Dendrite Growth Rate Research Articles

Influence of impurities, strontium addition and cooling rate on microstructure evolution in Al-10Si-0.3Fe casting alloys

The effect of impurities, Sr additions and cooling rate on the microstructural evolution of high-purity Al-10Si-0.3Fe and corresponding impure commercial alloys is examined by optical microscopy and in situ by X-ray synchrotron 3D tomography. At fast cooling rate (∼470 K min−1) the presence of impurities decreases the growth rate of primary Al dendrites and enables formation of the β phase. Besides a modification of the eutectic Si, the addition of Sr prevents the formation of the β phase and increases the growth rate of Al dendrites. A low cooling rate (∼1 K min−1) leads to the formation of α, γ, δ and β intermetallic phases. In all four alloys, the dominant phase is the δ phase, regardless of commercial impurities or the Sr level. Intermetallic phases formed during slow cooling rates are much coarser than those formed during fast cooling and they have different morphologies. Our results suggest that the velocity of growth and the final morphology and size of the intermetallic phases are mainly determined by diffusional processes which in turn are controlled by the cooling rate.

Study on the Thermal Contraction Behaviors of Mg and Al

By using a device which could achieve the 1-Dimensional shrinkage instead of 3D shrinkage of the casting during the solidification and cooling, the cooling curves and the relationship between the shrinkage / temperature and time were investigated. Then, the thermal shrinkage behaviors of pure Mg and Al ingots were obtained. As a result, the characterization of the thermal shrinkage behavior of Mg and Al was systematically investigated. The results indicate that the contraction rate of pure Mg ingot increases with the increase of growth rate of dendrites in quantity during solidification. The maximum contraction rate νl, max will be reached when the formationof primary dendrite ends, and the value of νl, max is 1.12×10-2 mm·s-1. However, the shrinkage will not be present until the solid state cooling starting for the pure Al ingot. The contraction of pure Mg ingot in solid state can be divided into two stages: high temperature solid shrinkage stage and low temperature solid shrinkage stage. And the rate of shrinkage firstly increases then decreases with the temperature dropping at the stage of high temperature solid shrinkage. The rate of solid shrinkage of pure Al ingot is present at the beginning of solid-state cooling. And its value is 9.69×10-3 mm·s-1, which is more than 3 times of that of pure Mg ingot. The shrinkage behaviors of both pure Mg and Al ingots tend to be uniform shrinkage when the temperature is below 400°C.

Lithium Dendrite Suppression through Controlled Mass Transfer

Lithium dendrite formation is a critical challenge that limits the lifetime of lithium (Li) metal batteries including lithium oxygen, and lithium sulfur systems. Dendrite growth occurs at the interface between the electrolyte and the Li metal anode. The formation of dendrites affects the local transport properties, the scale of the critical physics of the anode, leads to increased Joule heating and eventual breakage of the dendrites, which can cause short circuits leading to overheating and possible fires. Dendrite formation also decreases the Li available for the electrochemical reactions of the battery, which causes a decrease in cell capacity. In addition to Li metal batteries, dendrite growth is also a problem for other battery chemistries, such as Li-ion, NaS and NiCd. Among previous experimental studies, it has been shown that the dendrites in Li batteries can form one of two structures depending on the operating conditions. Under high current conditions, dendrite formation is typically dense and considered mossy or bush-like in structure; while under low current conditions dendrites form dense needle-like structures. In addition to charging current, many other factors are reported to influence dendrite growth. A close relation between Li ion mass transfer and dendrite growth rate have been shown; while other groups have found a strong influence between convection effects and the morphology of the dendritic structure. Measuring dendrite growth in situ is challenging, and examining the dendrites post mortem is difficult as they can fall apart during disassembly. To better understand dendrite growth and morphology, computational modeling can be used to investigate the electrode-electrolyte interface and how operating conditions and battery design effect dendrite growth and morphology. In this work, we use a Lagrangian particle based method known as smoothed particle hydrodynamics (SPH) to simulate the reactive transport in the interfacial region to understand Li ion transport effects on dendrite growth. This includes investigation of material properties and battery design, including the use of hybrid electrolytes and separators to restrict Li ion transport. Further studies are conducted to consider how operation and charging rates effect growth. Results of these computational studies show that restricted Li ion transport can suppress dendrite growth without sacrificing battery performance. This can be accomplished by changing the materials of electrolyte or using hybrid electrolyte designs. Additionally the charging profile can also be used to effectively suppress dendrites.

Thermoelectric magnetohydrodynamic effects on the crystal growth rate of undercooled Ni dendrites.

In the undercooled solidification of pure metals, the dendrite tip velocity has been shown experimentally to have a strong dependence on the intensity of an external magnetic field, exhibiting several maxima and minima. In the experiments conducted in China, the undercooled solidification dynamics of pure Ni was studied using the glass fluxing method. Visual recordings of the progress of solidification are compared at different static fields up to 6 T. The introduction of microscopic convective transport through thermoelectric magnetohydrodynamics is a promising explanation for the observed changes of tip velocities. To address this problem, a purpose-built numerical code was used to solve the coupled equations representing the magnetohydrodynamic, thermal and solidification mechanisms. The underlying phenomena can be attributed to two competing flow fields, which were generated by orthogonal components of the magnetic field, parallel and transverse to the direction of growth. Their effects are either intensified or damped out with increasing magnetic field intensity, leading to the observed behaviour of the tip velocity. The results obtained reflect well the experimental findings.This article is part of the theme issue ‘From atomistic interfaces to dendritic patterns’.

Thin-Film Protected Lithium Metal Anode for Lithium Batteries

Li batteries based on Li metal anode are expected to provide 2-3 times more energy density than today's Li-ion battery. However, uncontrolled dendritic Li growth (continuous loss of Li) and limited Coulombic efficiency (CE) during Li deposition/stripping of a bulk Li anode, are the main reasons behind low cycle life, which has limited their practical applications [1,2]. Although low CE can be partially compensated by an excess amount of Li, it is achieved at the expense of lower useable Li energy density and elevated risks of fire and other hazards associated with Li. Therefore, efforts should be directed to reduce the use of Li, limited in weight to balance cathode capacity. For example in an effort to balance the sulfur (S)-cathode capacity with Li-anode capacity, we recently demonstrated that 20 µm thick Li can replace 380 µm thick Li and achieve better CE (Figure 1 (a)). Furthermore, the rate of dendrite growth gets amplified when the cell is cycled at high charge-discharge current densities, high charge voltages, and high areal discharge currents. Increasing anode surface area would reduce the effective charge/discharge current densities. We have shown that a 20 mm thick Li deposited on Cu current collector (CC) using thermal evaporation technique showed particulate morphology (high surface area) compared to featureless (low surface area) morphology of commonly used thick Li (Figure 1 (b)) that may have provided improved results as shown in Figure 1 (a). However, with increased surface area, more surface of thin Li is expected to be exposed to electrolytes or catholytes (especially in Li-S battery), and thus react faster than bulk Li causing cell performance degradation during long-term battery cycling. To reduce the high reactivity of thermally evaporated thin film Li, we will apply an artificial SEI (ASEI) consisting of a conducting polymer (poly(3-hexylthiophene) (P3HT)/fullerene (PCBM)/graphite layer developed at UDRI [3]. In thick Li metal based LIB, we have demonstrated that this ASEI as Li protective layer remarkably reduces dendrite growth, enhances cycle life more than 300% and protects Li by physically separating it from the electrolyte (Figure 1 (c)). The protected Li anode may be incorporated into wet cell batteries or solid state batteries having transition metal oxide cathodes (specifically, state-of-the-art Li-ion cathodes), sulfur cathode (Li-S battery), or oxygen cathode (Li-O2 battery). Moreover, similar methodologies can be applied to other rechargeable metal anodes (Na, Al, Mg, etc.) for next generation rechargeable batteries. We will present and discuss detailed research being conducted in our laboratories in order to optimize thin-film Li fabrication parameters, protection of thin-film Li anode and their use in Li batteries including Li-ion and Li-S batteries.

Characterizing Li-Ion Batteries Using in Situ MRI

We will describe our work on the development of techniques for assessing Li-ion batteries and battery materials via magnetic resonance imaging (MRI). The goal of these studies is to analyze battery degradation and energy storage mechanisms in situby imaging changes in both the electrolyte and the electrodes in a noninvasive fashion while a cell is charged or discharged. We will discuss recent breakthroughs we have made in performing MRI experiments on commercial pouch and coin cell batteries that allow diagnostic tests to be obtained without disassembling the cells. These tests can be run in situ or ex situ and can be used to indicate cycling properties of new battery components, to give a ‘fingerprint’ of manufactured cells for comparison and quality control purposes, or to offer engineering insights into the physical properties of the battery. We will also report on specific applications of MRI in functioning lithium metal batteries [1], where the technique can be used to correlate the behavior of the electrolyte concentration gradient to the type and rate of dendrite growth on the surface of the Li metal electrode. These studies have confirmed the existence of separate growth mechanisms in different charging regimes. The methodology is extremely sensitive to Li dendrite formation, opening the possibility of testing a broad range of materials and operating conditions to understand when dendrites grow and how they can be prevented. By developing a detailed understanding of the impact lithium dendrites have on their surroundings, we have been able to design fast 1H MRI experiments that are extremely sensitive to local dendrite growth [2]. The experiments allow for the 3D structure of the dendrites to be reconstructed, and are fast enough to capture the dendrite growth in real time, shedding light on the growth behavior, rate and morphology of the structures formed. Moreover, through simulations and a detailed analysis of the results obtained, we show how the MRI can be used to quantify the volume of dendrites grown. 1. Chang, H. J.; Ilott, A. J.; Trease, N. M.; Mohammadi, M.; Jerschow, A.; Grey, C. P. J. Am. Chem. Soc. 2015, 137(48), 15209–15216. 2. Ilott, A. J.; Mohammadi, M.; Chang, H. J.; Grey, C. P.; Jerschow; Proc. Natl. Acad. Sci. 2016 113 (39), 10779–10784. Figure caption: Regions of microscopic lithium dendrites that have grown between two lithium metal electrodes. The image is reconstructed from a 15 minute 1H MRI scan. Figure 1

A Model of Concurrent Lithium Dendrite Growth, SEI Growth, SEI Penetration and Regrowth

We present a model that enables simulations of concurrent coupled lithium dendrite growth, SEI formation and growth on the dendrite surface, dendrite penetration of the SEI layer, and SEI regrowth. The concept of SEI coverage density is introduced, which is conceptually similar to SEI thickness but broader. SEI and dendrite growth interacts with each other by evolving dendrite morphology, stretching of SEI by the dendrite surface, and the effect of evolving SEI thickness (coverage density) on lithium deposition reaction. The model shows that even under low current density, small protrusions on lithium surface can still grow into dendrites by penetrating SEI and causing inhomogeneous electrical resistance on dendrite surface. We find that dendrite growth shows two distinct stages: fast acceleration during SEI penetration, and stable growth after reaching an equilibrium SEI thickness at the dendrite tip. Under low current density, increasing current scales up the growth rate of dendrite without much shape change. High current density causes dendrite morphology to change significantly, forming a needle shape. We show that local SEI defects/inhomogeneity can induce dendrite formation and growth. Our study suggests that flat lithium surface, uniform SEI layer, and low SEI resistivity are important to reduce dendrite formation.

Investigating the effect of ionic strength on the suppression of dendrite formation during metal electrodeposition.

The effect of ionic strength on the electrodeposition of silver has been investigated in acetonitrile (MeCN) containing TBAPF6 or in the ionic liquid [EMIm][OTf]. The use of an ionic liquid allows a greater ionic strength to be investigated as the solubility limits of supporting electrolytes in organic solvents can be overcome using neat ionic liquid. The SEM and XRD data show that polycrystalline silver is deposited in a fcc structure and that dendrite formation is retarded at high ionic strength. Electrochemical measurements undertaken in electrolytes of low ionic strength indicate that the deposition and growth of a few nuclei is preferred and leads to dendrite formation. However, at higher ionic strength, the deposition and growth of significantly more nuclei is observed and therefore dendrite growth rates and tip currents are lower leading to the deposition of spherical particulates. Crucially, the data shows that if the ionic strength of the electrolyte is controlled there are no differences between ionic liquids and molecular solvents for the electrodeposition of silver.

Single lithium-ion conducting solid polymer electrolytes: advances and perspectives.

Electrochemical energy storage is one of the main societal challenges to humankind in this century. The performances of classical Li-ion batteries (LIBs) with non-aqueous liquid electrolytes have made great advances in the past two decades, but the intrinsic instability of liquid electrolytes results in safety issues, and the energy density of the state-of-the-art LIBs cannot satisfy the practical requirement. Therefore, rechargeable lithium metal batteries (LMBs) have been intensively investigated considering the high theoretical capacity of lithium metal and its low negative potential. However, the progress in the field of non-aqueous liquid electrolytes for LMBs has been sluggish, with several seemingly insurmountable barriers, including dendritic Li growth and rapid capacity fading. Solid polymer electrolytes (SPEs) offer a perfect solution to these safety concerns and to the enhancement of energy density. Traditional SPEs are dual-ion conductors, in which both cations and anions are mobile and will cause a concentration polarization thus leading to poor performances of both LIBs and LMBs. Single lithium-ion (Li-ion) conducting solid polymer electrolytes (SLIC-SPEs), which have anions covalently bonded to the polymer, inorganic backbone, or immobilized by anion acceptors, are generally accepted to have advantages over conventional dual-ion conducting SPEs for application in LMBs. A high Li-ion transference number (LTN), the absence of the detrimental effect of anion polarization, and the low rate of Li dendrite growth are examples of benefits of SLIC-SPEs. To date, many types of SLIC-SPEs have been reported, including those based on organic polymers, organic-inorganic hybrid polymers and anion acceptors. In this review, a brief overview of synthetic strategies on how to realize SLIC-SPEs is given. The fundamental physical and electrochemical properties of SLIC-SPEs prepared by different methods are discussed in detail. In particular, special attention is paid to the SLIC-SPEs with high ionic conductivity and high LTN. Finally, perspectives on the main challenges and focus on the future research are also presented.

Real-Time X-ray Imaging Reveals Interfacial Growth, Suppression, and Dissolution of Zinc Dendrites Dependent on Anions of Ionic Liquid Additives for Rechargeable Battery Applications.

The dynamic interfacial growth, suppression, and dissolution of zinc dendrites have been studied with the imidazolium ionic liquids (ILs) as additives on the basis of in situ synchrotron radiation X-ray imaging. The phase contrast difference of real-time images indicates that zinc dendrites are preferentially developed on the substrate surface in the ammoniacal electrolytes. After adding imidazolium ILs, both nucleation overpotential and polarization extent increase in the order of additive-free < EMI-Cl < EMI-PF6 < EMI-TFSA < EMI-DCA. The real-time X-ray images show that the EMI-Cl can suppress zinc dendrites, but result in the formation of the loose deposits. The EMI-PF6 and EMI-TFSA additives can smooth the deposit morphology through suppressing the initiation and growth of dendritic zinc. The addition of EMI-DCA increases the number of dendrite initiation sites, whereas it decreases the growth rate of dendrites. Furthermore, the dissolution behaviors of zinc deposits are compared. The zinc dendrites show a slow dissolution process in the additive-free electrolyte, whereas zinc deposits are easily detached from the substrate in the presence of EMI-Cl, EMI-PF6, or EMI-TFSA due to the formation of the loose structure. Hence, the dependence of zinc dendrites on anions of imidazolium IL additives during both electrodeposition and dissolution processes has been elucidated. These results could provide the valuable information in perfecting the performance of zinc-based rechargeable batteries.

A Thermal Simulation Method for Solidification Process of Steel Slab in Continuous Casting

Eighty years after the invention of continuous cast of steels, reproducibility from few mm3 samples in the laboratory to m3 product in plants is still a challenge. We have engineered a thermal simulation method to simulate the continuous casting process. The temperature gradient (GL) and dendritic growth rate (v) of the slab were reproduced by controlling temperature and cooling intensity at hot and chill end, respectively, in our simulation samples. To verify that our samples can simulate the cast slab in continuous casting process, the heat transfer, solidification structure, and macrosegregation of the simulating sample were compared to those of a much larger continuous casting slab. The morphology of solid/liquid interface, solidified shell thickness, and dendritic growth rate were also investigated by in situ quenching the solidifying sample. Shell thickness (δ) determined by our quenching experiment was related to solidification time (τ) by equation: δ = 4.27 × τ0.38. The results indicated that our method closely simulated the solidification process of continuous casting.

Simulation of Lithium Dendrite Growth: Surface Morphology Effect

Li metal has been considered as an anode material for lithium secondary batteries (LSBs) for over four decades. However, its uncontrolled chemical reactivity towards organic solvents and dendritic growth at random in electrochemical cells delayed its commercialization in the secondary battery market. Thus, without developing stable Li metal electrode, we will not encounter lithium-oxygen (Li-O2) and lithium-sulfur (Li-S) batteries as well as lithium metal secondary battery forever. However, regardless of tremendous endeavour to control the Li metal, unfortunately, the dendrite formation and growth mechanism has not been fully understood owing to the limited analysis in operando. Therefore, we try to contruct a mathematical model to study the current distribution upon the Li metal and the growth rate of dendrites with a simulation tool, COMSOL Multiphysics 5.2. Since this simulation contains a moving electrode surface during electrodeposition process, the deformed geometry should be considered carefully. From these simulated results, we can have different dendrite growth behaviours depending on initial Li metal surficial morphology.

In-Situ Scanning Electron Microscopy Observation of Lithium Dendrite Growth on Carbon Electrodes

The formation of lithium dendrites at the anode during charging can result in capacity fade and potentially lead to internal short circuits. Adopting high capacity next-generation technologies, such as Li-air or Li-S, exacerbates this problem. Although this phenomenon has been known to be a primary safety concern and source of failure for decades, limited remedies exist to suppress dendrites growth. This partially stems from our limited understanding of the mechanism(s) governing Li dendrite nucleation and growth during cycling. While several models have been proposed, a general agreement has yet to be reached, which motivates our current work. In this work, in-situ scanning electron microscopy was used to investigate the growth of lithium dendrites. The cell was composed of lithium cobalt oxide as the cathode, carbon film as the anode, and a Li imide salt dissolved in P13TFSI ionic liquid as the electrolyte. We were able to observe operando growth of lithium dendrites on the carbon film. The experimental configuration differs from typical Li-ion systems, since the dendrites do not grow along the field gradient, suggesting that this is not the primary source of dendrite growth. The morphologies and growth rates of lithium dendrites were characterized as a function of time and applied voltage. Additionally, the focused ion beam was utilized to cross section the lithium dendrites in-situ in order to characterize the internal morphology. The results provide new insights into the roles of surficial reactions and stresses in driving the formation of dendrites.

Study on the Solidification of Sn-Pb Alloy under Direct Current Field by Synchrotron X-Ray Radiography

In this paper, synchrotron X-ray radiography was used to study the grain refining performance of direct current (DC) during the solidification of Sn-50 wt.% Pb alloy. Based on the radiographs, the variations of equiaxed grain number and dendrite growth rate with time were measured and analyzed. It is shown that DC mainly affects the microstructure formation through changing the nucleation behavior and the dendrite growth rate. When DC was applied during the whole process of solidification, the nucleation was enhanced and the dendrite growth was suppressed, which resulted in significant grain refinement.

Prediction of Melt Flow Effects on Dendrite Growth

The effects of melt flow on dendrite growth during solidification are studied by the quantitative phase field model coupling the Navier-Stokes equations. Through analyzing the relationship between flow velocity and dendrite growth rate in simulations, a flow Péclet number involving with characteristic flow velocity, characteristic length of the zone affected by flow and thermal (solute) diffusion coefficient, is suggested for dendrite growth under convections. The growth rate increment due to flow follows a power-law relationship with the Péclet number. As the Péclet number is much higher than one, the influence of convection on dendrite growth is apparent, whereas as it is below one, the flow effects can be neglected.

Phase field simulation of dendrite growth in binary Ni–Cu alloy under the applied temperature gradient

Phase field method was employed to investigate the effect of the applied temperature gradient on the microstructure evolution for the binary Ni–Cu alloy during the solidification process. As a comparison group, the isothermal solidification and the non-isothermal solidification taking into account the effect of latent heat were simulated at the same time. Simulation results showed that temperature distribution could greatly influence dendrite morphology, concentration distribution and tip velocity. Under the applied temperature gradient, a cone shape of dendrite, instead of the normal dendrite with four symmetric arms, could be found easily. The growth rate of dendrite and the segregation of concentration field increased with the increasing temperature gradient. This model was able to give a clear outlook for the influence of temperature gradient on the dendrite morphology and help a better design for the microstructure control in the field of direct chill casting.

Study on Dendritic Growth in Pressurized Solidification of Mg–Al Alloy Using Phase Field Simulation

A multi-grain phase field model coupled with thermodynamic calculation was adopted to describe the dendritic growth in pressurized solidification of Mg–Al alloy during squeeze casting, in which the effects of the pressure on the Gibbs free energy and chemical potential of solid and liquid phases, the solute diffusion coefficient, and the solute partition coefficient were considered. The individual effect of solute diffusion coefficient, and the Gibbs free energy on the dendritic growth was studied. With the comparison of the dendritic growth under atmospheric and elevated pressures, the effect of pressure on the microstructure evolution was discussed. The results showed that the grains are refined, the dendritic growth rate tends to increase and the secondary dendrite arms are more developed as the pressure is increased from 0.1 to 100 MPa, which showed a good agreement with the experimental results of direct squeeze casting of Mg–Al alloy. As the pressure increases, the largest dendritic growth rate can be obtained under the pressure between 200 and 250 MPa, while the growth rate decreases with a further increase of pressure.

Investigating the Effects of Transport and Mixing on Dendrite Growth in High Energy Density Lithium Batteries

High Energy density lithium (Li) batteries are promising advanced energy technologies for applications in the automotive and portable power markets. Advanced Li batteries have several design and operational challenges to overcome before becoming commercially viable. One of these challenges is the formation of dendrites on the anode surface of the Li-air battery over multiple charge and discharge cycles. Dendrite formation decreases cell performance and causes safety concerns due to short circuiting. In this research computational methods are adopted to investigate the physics of dendrite formation in Li-air batteries and to develop strategies to suppress dendrite growth. The effects of convection and anisotropic transport properties in the electrolyte on the morphology and growth rate of dendrites are investigated using a two-dimensional (2D) smoothed particle hydrodynamics (SPH) model. SPH is a mesh-free Lagrangian computational fluid dynamics method, which allows for easy implementation of complex physics at the dendrite surface. Anisotropic diffusion is modeled to study the effects of mixing near the anode-electrolyte interface on dendrite growth. The simulation results suggest that high anisotropy in the electrolyte promotes a robust, compact dendrite structure versus isotropic diffusion cases. Convection effects are also considered to study the electro-osmosis effect on dendrite growth. Circulating flows are included in the model to simulate electro-osmotic flows near the anode-electrolyte interface. The simulation results reveal that the convection effect will increase the mixing near the dendrite and lead to significant changes in dendrite morphology. The results of these computational studies are being used to develop mitigation strategies for dendrite growth in Li-air batteries. These strategies include the use of novel electrolyte solutions and hybrid electrolyte designs, which allow for the tuning of transport properties through the electrolyte. The computational models are being used to investigate these designs and optimize material properties to increase battery performance and lifetime. Figure 1

In Situ Optical Microscopy Studies of Spontaneous Oscillatory Growth of Zinc Dendrites

Rechargeable batteries have been an integral part of our society since the invention of lead-acid batteries in mid-1800s.[1] With the recent demand for clean and sustainable energy, recyclable and rechargeable batteries have received renewed attention. Among the rechargeable batteries, metal-air batteries are considered promising because of their high energy density, reliable lifetime, minimal environmental impact, and low cost.[2, 3] Of all the metal-air battery systems, Li-air and Zn-air batteries are extremely attractive because of their high energy densities. While Li is highly reactive, Zn is relatively more stable and desirable because of its mild response to moisture, low cost, abundance, and easy manufacturability. However, the drawbacks associated with Zn-based batteries include relatively low operating potential, poor stability of the air electrode, and poor recyclability.[2] One of the critical factors affecting the rechargeability of Zn-air batteries is the formation of unstable growth morphologies, notably dendrites -- highly anisotropic and porous structures. These dendrites are vulnerable to mechanical vibrations and can cause catastrophic failure by shorting the anode and the cathode. Here, we present results from in situ optical microscopy studies of Zn dendrite growth during electrodeposition of Zn in zinc sulfate (ZnSO4) solutions. All of our experiments are carried out using an electrochemical Hele-Shaw cell. The cell consists of a circular (10 cm in diameter) Zn wire (99.994% purity) anode sandwiched between two Plexiglas plates, separated by a polydimethylsiloxane (PDMS) spacer and filled with 100-μm-thick solution of ZnSO4. A vertical Zn electrode (0.25 mm in diameter) placed at the center of the cell serves as the cathode. In order to prevent bubble formation, the electrolyte is introduced slowly into the cell. In our experiments, we observe an interesting phenomenon: oscillatory growth of the dendrites, during electrodeposition in 0.1 M ZnSO4 with pH = 3.0. Upon applying a constant potential across the electrodes, the dendrites start to grow with a self-similar structure (see Figure 1). From the images of the dendrites obtained as a function of time, we find that the dendrite growth rate changes periodically. Concomitant with this oscillatory growth, we observe a periodic variation in deposition current. Scanning electron microscopy (SEM) images (see Figure 2(a)) of the dendrites reveal that they have self-similar morphology. Tilted (Figure 2(b)) and cross-sectional images (not included here) also show that the dendrites are made of micro-crystallites sticking out of a main arm, and that the dendrite thickness varies periodically along the length of the dendrites. Transmission electron diffraction patterns acquired from the sample show that the growth structures are single-crystalline and with the same crystal orientation. Based upon these initial results, we propose a growth model to explain the oscillatory growth. We gratefully acknowledge funding from the National Science Foundation (NSF-GOALI: DMR-1310639). Figure caption: Figure 1. In situ optical micrograph acquired during electrodeposition of Zn in 0.1 M ZnSO4 solution with pH = 3 at Ewe = -2.5 V for 20 minutes. The scale bar is 100 µm in length. Figure 2. (a) Top and (b) 58°-tilt view scanning electron microscopy images of the dendrites extracted from the solution. The layered morphology is characteristic of oscillatory growth. The scale bars are 40 µm in length. Figure 1

Influence of solidification on the impact of supercooled water drops onto cold surfaces

This study presents an experimental investigation of the impact of a supercooled drop onto hydrophilic and superhydrophobic substrates. The aim is to better understand the process of airframe icing caused by supercooled large droplets, which has been recently identified as a severe hazard in aviation. The Weber number and Reynolds number of the impinging drop ranged from 200 to 300 and from 2600 to 5800, respectively. Drop impact, spreading, and rebound were observed using a high-speed video system. The maximum spreading diameter of an impacting drop on hydrophilic surfaces was measured. The temperature effect on this parameter was only minor for a wide range of the drop and substrate temperatures. However, ice/water mixtures emerged when both the drop and substrate temperatures were below 0 °C. Similarly, drop rebound on superhydrophobic substrates was significantly hindered by solidification when supercooled drop impacted onto substrates below the freezing point. The minimum receding diameter and the speed of ice accretion on the substrate were measured for various wall temperatures. Both parameters increased almost linearly with decreasing wall temperature, but eventually leveled off beyond a certain substrate temperature. The rate of ice formation on the substrate was significantly higher than the growth rate of free ice dendrites, implying that multiple nucleation sites were present.