Penger Tong

Turbulent thermal convection in a cell with ordered rough boundaries

A novel convection experiment is conducted in a cell with rough upper and lower surfaces. The measured heat transport in the rough cell is found to be increased by more than 76%. Flow visualization and near-wall temperature measurements reveal new dynamics for the emission of thermal plumes. The experiment shows that the interaction between the horizontal shear flow due to the large-scale circulation and the ordered rough surface creates a secondary flow (eddies) in the groove region. The secondary flow together with the large-scale circulation enhance the detachment of the thermal boundary layer from the tip of the rough elements. These extra thermal plumes are responsible for the enhanced heat transport in the rough cell. The discovery of the enhanced heat transport has important applications in engineering for more ecient heat transfer.

Velocity oscillations in turbulent Rayleigh–Bénard convection

A systematic study of velocity oscillations in turbulent thermal convection is carried out in small aspect-ratio cells filled with water. Local velocity fluctuations and temperature-velocity cross-correlation functions are measured over varying Rayleigh numbers and spatial positions across the entire convection cell. These structural measurements reveal how the thermal plumes interact with the bulk fluid in a closed cell and provide an interesting physical picture for the dynamics of the temperature and velocity oscillations in turbulent convection.

Short-time self-diffusion of nearly hard spheres at an oil–water interface

Optical microscopy and multi-particle tracking are used to study hydrodynamic interactions of monodisperse polymethylmethacrylate (PMMA) spheres at a decalin–water interface. The short-time self-diffusion coefficient measured at low surface coverage has the form D S S ( n ) = α D 0 (1 − β n ), where n is the area fraction occupied by the particles, and D 0 is the Stokes–Einstein diffusion coefficient in the bulk suspension of PMMA spheres in decalin. The measured values of α are found to be in good agreement with the numerical calculation for the drag coefficient of interfacial particles. The measured values of β differ from that obtained for bulk suspensions, indicating that hydrodynamic interactions between the particles have interesting new features at the interface.

Dual-beam incoherent cross-correlation spectroscopy

An incoherent dynamic light-scattering technique is developed to measure the local velocity and its statistics. By employing two parallel laser beams of different colors, the technique measures the cross-correlation function of the scattered intensities from two separate illuminating volumes. Because there is no phase coherence between the two laser beams, the measured cross-correlation function is sensitive only to the intensity fluctuations caused by a seed particle that crosses the two beams in succession. The flow velocity is obtained from the measured particle transit time. We frame the scattering theory so as to account for the two-beam scattering geometry. Our experiment verifies the calculation and demonstrates applications of the technique. The method has the unique feature of being able to measure simultaneously the local velocity in two opposite directions perpendicular to the incident laser beams. Its advantages are high spatial resolution and accuracy, fast temporal response, and ease of use. The technique is useful in studies of turbulent flows, sedimentation of heavy particles, and flow phenomena in complex fluids.

Experimental study of velocity boundary layer near a rough conducting surface in turbulent natural convection

Dual-beam intensity cross-correlation spectroscopy is used to measure the viscous boundary layer over a rough conducting surface in turbulent convection in water. It is found that the velocity boundary layer near the rough surface can be characterized by three quantities: the maximum velocity v m , the shear rate γ v and the total boundary layer thickness d+δ v ; all of them obey power laws of Rayleigh number Ra. The maximum velocity v m is located outside the rough surface and its numerical value remains the same as that in the smooth cell. The Ra-dependence of d+δ v is found to be the same as that in the smooth cell, but the power-law amplitude is increased by a factor close to 2. Because of the formation of small eddies inside the roughening grooves, the measured γ v is reduced considerably in the groove region. It is also found that the Nusselt number Nu ∼ Ra 0.35, which is larger in both magnitude and scaling exponent as compared to the smooth case.

Dynamic heterogeneity and non-Gaussian statistics for acetylcholine receptors on live cell membrane

The Brownian motion of molecules at thermal equilibrium usually has a finite correlation time and will eventually be randomized after a long delay time, so that their displacement follows the Gaussian statistics. This is true even when the molecules have experienced a complex environment with a finite correlation time. Here, we report that the lateral motion of the acetylcholine receptors on live muscle cell membranes does not follow the Gaussian statistics for normal Brownian diffusion. From a careful analysis of a large volume of the protein trajectories obtained over a wide range of sampling rates and long durations, we find that the normalized histogram of the protein displacements shows an exponential tail, which is robust and universal for cells under different conditions. The experiment indicates that the observed non-Gaussian statistics and dynamic heterogeneity are inherently linked to the slow-active remodelling of the underlying cortical actin network.

Studies of phospholipid vesicle deposition/transformation on a polymer surface by dissipative quartz crystal microbalance and atomic force microscopy.

Polymer-supported phospholipid bilayers (PLBs) are popular model systems for the study of transmembrane proteins under conditions close to cellular membrane environments. In this work, by combining the techniques of dissipative quartz crystal microbalance and atomic force microscopy, we investigate the deposition of vesicles on a hydrated cationic poly(diallyldimethylammonium chloride) (PDDA) layer as a function of phospholipid composition and sodium chloride concentration. The vesicles used consist of phospholipid mixtures with varying amounts of net negative charge. Uniform PLBs are formed by either increasing the negative charge density of the vesicles or decreasing sodium chloride concentration, suggesting that the electrostatic attraction between the vesicle and PDDA layer is the driving force for the formation of the PLBs. Our results indicate that the PLB formation is a fast adsorption-rupture process of the vesicles, without passing through a critical vesicle density. We further contend that the fluctuating PDDA support plays a central role for this process. This work provides a framework for understanding the key factors that influence the formation of PLBs.

Molecular transfer of surfactant bilayers: widening the range of substrates.

Handling nanometer-thick films and nano-objects remains a challenge. Applying self-assembly properties of surfactants to nanomaterials manipulation may be the key to the fast, easy, cost-effective growth of 2D and 3D nanostructures. Newton black films (NBFs) are self-assembled bilayers of surfactant, well-organized, but fragile objects. To render such films amenable to practical applications, it is necessary to find ways to transfer them onto solid substrates. A method developed recently to transfer NBFs onto a solid substrate while preserving their molecular organization (Benattar, J.-J.; Nedyalkov, M.; Lee, F. K.; Tsui, O. K. C. Angew. Chem., Int. Ed. 2006, 45, 4186) is broadened here to different surfaces. The method requires hydrophobic, planar, atomically smooth surfaces. This study presents the adhesion of a fluorinated NBF surfactant onto hydrophobically treated silica and silicon surfaces (with etching or silanization). The structures of the free-standing film, bare substrates, and transferred films are investigated using X-ray reflectivity. The homogeneity of the surfaces before and after bilayer deposition is examined by atomic force microscopy (AFM). Multiple transfers are tested and described for the future development of more complex architectures involving many surfactant layers and inserted nanosized objects.

Relative velocity fluctuations in turbulent flows at moderate Reynolds numbers. I. Experimental

Turbulent pipe flow and grid flow have been explored by the scattering of light from small particles suspended in a fluid. Laser Doppler velocimetry and visual observation were used to characterize the gross features of the flows. However, novel information came from the homodyne correlation function g(t), which was measured as a function of the Reynolds number, the photon momentum transfer, and the size of the scattering volume. In terms of these control variables, g(t) was found to be of scaling form. Using such measurements one can deduce from the probability distribution function, P(V,R), that two particles, separated by a distance R, have velocity difference V(R,t). For small‐velocity fluctuations, the scaling behavior of g(t) implies that P(V,R) has the form Q[V/u(R)]/u(R). This self‐similarity in P(V,R) is seen only when Re exceeds a transition Reynolds number Rec. The measured scaling velocity u(R) has the form u(R)∼Rζ, with ζ increasing from 0 at Re=Rec to ∼ (1)/(3) at the maximum attainable ...

Statistics of the locally averaged thermal dissipation rate in turbulent Rayleigh–Bénard convection

From the measured thermal dissipation rate in turbulent Rayleigh–Benard convection in a cylindrical cell, we construct a locally averaged thermal dissipation rate χ fτ by averaging over a time interval τ. We study how the statistical moments ⟨(χ fτ) p ⟩ depend on τ at various locations along the vertical axis of the convection cell. We find that ⟨(χ fτ) p ⟩ exhibits good scaling in τ, of about a decade long, with scaling exponents μ(p) for p = 1–6. For Rayleigh number (Ra) around 8×109, the scaling range is 1.4–21 s at the cell center and 4–21 s at the bottom plate. The dissipative and turnover times are about 0.8 s and 35 s respectively, while the timescale corresponding to the local Bolgiano scale is estimated to be about 31 s at the cell center and 3.5 s at the bottom plate. On the basis of several assumptions, we derive theoretical predictions for μ(p) at the different locations. The measured values of μ(p) are presented and shown to be in good agreement with our theoretical predictions.