Michael A. Lomholt

+45 65 503 475

I am a theoretical physicist working within the fields of statistical mechanics, stochastic processes, biophysics, and soft condensed matter physics. I have included a few selected research highlights below to illustrate the kind of work I do.

Further info: Publications (pdf) / Preprints

Elastic moderation of intrinsically applied tension in lipid membranes

Tension in lipid membranes is often controlled externally, by pulling on the boundary of the membrane or changing osmotic pressure across a curved membrane. But modifications of the tension can also be induced in an internal fashion, for instance as a byproduct of changing a membrane’s electric potential or, as observed experimentally, by activity of membrane proteins. Here we develop a theory that demonstrates how such internal contributions to the tension are moderated through elastic stretching of the membrane when the membrane is initially in a low-tension floppy state.

Michael A. Lomholt, Bastien Loubet, and John H. Ipsen, Phys. Rev. E 83, 011913 (2011).


Dissimilar bouncy walkers

We consider the dynamics of a one-dimensional system consisting of dissimilar hardcore interacting (bouncy) random walkers. The walkers’ (diffusing particles’) friction constants are drawn from a distribution. We provide an approximate analytic solution to this recent single-file problem by combining harmonization and effective medium techniques. Two classes of systems are identified: when the distribution is heavy-tailed we identify a new universality class in which density relaxations, characterized by the dynamic structure factor, follows a Mittag-Leffler relaxation, and the mean square displacement (MSD) of a tracer particle grows as a subdiffusive powerlaw with time. If instead the distribution is light-tailed such that the mean friction constant exist, the structure factor decays exponentially and the MSD scales with the square root of time. We also derive tracer particle force response relations. All results are corroborated by simulations and explained in a simplified model.

Michael A. Lomholt, Ludvig Lizana, and Tobias Ambjornsson, J. Chem. Phys. 134, 045101 (2011).


Facilitated diffusion with DNA coiling

When DNA-binding proteins search for their specific binding site on a DNA molecule they alternate between linear 1-dimensional diffusion along the DNA molecule, mediated by nonspecific binding, and 3-dimensional volume excursion events between successive dissociation from and rebinding to DNA. If the DNA molecule is kept in a straight configuration, for instance, by optical tweezers, these 3-dimensional excursions may be divided into long volume excursions and short hops along the DNA. These short hops correspond to immediate rebindings after dissociation such that a rebinding event to the DNA occurs at a site that is close to the site of the preceding dissociation. When the DNA molecule is allowed to coil up, immediate rebinding may also lead to so-called intersegmental jumps, i.e., immediate rebindings to a DNA segment that is far away from the unbinding site when measured in the chemical distance along the DNA, but close by in the embedding 3-dimensional space. This effect is made possible by DNA looping. The significance of intersegmental jumps was recently demonstrated in a single DNA optical tweezers setup. Here we present a theoretical approach in which we explicitly take the effect of DNA coiling into account. By including the spatial correlations of the short hops we demonstrate how the facilitated diffusion model can be extended to account for intersegmental jumping at varying DNA densities. It is also shown that our approach provides a quantitative interpretation of the experimentally measured enhancement of the target location by DNA-binding proteins.

Michael A. Lomholt, Bram van den Broek, Svenja-Marei J. Kalisch, Gijs J. L. Wuite, and Ralf Metzler, Proc. Natl. Acad. Sci. USA 106, 8204, (2009).


Lévy strategies in intermittent search processes are advantageous

Intermittent search processes switch between local Brownian search events and ballistic relocation phases. We demonstrate analytically and numerically in one dimension that when relocation times are Lévy distributed, resulting in a Lévy walk dynamics, the search process significantly outperforms the previously investigated case of exponentially distributed relocation times: The resulting Lévy walks reduce oversampling and thus further optimize the intermittent search strategy in the critical situation of rare targets. We also show that a searching agent that uses the Lévy strategy is much less sensitive to the target density, which would require considerably less adaptation by the searcher.

Michael A. Lomholt, Tal Koren, Ralf Metzler, and Joseph Klafter, Proc. Natl. Acad. Sci. USA 105, 11055 (2008).