An overview of the state-of-the-art for capturing CO2via electrochemical routes.

We investigate the coupled dynamics of the local hydrodynamics and global electric response of an electrodialysis system, which consists of an electrolyte solution adjacent to a charge selective membrane under electric forcing. Under a dc electric current, counterions transport through the charged membrane while the passage of co-ions is restricted, thereby developing ion concentration polarization (ICP) or gradients. At sufficiently large currents, simultaneous measurements of voltage drop and flow field reveal several distinct dynamic regimes. Initially, the electrodialysis system displays a steady Ohmic voltage difference (ΔV_{ohm}), followed by a constant voltage jump (ΔV_{c}). Immediately after this voltage increase, microvortices set in and grow both in size and speed with time. After this growth, the resultant voltage levels off around a fixed value. The average vortex size and speed stabilize as well, while the individual vortices become unsteady and dynamic. These quantitative results reveal that microvortices set in with an excess voltage drop (above ΔV_{ohm}+ΔV_{c}) and sustain an approximately constant electrical conductivity, destroying the initial ICP with significantly low viscous dissipation.

The present study focuses on the effect of ultrasound (42 kHz, 7.1 W cm −2 ) on the free drift precipitation of CaCO 3 from a clear, supersaturated solution. To delineate the way ultrasound exerts its effect, we applied different treatment periods (time windows). Applying ultrasound during the first 10 min of the experiment did not result in any significant effect which rules out an influence on primary nucleation. The application of ultrasound starting later in the experiment enhanced precipitation of CaCO 3 . The dominant mechanism responsible for enhanced precipitation is deaggregation during the early growth phase (nuclei to crystals conversion regime). This effect is attributed to shear induced by micromixing and/or shear/stress induced by (supersonic) shockwaves, as a result of cavitation. With ultrasound applied, online pH and scattering measurements displayed a reduction in induction time and an increase in the volumetric precipitation rate. Scanning electron microscopy analysis shows that ultrasound increases the total number of particles that has, in addition, a more uniform size distribution compared with the untreated experiment. Consequently the available surface area for growth is higher resulting in a higher volumetric precipitation rate. With and without ultrasound the formed polymorph was predominantly vaterite with small amounts of calcite. ■ INTRODUCTIONCalcium carbonate is one of the most abundant minerals on earth, and its scaling propensity is a problem in many industrial water treatment processes. Moreover it is an important raw material in a wide range of industries. Controlling the formation of calcium carbonate is therefore of great interest, and as a result the crystallization of CaCO 3 from clear solution (spontaneous precipitation) has been investigated extensively. 1−15 In the initial phase of precipitation, or nucleation, clusters of calcium carbonate obtain a critical size. In the past decade more evidence has pointed to the existence of a two-step nucleation mechanism 16 in which the nucleation of a crystal occurs within stable mesoscopic clusters of dense liquid. These prenucleation phases were also found in calcium carbonate experiments. 17,18 Calcium carbonate nucleation, in this case the vaterite polymorph, consists of several steps. Prenucleation clusters are formed (∼1 nm) and aggregate to nanoparticles with a size distribution of around 30 nm. 18 These particles aggregate and grow out at the expense of others. The nanocrystalline domains share the same three-dimensional (3D) orientation, resulting in the development of a single crystalline structure. 18,19 Mostly, some time elapses before a measurable amount of the newly formed material is detected, the induction time, or induction period, and it marks the ability of the solution to stay in a metastable state. This induction time is heavily dependent on the measurement method and various methods have been used by others. [4][5][6][7]20,21 The application of ultrasound during crystallization and precipitation processes rec...

The acoustic cavitation (42,080 Hz, 7.1 W cm(-2) or 17 W) effects on suspended calcite crystals, sized between 5 and 50 μm, have been visualized for the first time using high speed photography. High speed recordings with a duration of 1 s containing up to 300,000 frames per second, revealed the effect of cluster and streamer cavitation on several calcite crystals. Cavitation clusters, evolved from cavitation inception and collapse, caused attrition, disruption of aggregates and deagglomeration, whereas streamer cavitation was observed to cause deagglomeration only. Cavitation on the surface gave the crystals momentum. However, it is shown that breakage of accelerated crystals by interparticle collisions is unrealistic because of their small sizes and low velocities. Crystals that were accelerated by bubble expansion, subsequently experienced a deceleration much stronger than expected from drag forces, upon bubble collapse. Experiments with pre-dried crystals seemed to support the current theory on bubble nucleation through the presence of pre-existing gas pockets. However, experiments with fully wetted crystals also showed the nucleation of bubbles on the crystal surface. Although microjet impingement on the crystal surface could not be directly visualized with high speed photography, scanning electron microscopy (SEM) analysis of irradiated calcite seeds showed deep circular indentations. It was suggested that these indentations might be caused by shockwave induced jet impingement. Furthermore, the appearance of voluminous fragments with large planes of fracture indicated that acoustic cavitation can also cause the breakage of single crystal structures.

We present a new and simple method to produce superhydrophobic surfaces with ultralow hysteresis. The method involves surface modification of SU-8 using an excimer laser treatment. The modified surface is coated with a hydrophobic plasma-polymerized hexafluoropropene layer. The advancing and receding water contact angles were measured to be approximately 165 degrees . The achieved water contact angle hysteresis was below the measurement limit. This low hysteresis can be ascribed to nanoscale debris generated during the excimer laser process.