightly more than two frames. One frame shows an instant of bubble growth followed by the next frame showing an instant of bubble collapse.

Results and discussion

Seed acceleration by bubble expansion and collapse

For each frame the centre position of the particle of interest was measured, the displacement from frame to frame was calculated and multiplied by the frame rate, fps (s-1), the particle velocity (m·s-1)is presented as:

where x (m) is the x-coordinate of the particle of interest and y (m) is the y-coordinate.

In a similar fashion the acceleration (m·s-2)was determined from the acquired velocities:

Results and discussion

The drag force on small particles moving through fluids at relatively low speeds, where there is no turbulence, can be described by Stokes’s law. From this law, a general equation of motion can be derived, assuming the liquid velocity to be zero:

The relaxation time is a constant, defined by material and fluid properties:

ρs (kg/m3) - the density of the solid; η (Pa·s) - the liquid viscosity; d (m)- the spherical particle diameter.

Results and discussion

Stokes’s law can only be used for Reynolds numbers smaller than 1. At maximum velocity the particle’s Reynolds number is 65 and Newton’s law has to be applied, resulting in the following relaxation time:

where Re (–) is Reynolds number and CD (–) the drag coefficient, described by an empirical relation for 1 < Re < 1000.

In reality the velocity profile will be between Stokes’ law and the outer limit of maximum drag calculated with Newton’s law, defined as the drag window.

Results and discussion

Seed acceleration by bubble expansion and collapse

Fig. 8. Velocity profile of accelerated crystal B1 from Fig. 7. The velocity at t =0μs corresponds to the velocity between t = 110 and 120μs in Fig. 7. The arrow indicates the large decrease in velocity right after bubble collapse. The dashed line is calculated with Stokes, the dotted line with Newton (outer limit of maximum drag). Calculation based on the (initial) velocity at, (A) t =0s(Re = 65), and (B) t =10 ls(Re = 28).

not only

related to drag

crack growth

Results and discussion

Effect of cavitation on crystal habit

Fig. 9. SEM pictures of voluminous fragments of seeds B1 with large planes of fracture

Fig. 10. SEM pictures of damaged calcite seeds. Possible broken aggregates (A, B); circular indentations caused by ultrasound (B, C) and by laser ablation (D) in calcite.

Fig. 11. Microjets captured at >250,000 fps.

plastic deformation

Griffith cracks

Conclusions

Cavitation clusters, evolved from cavitation inception and collapse, caused attrition, disruption of aggregates and deagglomeration.

Crystals that were accelerated by bubble expansion, subsequently experienced a deceleration much stronger than expected from drag forces, upon bubble collapse.

The appearance of voluminous fragments with large planes of fracture indicated that acoustic cavitation can cause the breakage of single crystal structures, and these indentations might be caused by shockwave induced jet impingement.

Disadvantage: no direct record of jet impingement on a suspended crystal

Ultrasonic Technology

concentration in the liquid state

crystal growth kinetics

CSD & suspension density

Fig. 12 Experimental setup with ultrasound sensor, controller, and electric stirrer

crystal growth kinetics

In liquids, the spread velocity ν of ultrasound waves through a liquid depends on the density ρL of the liquid and its adiabatic compressibility βad:

For determining the crystal growth kinetics, the following four steps were performed:

I. Measure the saturation point and the nucleation point by the ultrasonic technology.

II. Determine the correlation between the concentration of a solution and ultrasonic velocity and temperature.

III. Measure the decrease in supersaturation versus time using ultrasonic techniques.

IV. Calculate the kinetic parameters.

crystal growth kinetics

Determination of the kinetic parametersCrystal growth rate: The mass flux:

CSD & suspension density

Data Validation experimental velocity data:

the attenuation:φ - suspension density;βn - experimentally determined parameters

CSD & suspension density

Model Identification & Verification

LOGO