The Laboratory of Acoustics at KU Leuven was founded more than 50 years ago. Until today, various acoustics-related topics, ranging from physical acoustics through building and room acoustics up to environmental acoustics and noise-related health issues, have been investigated. In the second half of the 20th century, the laboratory was one of the main centres of expertise in acoustics in Belgium, contributing to consulting and establishing legislation in building and environmental acoustics. In the 1990s, it also consolidated expertise in the characterisation of porous materials and was one of the driving research groups in the field of photoacoustics. During the past 15 years, additional research directions have been taken thanks to interdisciplinary collaborations, including psychoacoustics, perception of sound, sound quality assessment, archaeo-acoustics, tackling acoustic issues in building retrofit, and characterisation of walls materials in the framework of sustainable development (recycled materials, biomaterials etc.) This paper first brings a brief historical overview of the past activities of the Laboratory of Acoustics (and Thermal Physics) (ATF), its involvement in national and international collaborations and its main recent scientific and educational activities.

Correlations between material properties are useful in engineering, and in addition, the underlying common mechanisms allow for a better understanding of the origins of the properties. Properties related to phase changes are an example, being important, e.g., in thermodynamic applications. For n-alkanes used, e.g., as phase change materials for thermal energy storage, linear correlations between enthalpy changes and entropy changes in phase transitions, as well as the number of carbon atoms n, have been observed and described by many researchers. Different correlations for odd and even n were found, though still with significant outliers. In this work, data from high-accuracy and high-resolution calorimetry were used for the analysis of enthalpy and entropy changes for alkanes with n = 14 to 30. The analysis shows more, and different, correlations than described in the previously published literature. Specifically, the ‘outliers’ have a physical and chemical origin, rooted in the phase transitions present in the specific n-alkanes, not just in whether n is odd or even. These detailed findings promise a better understanding of the thermodynamics of phase transitions.

The idea that rod-like molecules possessing an electric dipole moment could exhibit a ferroelectric nematic phase was suggested more than a century ago. However, only recently such a phase has been reported for two quite different liquid crystals: RM734 (4-[(4-nitrophenoxy)carbonyl)]phenyl 2,4-dimethoxybenzoate) and DIO (2.3',4',5'-tetrafluoro[1,1'-biphenyl]-4-yl 2.6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate). For RM734 a direct ferroelectric nematic (NF) to classical nematic N transition was reported, whereas for DIO an intermediate phase Nx was discovered between the NF and the N phases. Here we present high-resolution calorimetric evidence that an intermediate Nx phase also exists in RM734 along a narrow temperature range between the NF and the N phases.

The discovery of the cyanobiphenyl and related compounds in 1973 not only played a significant role in important innovations in displays and related technologies but also opened opportunities for testing of novel insights in understanding phase transitions and critical phenomena. In the study of these phenomena, high-resolution calorimetry made substantial contributions. In this paper, we present an overview of the high-resolution calorimetry results obtained during the past 50 years on the phase transitions that occur in the cyanobiphenyl compounds. For the isotropic-nematic and isotropic-smectic A transitions, their (weakly) first-order nature is well established and the temperature dependence of the pretransitional-specific heat capacity strongly points in the direction of a nearby tricritical point. For the nematic-smectic A transition, precise latent heat values of several binary liquid crystal (mainly of cyanobiphenyl type) mixtures give significant evidence on the Halperin, Lubensky and Ma prediction of the always weakly first-order nature of this transition. The dependence of the specific heat capacity critical exponent alpha on the width of the nematic range, resulting from coupling between the nematic and the smectic order parameters as pointed out by de Gennes, is well confirmed.

In this work, a Peltier-element-based adiabatic scanning calorimeter was used to investigate, with high-resolution and accuracy, the temperature dependence of the specific heat capacity and specific enthalpy difference of a series of linear alkanes. More specifically, results are reported for the even n-alkanes; namely n-eicosane, n-octadecane, n-hexadecane and for the odd n-alkanes, namely n-nonadecane, n-heptadecane and n-pentadecane. The measurements cover a temperature range from well in the crystalline solid phase across the solid–solid transitions (when present) in to the liquid phase across the melting transition. Accurate transition temperatures and heat of transitions were derived from the direct experimental data in view of the potential use of n-alkanes as phase change materials.

Hypothesis: The micellization of block copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) is driven by the dehydration of PPO at elevated temperatures. At low concentrations, a vis-cous solution of isolated micelles is obtained, whereas at higher concentrations, crowding of micelles results in an elastic gel. Alternating PEO-PPO multiblock copolymers are expected to exhibit different phase behavior, with altered phase boundaries and thermodynamics, as compared to PEO-PPO-PEO tri-block copolymers (Pluronics (R)) with equal hydrophobicity, thereby proving the pivotal role of copolymer architecture and molecular weight.Experiments: Multiple characterization techniques were used to map the phase behavior as a function of temperature and concentration of PEO-PPO multiblock copolymers (ExpertGel (R)) in aqueous solution. These techniques include shear rheology, differential and adiabatic scanning calorimetry, isothermal titration calorimetry and light transmittance. The micellar size and topology were studied by dynamic light scattering.Findings: Multiblocks have lower transition temperatures and higher thermodynamic driving forces for micellization as compared to triblocks due to the presence of more than one PPO block per chain. With increasing concentration, the multiblock copolymers in solution gradually evolve into a viscoelastic network formed by soluble bridges in between micellar nodes, whereas hairy triblock micelles jam into liquid crystalline phases resembling an elastic colloidal crystal.(c) 2023 Elsevier Inc. All rights reserved.

High-resolution calorimetry has played a significant role in providing detailed information on phase transitions in liquid crystals. In particular, adiabatic scanning calorimetry (ASC), capable of providing simultaneous information on the temperature dependence of the specific enthalpy h(T) and on the specific heat capacity c_{p}(T), has proven to be an important tool to determine the order of transitions and render high-resolution information on pretransitional thermal behavior. Here we report on ASC results on the compound 2,3',4',5'-tetrafluoro[1,1'-biphenyl]-4-yl 2,6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate (DIO) and on mixtures with 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734). Both compounds exhibit a low-temperature ferroelectric nematic phase (N_{F}) and a high-temperature paraelectric nematic phase (N). However, in DIO these two phases are separated by an intermediate phase (N_{x}). From the detailed data of h(T) and c_{p}(T), we found that the intermediate phase was present in all the mixtures over the complete composition range, albeit with strongly decreasing temperature width for that phase with decreasing mole fraction of DIO (x_{DIO}). The x_{DIO} dependence on the transition temperatures for both transitions could be well described by a quadratic function. Both these transitions were weakly first order. The true latent heat of the N_{x}-N transition of DIO was as low as L=0.0075±0.0005J/g and L=0.23±0.03J/g for the N_{F}-N_{x} transition, which is about twice the previously reported value of 0.115 J/g for the N_{F}-N transition in RM734. In the mixtures both transition latent heats decrease gradually with decreasing x_{DIO}. At all the N_{x}-N transitions pretransition fluctuation effects are absent and these transitions are purely but very weakly first order. As in RM734 the transition from the N_{F} to the higher-temperature phase exhibits substantial pretransitional behavior, in particular, in the high-temperature phase. Power-law analysis of c_{p}(T) resulted in an effective critical exponent α=0.88±0.1 for DIO and this value decreased in the mixtures with decreasing x_{DIO} toward α=0.50±0.05 reported for RM734. Ideal mixture analysis of the phase diagram was consistent with ideal mixture behavior provided the total transition enthalpy change was used in the analysis.

In high-resolution adiabatic scanning calorimetry (ASC) experiments, data for the temperature dependence of the specific enthalpy, h(T), and of the specific heat capacity, c(p)(T), are simultaneously obtained, from which the order of the phase transition and critical behaviour can be evaluated. ASC was applied to study the nematic to ferroelectric nematic phase transition (N-N-F) in the liquid crystal molecule 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734). The N-N-F was found to be very weakly first order with a latent heat Delta h = 0.115 +/- 0.005 J/g. The pretransitional specific heat capacity behaviour is substantially larger in the high-temperature N phase than in the low-temperature N-F phase. In both phases the power-law analysis of c(p)(T) resulted in a critical exponent alpha = 0.50 +/- 0.05 and amplitude ratio A(NF)/A(N) = 0.42 +/- 0.03. The very small latent heat and the value of alpha indicate that the N-N-F transition is close to a tricritical point. This is confirmed by a value of the order parameter exponent beta approximate to 0.25, recently obtained from electric polarisation measurements. Invoking two-scale-factor universality, it follows from the low value of A(NF)/A(N) ratio that the size of the critical fluctuations is much larger in the N-F phase than in the N phase.

The electrocaloric heat production in $$\hbox {BaTiO}_3$$ -based multilayer ceramic capacitors with Y5V specification was measured in a direct way by means of an adiabatic calorimeter setup. Applying an electric field of 30 $$\hbox {MV}\,\hbox {m}^{-1}$$ is found to result in a heat release of 0.94 $$\hbox {J}\,\hbox {g}^{-1}$$ and an electrocaloric temperature change of 0.46 K, in good agreement with direct results in previous studies.

High-resolution Peltier-element-based adiabatic scanning calorimetry (pASC) has been used to investigate the temperature dependence of the specific heat capacity and specific enthalpy of two mixture systems of the liquid crystal smectic A1 compound 5-n-nonyl-2-(4′-isothiocyanatophenyl)dioxane-1.3 (9DBT), and one of the smectic Ad compounds 4-n-octyloxy-4′-cyanobiphenyl (8OCB) and 4-nonyloxy-4′cyanobiphenyl (9OCB). For both mixture systems, measurements have been carried out over a large temperature range, from the crystalline solid phase over the smectic and nematic phases into the isotropic liquid phase. Both systems exhibit substantial, mixing-induced, enhanced nematic ranges and large changes in the composition dependence of the transition temperatures between the different phases. In both systems, eutectic melting points have been located and, for the different mixtures, the melting and eutectic transition heats have been obtained. The nematic to isotropic (NI) transitions are weakly first order with latent heat values in the range usually observed for this transition in other liquid crystals. The critical behavior of the specific heat capacity at the NI transition is described by exponent values near the tricritical one of 0.5. Along the smectic A to nematic (AN) transition lines, strong composition dependence was observed for the latent heat, from almost zero to values comparable to those observed at the nematic to isotropic transition. The concentration dependence of these AN latent heats was adequately fitted with a crossover function consistent with a mean-field free-energy expression that has a nonzero cubic term; the latter is induced by the Halperin-Lubensky-Ma coupling between the smectic A order parameter and orientational order director fluctuations. The fitting analysis resulted in the location of a Landau-tricritical point along one branch of the transition line in the system 9DBT-8OCB and one in each branch (for low and high 9OCB mole fractions) of the NA transition line of the 9DBT-9OCB system. The effective critical exponent values for the specific heat capacity of the AN transitions follow the McMillan ratio-dependence observed for other liquid crystal mixtures.

A novel Peltier-element-based adiabatic scanning calorimeter has been used to investigate the temperature dependence of the specific heat capacity and the specific enthalpy for n-alkanes heptadecane and nonadecane and for a set of their binary mixtures. The measurements have covered a broad temperature range from well into the ordered solid phase to well into the liquid phase. The analysis of the experimental data has allowed us to obtain accurate results for the phase transition temperatures, the transition enthalpies and the specific heat capacity values in the different phases. (c) 2021 Elsevier Ltd.

Recent advances in experimental studies of nanoparticle-driven stabilization of chiral liquid-crystalline phases are highlighted. The stabilization is achieved via the nanoparticles’ assembly in the defect lattices of the soft liquid-crystalline hosts. This is of significant importance for understanding the interactions of nanoparticles with topological defects and for envisioned technological applications. We demonstrate that blue phases are stabilized and twist-grain boundary phases are induced by dispersing surface-functionalized CdSSe quantum dots, spherical Au nanoparticles, as well as MoS2 nanoplatelets and reduced-graphene oxide nanosheets in chiral liquid crystals. Phase diagrams are shown based on calorimetric and optical measurements. Our findings related to the role of the nanoparticle core composition, size, shape, and surface coating on the stabilization effect are presented, followed by an overview of and comparison with other related studies in the literature. Moreover, the key points of the underlying mechanisms are summarized and prospects in the field are briefly discussed.

A study of phase transitions of photochromic p,p’-n-alkylazobenzenes compounds was carried out by means of adiabatic scanning calorimetry (ASC) and photopyroelectric calorimetry (PPE) techniques, yielding accurately the temperature dependence of the enthalpy and of the specific heat capacity across the observed phase transitions, and the corresponding enthalpy of transition values. In addition, polarization microscopy imaging of the sample texture was performed together with the PPE calorimetric evaluations to establish the nature of the involved phases. By performing PPE calorimetric evaluations during UV irradiation of the samples, the influence of the photoinduced trans to cis isomerization of the nAB molecules on the occurrence of the phase transitions was assessed.

Current applications of PCM focus on the temperature range from about − 40 °C to about + 80 °C, with the interest expanding up to about 400 °C for future applications in power plants and industry. Calorimetric measurements to determine the ability of PCM to store heat as a function of temperature are done with a variety of methods and instruments. Some of them have been developed only in the last decade, especially to allow characterization of larger samples, even encapsulated PCM with volume in the order of 100 ml and larger. Currently there are not enough reference materials for calibration and testing available to sufficiently cover the required temperature range, being suitable for the used variety of samples regarding size, as well as other requirements of the new methods and instruments. In this paper, we give an introduction, review, and recommendations for new reference materials for calibration and testing.

Itraconazole, an antifungal drug, is a thermotropic liquid crystal that exhibits nematic (N) and smectic A (SmA) phases, when cooled from the melt. By means of high-resolution adiabatic scanning calorimetry (ASC), we have obtained the temperature dependence of the heat capacity as well as the enthalpy (including latent heats) of the nematic to smectic A (N-SmA) and the isotropic to nematic (I-N) phase transitions. The N-SmA transition is weakly first-order, with substantial pretransitional heat capacity increases. The critical exponent cx, obtained from power law fits to the heat capacity data, is 0.50 +/- 0.05, suggesting that the N-SmA transition must be very dose to a tricritical point. Indeed, with this character, the small molecule dopant glycerol (in binary mixtures with ITZ), causes interesting changes to the mesomorphic phase sequence and to the order of the phase transitions. With increasing glycerol content, the temperature width of the nematic phase systematically reduces, until a critical concentration, at which the nematic phase disappears, leading to a direct isotropic-smectic A (I-SmA) transition. The I-SmA transitions of the ITZ-glycerol mixtures show stronger first-order characte r with substantial latent heats and wide two-phase regions, when compared to the (N-SmA and I-N) transitions of neat itraconazole. The ability of glycerol to drive the ITZ transitions to stronger first-order character indicates a possible coupling of the additive concentration to the smectic order parameter, which leads to the development of highly ordered, stable smectic structures. (C) 2020 Elsevier B.V. All rights reserved.

The collagen denaturation process in several artificially aged modern parchment samples were studied by combining Adiabatic Scanning Calorimetry and Light Transmission Analysis investigations. It was found that the calorimetric evaluations enabled not only to confirm the previously reported specific heat results concerning the collagen denaturation peaks, but also to identify a novel broad feature most likely associated with additional transformations occurring once denaturation takes place. Moreover, the combination of the investigations by the two adopted techniques allowed to validate the Light Transmission Analysis as a novel simple method capable of recording the entire collagen denaturation temperature path of the analyzed samples. Finally, the polarization microscopy imaging, performed simultaneously with the optical transmission characterization, enabled to perform a direct correlation of the morphology changes of the parchment with the different stages of the collagen denaturation.

We address the question on the validity of the ideal volumetric mixing approximation, a customarily used approach to analyze liquid-liquid phase transitions. Thus, liquid-liquid coexistence curves for the binary mixture composed by nitromethane and 3-pentanol were obtained from direct single-phase mass density measurements as a function of temperature. A reasonable agreement with previously reported data obtained from refractive index measurements has been found, indicating that assuming ideal mixing is rather reasonable. Furthermore, the implications of the ideal volumetric mixing approximation on asymmetric liquid-liquid criticality are explored in the framework of complete scaling formulation, with emphasis on the coexistence curves in the mole fraction-temperature and volume fraction-temperature planes as well as on the behaviour of the isobaric heat capacity per particle and per unit volume and the associated Yang-Yang-type anomalies. (C) 2018 Published by Elsevier Ltd.

We present new high-resolution experimental data for the temperature behavior of optical birefringence for a series of mixture of the liquid crystals octyloxycyanobiphenyl (8OCB) and nonyloxycyanobiphenyl (9OCB) by using a rotating analyzer technique. The birefringence data have been used to probe the temperature dependence of the nematic order parameter \( S(T)\). We have then arrived at values for possible entropy discontinuities at the nematic-smectic A transition temperature \( T_{NA}\) from the detailed inspection of \( S(T)\) data in the immediate vicinity of \( T_{NA}\). The 9OCB mole fraction dependence of the obtained reduced entropy discontinuities has been shown to be well fitted with a crossover function which is itself consistent with the mean-field free energy expression with a non-zero cubic term arising from the Halperin-Lubensky-Ma (HLM) coupling. The obtained results are in good accordance with existing results from adiabatic scanning calorimetry (ASC). Our birefringence results and determined entropy discontinuities (consistent with calorimetry results) are in striking contrast with the recent birefringence results of Barman et al. (Phase Transit. 91, 58 (2018) published online 16 Aug. 2017) claiming second-order nematic-to-smectic A transitions for all mixtures. In this paper we present a possible explanation for this discrepancy. We have also extracted the effective critical exponent values \( \alpha_{eff}\) characterizing the critical fluctuations near the N-SmA transition for all compositions by using the fact that the temperature derivative of the order parameter \( S(T)\) near \( T_{NA}\) exhibits the same power-law divergence as the specific heat capacity. Measurable latent heat values were extracted from optical birefringence data for mole fractions of 9OCB where the \( \alpha_{eff}\) values are as low as 0.2, which is substantially lower than the tricritical value \( \alpha_{TCP}=0.5\). This is qualitatively different from what has been observed so far in other liquid-crystal systems. Together with ASC data, these pecuilarities of the 8OCB+9OCB system render further convincing evidence for the presence of the HLM coupling effect at the N-SmA transition phase transition line.

The study of the nature of various phase transitions between rotator phases in several linear alkanes was performed by analyzing the hysteretic behavior of the specific heat between heating and cooling measurements. The investigations have been carried out by both adiabatic scanning calorimetry and photopyroelectric calorimetry techniques, whose combined use has provided complementary information concerning the changes occurring in the samples’ structure during their temperature change. The study enabled to establish that, unlike what previously reported, the Riii-Riv and the Rii-Riv transitions are of first order, despite them not showing any sharp peak profile in the temperature dependence of the specific heat. The first order of the Rv-Rii transition was confirmed also in the present study and, finally the Rv-Riii transition could be observed by calorimetric detection for the first time and shown to be of second order. The obtained results were discussed in terms of order parameters power terms in the Landau free energy expansion.