抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

A comprehensive review of spatiotemporal pattern formation in systems driven away from equilibrium is presented, with emphasis on comparisons between theory and quantitative experiments. Examples include patterns in hydrodynamic systems such as thermal convection in pure fluids and binary mixtures, Taylor-Couette flow, parametric-wave instabilities, as well as patterns in solidification fronts, nonlinear optics, oscillatory chemical reactions and excitable biological media. The theoretical starting point is usually a set of deterministic equations of motion, typically in the form of nonlinear partial differential equations. These are sometimes supplemented by stochastic terms representing thermal or instrumental noise, but for macroscopic systems and carefully designed experiments the stochastic forces are often negligible. An aim of theory is to describe solutions of the deterministic equations that are likely to be reached starting from typical initial conditions and to persist at long times. A unified description is developed, based on the linear instabilities of a homogeneous state, which leads naturally to a classification of patterns in terms of the characteristic wave vector q0 and frequency ω0 of the instability. Type Is systems (ω0=0, q0≠0) are stationary in time and periodic in space; type IIIo systems (ω0≠0, q0=0) are periodic in time and uniform in space; and type Io systems (ω0≠0, q0≠0) are periodic in both space and time. Near a continuous (or supercritical) instability, the dynamics may be accurately described via "amplitude equations," whose form is universal for each type of instability. The specifics of each system enter only through the nonuniversal coefficients. Far from the instability threshold a different universal description known as the "phase equation" may be derived, but it is restricted to slow distortions of an ideal pattern. For many systems appropriate starting equations are either not known or too complicated to analyze conveniently. It is thus useful to introduce phenomenological order-parameter models, which lead to the correct amplitude equations near threshold, and which may be solved analytically or numerically in the nonlinear regime away from the instability. The above theoretical methods are useful in analyzing "real pattern effects" such as the influence of external boundaries, or the formation and dynamics of defects in ideal structures. An important element in nonequilibrium systems is the appearance of deterministic chaos. A greal deal is known about systems with a small number of degrees of freedom displaying "temporal chaos," where the structure of the phase space can be analyzed in detail. For spatially extended systems with many degrees of freedom, on the other hand, one is dealing with spatiotemporal chaos and appropriate methods of analysis need to be developed. In addition to the general features of nonequilibrium pattern formation discussed above, detailed reviews of theoretical and experimental work on many specific systems are presented. These include Rayleigh-Benard convection in a pure fluid, convection in binary-fluid mixtures, electrohydrodynamic convection in nematic liquid crystals, Taylor-Couette flow between rotating cylinders, parametric surface waves, patterns in certain open flow systems, oscillatory chemical reactions, static and dynamic patterns in biological media, crystallization fronts, and patterns in nonlinear optics. A concluding section summarizes what has and has not been accomplished, and attempts to assess the prospects for the future.

/pdf/pattern-formation-outside-of-equilibrium-1601b9znum.pdf

Pattern formation outside of equilibrium

Images and force measurements taken by an atomic‐force microscope (AFM) depend greatly on the properties of the spring and tip used to probe the sample’s surface. In this article, we describe a simple, nondestructive procedure for measuring the force constant, resonant frequency, and quality factor of an AFM cantilever spring and the effective radius of curvature of an AFM tip. Our procedure uses the AFM itself and does not require additional equipment.

Calibration of atomic‐force microscope tips

N G van Kampen 1981 Amsterdam: North-Holland xiv + 419 pp price Dfl 180 This is a book which, at a lower price, could be expected to become an essential part of the library of every physical scientist concerned with problems involving fluctuations and stochastic processes, as well as those who just enjoy a beautifully written book. It provides an extensive graduate-level introduction which is clear, cautious, interesting and readable.

https://iopscience.iop.org/article/10.1088/0031-9112/34/4/040/pdf

Stochastic Processes in Physics and Chemistry

http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

Within a phase-field model that couples a nonconserved order parameter to the temperature field, we study the crystal growth velocity υ in a supercooled liquid as a function of the undercooling Δ. The velocity shows critical behavior in Δ depending on a the ratio p of effective diffusion constants and on δ, which measures the coupling between the order parameter and the temperature field. As Δ becomes smaller, there is a transition from steady-state growth to a diffusive regime. At the phase boundary, υ remains nonzero for p>pc; otherwise, it vanishes as (Δ - Δc)ν with ν = 1 for p<pc, ν = 1/2 for p = pc, and ν = 1/3 for p = pc and δ = δc, which corresponds to a critical point (Δc, pc, δc) in the (Δ, p, δ) parameter space. Typical experimental systems have p<pc; however, if impurity concentration gradients dominate over temperature variations, p is much larger and δ may be tuned by varying the overall impurity concentration.

http://iopscience.iop.org/article/10.1209/0295-5075/16/2/013/pdf

Critical Behavior of Crystal Growth Velocity

On the properties of large banded spherulites in a maleic anhydride–polyacrylonitrile mixture

We have investigated the smectic-to-crystal phase transition of the liquid crystal 1O OCB The smectic plays the role of a viscous liquid, and the system shares essential characteristics with the freezing of polymers, viscous liquids, and liquid metals The 10 OCB material has the advantage of convenient time scales: “rapid”-solidification effects occur at freezing velocities of 10–100 μm/s, versus 10 μm/h for polymers and 1 m/s for metals We observe six different solidification modes, each characterized by a distinct micro- and meso-structure In addition, each mode is correlated with a separate branch of the front-velocity versus undercooling curve This curve has a discontinuity in the slope at mode transitions In one case, the velocity curve itself is discontinuous A suggested morphology-selection hypothesis — “the fastest mode wins” — is explicitly ruled out

Many modes of rapid solidification in a liquid crystal

Feedback traps can manipulate particles arbitrarily In a feedback trap, a position detector detects the particle’s position, a computer calculates the necessary force to be applied based on the position in the “virtual potential,” which is applied to the particle The process is repeated with as fast a loop rate as practical Previous feedback traps have used electrokinetic or hydrodynamic forces to manipulate particles Here, an optical trap creates the force used by the feedback trap to impose arbitrary potentials We create feedback forces on optically trapped particles by moving the trap position rapidly in response to observed fluctuations with the help of an acoustooptic deflector (AOD) In preliminary experiments, we have confined a 15 μm silica bead in a virtual potential that is 35-40 times stiffer than the underlying optical trap, whose laser power is kept constant We also create a virtual double-well potential with independent control over the well separation and barrier height, which is impossible to do with time-sharing optical tweezers

Optical feedback tweezers

For an overdamped colloidal particle diffusing in a fluid in a controllable, virtual potential, we show that arbitrarily slow transformations, produced by smooth deformations of a double-well potential, need not be reversible. The arbitrarily slow transformations do need to be fast compared to the barrier crossing time, but that time can be extremely long. We consider two types of cyclic, isothermal transformations of a double-well potential. Both start and end in the same equilibrium state, and both use the same basic operations---but in different order. By measuring the work for finite cycle times and extrapolating to infinite times, we found that one transformation required no work, while the other required a finite amount of work, no matter how slowly it was carried out. The difference traces back to the observation that when time is reversed, the two protocols have different outcomes, when carried out arbitrarily slowly. A recently derived formula relating work production to the relative entropy of forward and backward path probabilities predicts the observed work average.

John Bechhoefer

Papers

Critical Behavior of Crystal Growth Velocity

On the properties of large banded spherulites in a maleic anhydride–polyacrylonitrile mixture

Many modes of rapid solidification in a liquid crystal

Optical feedback tweezers

Arbitrarily slow, non-quasistatic, isothermal transformations