258 research outputs found
Diffusion of active tracers in fluctuating fields
The problem of a particle diffusion in a fluctuating scalar field is studied.
In contrast to most studies of advection diffusion in random fields we analyze
the case where the particle position is also coupled to the dynamics of the
field. Physical realizations of this problem are numerous and range from the
diffusion of proteins in fluctuating membranes and the diffusion of localized
magnetic fields in spin systems. We present exact results for the diffusion
constant of particles diffusing in dynamical Gaussian fields in the adiabatic
limit where the field evolution is much faster than the particle diffusion. In
addition we compute the diffusion constant perturbatively, in the weak coupling
limit where the interaction of the particle with the field is small, using a
Kubo-type relation. Finally we construct a simple toy model which can be solved
exactly.Comment: 13 pages, 1 figur
A New Phase of Tethered Membranes: Tubules
We show that fluctuating tethered membranes with {\it any} intrinsic
anisotropy unavoidably exhibit a new phase between the previously predicted
``flat'' and ``crumpled'' phases, in high spatial dimensions where the
crumpled phase exists. In this new "tubule" phase, the membrane is crumpled in
one direction but extended nearly straight in the other. Its average thickness
is with the intrinsic size of the membrane. This phase
is more likely to persist down to than the crumpled phase. In Flory
theory, the universal exponent , which we conjecture is an exact
result. We study the elasticity and fluctuations of the tubule state, and the
transitions into it.Comment: 4 pages, self-unpacking uuencoded compressed postscript file with
figures already inside text; unpacking instructions are at the top of file.
To appear in Phys. Rev. Lett. November (1995
Fluctuation induced interactions between domains in membranes
We study a model lipid bilayer composed of a mixture of two incompatible
lipid types which have a natural tendency to segregate in the absence of
membrane fluctuations. The membrane is mechanically characterized by a local
bending rigidity which varies with the average local lipid
composition . We show, in the case where varies weakly with
, that the effective interaction between lipids of the same type can
either be everywhere attractive or can have a repulsive component at
intermediate distances greater than the typical lipid size. When this
interaction has a repulsive component, it can prevent macro-phase separation
and lead to separation in mesophases with a finite domain size. This effect
could be relevant to certain experimental and numerical observations of
mesoscopic domains in such systems.Comment: 9 pages RevTex, 1 eps figur
Interactions between proteins bound to biomembranes
We study a physical model for the interaction between general inclusions
bound to fluid membranes that possess finite tension, as well as the usual
bending rigidity. We are motivated by an interest in proteins bound to cell
membranes that apply forces to these membranes, due to either entropic or
direct chemical interactions. We find an exact analytic solution for the
repulsive interaction between two similar circularly symmetric inclusions. This
repulsion extends over length scales of order tens of nanometers, and contrasts
with the membrane-mediated contact attraction for similar inclusions on
tensionless membranes. For non circularly symmetric inclusions we study the
small, algebraically long-ranged, attractive contribution to the force that
arises. We discuss the relevance of our results to biological phenomena, such
as the budding of caveolae from cell membranes and the striations that are
observed on their coats.Comment: 22 pages, 2 figure
Fluctuations of Fluctuation-Induced "Casimir" Forces
The force experienced by objects embedded in a correlated medium undergoing
thermal fluctuations--the so-called fluctuation--induced force--is actually
itself a fluctuating quantity. We compute the corresponding probability
distribution and show that it is a Gaussian centered on the well-known Casimir
force, with a non-universal standard deviation that can be typically as large
as the mean force itself. The relevance of these results to the experimental
measurement of fluctuation-induced forces is discussed, as well as the
influence of the finite temporal resolution of the measuring apparatus.Comment: 4 pages, 2 figure
Engineered single- and multi-cell chemotaxis pathways in E. coli
We have engineered the chemotaxis system of Escherichia coli to respond to molecules that are not attractants for wild-type cells. The system depends on an artificially introduced enzymatic activity that converts the target molecule into a ligand for an E. coli chemoreceptor, thereby enabling the cells to respond to the new attractant. Two systems were designed, and both showed robust chemotactic responses in semisolid and liquid media. The first incorporates an asparaginase enzyme and the native E. coli aspartate receptor to produce a response to asparagine; the second uses penicillin acylase and an engineered chemoreceptor for phenylacetic acid to produce a response to phenylacetyl glycine. In addition, by taking advantage of a âhitchhiker' effect in which cells producing the ligand can induce chemotaxis of neighboring cells lacking enzymatic activity, we were able to design a more complex system that functions as a simple microbial consortium. The result effectively introduces a logical âAND' into the system so that the population only swims towards the combined gradients of two attractants
Genus Zero Correlation Functions in c<1 String Theory
We compute N-point correlation functions of pure vertex operator states(DK
states) for minimal models coupled to gravity. We obtain agreement with the
matrix model results on analytically continuing in the numbers of cosmological
constant operators and matter screening operators. We illustrate this for the
cases of the and models.Comment: 11 pages, LaTeX, IMSc--92/35. (revised) minor changes plus one
reference adde
Non-linear Structures in Non-critical NSR String
We investigate the Ward identities of the \W_{\infty} symmetry in the
super-Liouville theory coupled to the super-conformal matter of central charge
. The theory is classified into two chiralities.
For the positive chirality, all gravitationally dressed scaling operators are
generated from the gravitational primaries by acting one of the ring
generators in the R-sector on them repeatedly. After fixing the normalizations
of the dressed scaling operators, we find that the Ward identities are
expressed in the form of the {\it usual} \W_q algebra constraints as in the
bosonic case: \W^{(k+1)}_n \tau =0, , where the equations for even and odd come from the currents in the
NS- and the R-sector respectively. The non-linear terms come from the anomalous
contributions at the boundaries of moduli space. The negative chirality is
defined by interchanging the roles of and . Then we get the \W_p
algebra constraints.Comment: 22 pages, Latex file, YITP/U-94-16, UT-Komaba/94-1
On the amplitudes for non-critical n=2 supuerstrings
We compute correlation functions in non critical superstrings on the
sphere. Our calculations are restrained to the () bulk amplitudes. We show
that the four point function factorizes as a consequence of the non-critical
kinematics, but differently from the cases no extra discrete state
appears in the limit.Comment: 10 page
- âŠ